The Efficiency of a Fish Ladder for Salmonid

Upstream Migration in a Swedish Stream Potential Impact of a Hydropower Station on Connectivity and

Recruitment

Anton Larsson

Degree project for Master of Science in Biology

Animal Ecology, 30 hec, AT 2016

Department of Biological and Environmental Sciences University of Gothenburg

Supervisors: Johan Höjesjö, Lars-Olof Ramnelid, Daniel Johansson

Examiner: Charlotta Kvarnemo

1

Abstract Assessments of the function of fish passages are typically rare, although the approach is

frequently implemented to mitigate adverse effects of hydropower plants. In this study 249

electro fishing samples from 1979-2015, were used to assess the efficiency of a fish ladder to

allow upstream migration of salmonids past a hydropower station in Örekilsälven, Sweden.

Densities of brown trout (both young of the year, 0+, and older juveniles, >0+) did not

increase in the area upstream the hydropower station after construction of the fish passage;

neither did the densities of salmon 0+. >0+ salmon had a higher density upstream the

hydropower station after completion of the fish ladder, however this is most likely explained

by extensive fish translocations. 0+ salmon were only found in 5 % of the sampling occasions

upstream the power station when translocations were removed, whereas 0+ brown trout were

found in 44.3 %. No effect of discharge for ascension was found in the study. The efficiency

of the passage was determined low and non-satisfactory for Atlantic salmon and brown trout,

although the evaluation is more difficult for brown trout as a consequence of resident forms.

Smolt models indicate that contemporary smolt escapement of both salmon and brown trout

almost exclusively originate from the downstream areas. Improving the hydrological

connectivity will probably increase the smolt escapement from the area upstream the power

station, but the magnitude will depend on recolonization extent and mortality rates for smolts

migrating seawards. Although vast suitable spawning areas exist upstream the hydropower

station, natural features, including extensive migration distances and the presence of a lake

compose natural constraints in smolt escapement from the upstream area. Future studies

should include the aspect of downstream migration as a part of a holistic approach to improve

hydrological connectivity.

Keywords: Salmonid migration, fish passage, fish ladder, hydrological connectivity,

hydropower, electro fishing, smolt production

Cover photo: The fish ladder at the hydropower station in Torp, May 2016. Photo by the

author.

2

1. Introduction Freshwater ecosystems sustain high biodiversity compared to their limited area, but are

currently experiencing rapid declines in biodiversity, the rate exceeding those in the most

affected terrestrial ecosystems (Dudgeon et al. 2006). Dams and hydropower stations have

been extensively constructed in the last century (Graf 1999, WCD 2000, Nilsson et al. 2005)

and created impoundments by blocking transportation of water and altering the natural flow

regime of rivers (Nilsson and Berggren 2000, Nilsson et al. 2005, Moore et al. 2012). To date,

existing dams retain a volume of more than 10 000 km3 of water, equivalent of five times the

volume of all the world's rivers combined (Nilsson and Berggren 2000).

Although producing human services such as hydropower and water reservoirs, the foundation

of dams modifies streams and freshwater systems resulting in ecological impacts that affect a

wide array of taxa e.g. fish (e.g. Franchi et al. 2014, Poulos et al. 2014), amphibians (e.g.

Naniwadekar and Vasudevan 2014), macroinvertebrates (e.g. Benitez-Mora and Camargo

2014, Holt et al. 2015) and influencing stream nutrient levels (Zhou et al. 2015).

Perhaps the most imminent effect upon building a dam is the construction of a barrier to

migration, potentially impeding all movement across the dam and thus reducing hydrological

connectivity (Pringle 2001, Pringle 2003). The present study investigates the barrier effect of

a hydropower station on salmonid migration and smolt production in the river Örekilsälven on

the Swedish west coast.

The Atlantic salmon (Salmo salar) and the brown trout (Salmo trutta) are two anadromous

fish species migrating from marine environments to freshwater systems to breed and spawn

before returning to the ocean (Gibson 1993, Klemetsen et al. 2003). Strictly freshwater forms

of brown trout are common (Klemetsen et al. 2003) but typically rarer in Atlantic salmon

(Power 1958, Berg 1985, Klemetsen et al. 2003). Anadromous Atlantic salmon have a strong

natal homing behaviour, returning to their natal stream for spawning (Stabell 1984, Hansen et

al. 1993, Keefer et al. 2014). This trait restricts gene flow between different streams and

facilitates genetic divergence in rivers (Saunder 1981, Taylor 1991) not necessarily separated

by vast geographic distance (Verspoor 1997, Primmer et al. 2006, Vähä et al. 2007). Brown

trout has also been considered to have strong natal homing behaviour (Stuart 1957, Ferguson

1989) but recent studies suggest that straying can be considerable (Frank et al. 2012,

Degerman et al. 2012a, Östergren et al. 2012).

During the last centuries wild stocks of Atlantic salmon have decreased substantially or been

extirpated throughout their native range and are at present at the lowest levels in known

history (WWF 2001). The sources of decline are several and interconnected, including

pollution, overexploitation, acidification, aquaculture and dam constructions (Parrish et al.

1998, WWF 2001); the latter known to have caused multiple extirpations and severe

reductions of salmon populations (Limburg and Waldman 2009, Hall et al. 2012, Brown et al.

2013). Trends in population status of brown trout are limited and typically more complicated

to assess, but the species seem to experience declines in some areas whereas in other regions

it performs better (Pedersen et al. 2012, ICES 2013, Höjesjö et al. in press.A).

Due to the anadromous life cycle, dwindling stocks and commercial interest in salmonid

species, barrier effects of dams have often been addressed with the construction of different

types of fish passages as an attempt to allow upstream and downstream migration and ensure

population viability (Clay 1995, Schilt 2007, Calles and Greenberg 2009). Fish passages

include a variety of designs, i.e. fish ladders and bypasses, applied depending on location and

intention. However, despite extensive construction, studies assessing the function of these

3

passages are scarce (Schmutz et al. 1998, Roscoe and Hinch 2010, Bunt et al. 2012, Hatry et

al. 2013) and often indicate low efficiency (Noonan et al. 2012). Studies in this field use

confusingly resembling terminology but the difference is crucial to apprehend. The term

efficiency will in this report be applied as a quantitative concept defining the proportion of a

fish stock successfully migrating upstream a hydropower station, whereas the term

effectiveness will be used in a qualitative context, simply stating if target species are able to

pass the fish passage at some point (Larinier 2001). Fish passage efficiency is generally

determined by two aspects; (1) attraction efficiency meaning the proportion of individuals

present downstream the passage able to find the entrance of the fish passage and (2) passage

efficiency, the proportion of individuals locating the entrance that successfully ascend the fish

passage (Aarestrup et al. 2003, Bunt et al. 2012). In one of the few reviews assessing fish

passage efficiency, Noonan et al. (2012), reviewed 65 papers between 1960 to 2011 and

concluded that fish passage efficiency was about 50 % on average for both upstream and

downstream migration (although salmonids had slightly higher efficiency). Other studies have

also shown limited upstream migration success for salmonids (Linløkken 1993, Rivinoja et al.

2001, Croze et al. 2008, Lundqvist et al. 2008) although some fish passages seem to perform

better (Bryant et al. 1999, Gowans et al. 1999) indicating potential for improvement if dams

are constructed properly.

In Sweden, having approximately 2100 dams (Havs- och vattenmyndigheten 2014),

documentation and assessment of present fishways is also limited (Rivinoja 2015). One of

few reports assessing the efficiency of fishways was conducted in the county of Västra

Götaland (Andersson and Bäckstrand 2005). Investigating 62 fish passages (mostly pool and

weir passages), the authors concluded that only 53 % of these passages worked satisfactorily

(Andersson and Bäckstrand 2005). In other words, there is an increasing need to evaluate and

potentially modify fish passages in order to allow efficient migration for fish and other fauna

in lotic ecosystems with existing dams.

1.2 Study area

Örekilsälven is located on the Western coast of Sweden running 90 kilometres from the

county of Dalsland before entering the sea in the Gullmar fjord, close to Munkedal (Fig. 1). It

is one of the few rivers in the Western parts of Sweden still supporting a genetically distinct

salmon stock of high conservation value (Degerman et al. 1999). Brown trout is also present

in the area and is considered of high conservation value (Thorsson 2009). The catchment area

covers 1340 km2 and several larger tributaries add to the main stream channel. Forests cover

the majority of the catchment area (76 %) whereas the proportion of lakes is limited (3.7 %),

the largest lake being Kärnsjön (Fig. 1). Historically, Örekilsälven, has been subjected to

human alterations through different dams and mills constructions and modifications such as

log driving used for the transportation of timber for several centuries. At present, the river

system is mostly affected by various dam operations which compose one of the major impacts

in the system, potentially impeding fish migration and altering the natural flow regime of the

stream (Andersson and Bäckstrand 2005, Thorsson 2009). The hydropower station at Torp,

located approximately 10 kilometres upstream of the river mouth, constitute the first

anthropogenic obstacle in the system (Fig. 1). The present dam was built in 1984 but previous

dams and structures have existed at the current location for centuries. Due to human

modifications and degradation of the habitat for salmonid species, several conservation

actions have been carried out throughout the years including legal compensatory release of

foremost Atlantic salmon but also brown trout (of natural stock origin) and biotope

restorations. More recently (year 2000-2011) both 0+ (young of the year) and adult salmon

along with 0+ of brown trout have been transported manually upstream the hydropower

4

station of Torp. In 1991 a fish ladder, pool and weir type, was constructed to allow migration

past the hydropower station and give access to the extensive rearing habitats upstream the

dam. The fish ladder is 44 meters long, consisting of 22 chambers with a combined slope of

approximately 17 % and designed for a discharge of 0.25-0.5 m3/s (Thorsson 2009).

Quantitative assessment of passage efficiency is lacking but concerns about its efficiency

have been raised repeatedly (Andersson and Bäckstrand 2005, Länsstyrelsen 2005, Thorsson

2009).

1.3 Aim and hypothesis

The aim of this study is to assess the function (both effectiveness and efficiency) of the fish

ladder at the hydropower station of Torp in Örekilsälven, Sweden, for Atlantic salmon,

(Salmo salar) and brown trout (Salmo trutta), using data from electro fishing. The hypothesis

is that recruitment of these species is lower upstream of the dam compared to downstream.

The study also seeks to answer if the fish ladder is more functional during certain discharge.

Lastly, the study aims to estimate contemporary and potential increase in smolt production of

Atlantic salmon and brown trout if connectivity and accessibility to the suitable habitats

upstream of the hydropower station could be improved.

Figure 1. Örekilsälven and the electro fishing sampling sites included in the analysis. The location of Torp

hydropower station is marked by the red triangle.

5

2. Method

2.1 Data handling and analysis

Electrofishing is a widely used method for estimating density of territorial salmonid fish

species (Bohlin et al. 1989). In Sweden this method has been used at least since the 1940s

(Degerman and Sers 1999) and today it is estimated that more than 2000 sites are sampled

each year (Degerman et al. 2012b). To assess density of salmonids, data from the Swedish

national electrofishing register (Svenskt ElfiskeRegiSter, SERS, 2016) were extracted. A total

of 249 electrofishing samples, located downstream and upstream of the hydropower plant

conducted between 1979-2015 were included. Sampling sites were selected from the main

stream channel of Örekilsälven and from two major tributaries (Töftedalsån and

Hajumsälven). No sampling sites upstream of the dam at Torpfors, the northernmost

migration barrier 77 km from the sea, (Fig. 1) were included as suitable biotopes for

salmonids in this upper reach are very scarce. Also, only sampling sites located downstream

of anthropogenic barriers in the two tributaries, 58.7 km and 42.6 km respectively, were

included in order to exclude the impact of other dams in the analysis.

Usually salmonid density from electrofishing is estimated by fishing the same river stretch

repeatedly and successive removal of fish each time (Bohlin et al. 1989). However, in this

area, the majority of electrofishing occasions included only 1 fishing opportunity. For these

occasions, the density value calculated in SERS was used. The hypothesis of lower

recruitment of both salmon and brown trout was analysed with a Mann-Whitney U test,

comparing densities of 0+ (young of the year) in the areas downstream the hydropower station

to the areas upstream. To analyse the effectiveness and efficiency of the fish ladder, densities

during the episode before the fish ladder (1979-1991) and after construction of fish ladder

(1992-2015) were compared using a Mann-Whitney U test. A nonparametric test was used

since the data did not display normal distribution.

In order to investigate if the fish ladder had higher efficiency during certain discharge,

hydrological data from the Swedish meteorological and hydrological institute (SMHI) were

downloaded. As the adult salmonid fish in Örekilsälven are mainly autumn spawners,

(migrating upstream in September-Novembers), an average autumn discharge (calculated as a

monthly mean between September-November) was plotted against 0+ (young of the year)

densities upstream of the ladder the following year. Data were not normally distributed

(Kolmogorov-Smirnov P > 0.05, Shapiro-Wilk P > 0.05) and transformations using log10

(original value + 0.5) and √(original value + 0.5) both failed producing normality, hence no

regression was used. Instead, average autumn discharge was plotted against densities of 0+

following year and visualized for any linear trends. All statistics and analyses were made in

SPSS 22.

2.1.1 Correction of fish translocation and compensatory release

Due to historic compensatory release and extensive fish translocation, attempts to exclude

these effects were made. These corrections were only performed for density values of 0+ and

by excluding density values for those years and localities where translocations or

compensatory releases were conducted. Additionally, values of 0+ was removed the year after

release of adult fish for the location of release. When the locality of release was unknown, all

values from that year were removed from the analysis.

6

2.1.2 Smolt models

Nilsson et al. (2013) and Höjesjö et al. (in press.B) used a model called the SBS-model

(Swedish Biotope Survey) that combines electro fishing and biotope surveys of the whole

stream to estimate smolt escapement of sea trout, defined as the number of smolts reaching

the sea. The two smolt models used in this study follow the framework of Nilsson et al.

(2013) except for excluding the division >0+ (juveniles older than one summer) into further

year classes. Instead, a fixed smoltification value of >0+ was used for brown trout (30 %) and

a range of 40-50 % used for Atlantic salmon (E. Degerman, Swedish University of

Agricultural Science, personal communication). The smoltification value was multiplied with

the mean density of >0+ and the area of each stream section derived from the SBS (retrieved

from the Swedish Database for Biotope Surveys). The mean density value of >0+ for the area

upstream of the hydropower stations after completion of the fish ladder (year 1992-2015) was

used for present smolt escapement for all stream sections in this area, whereas the mean

density value of >0+ from the downstream area after construction of the fish ladder, was used

for all stream sections downstream the hydropower station. Two compensation factors, based

on spawning habitat and rearing habitat class according to Nilsson et al. (2013) were then

incorporated into the model. Depending on spawning habitat class, smolt production for each

stream section was multiplied with a value of 0.25-1 (spawning habitat class 0 = 0.25, class 1

= 0.5, class 2 = 0.75, class 3 = 1; Nilsson et al. 2013) whereas rearing habitat was multiplied

with a factor of 0-1 (rearing habitat class 0 = 0, class 1 = 0.26, class 2 = 0.57, class 3 = 1;

Nilsson et al. 2013). The most suitable habitat for salmonid parr is classified as 3 whereas a

non-suitable area receives the score of 0 (Halldén et al. 2002).

Finally, migration mortality is included in the model where lotic mortality is applied for the

stream sections and lentic mortality utilized for lake sections (only relevant for smolts

migrating from the upstream area). Two different lotic migration mortality scenarios were

used in the model presented here. The first one, called lotic mortality 1, used the same

mortality rate per kilometer as used in Nilsson et al. (2013). This mortality depends on the

rearing habitat class (habitat class 0 = 10-17 %, class 1 and 2 = 3-12 % and class 3 = 0-5 %).

The second lotic mortality rate, referred to as lotic mortality 2, used the same range of

mortality for all stretches and originates from the review of Thorstad et al. (2012) where

migration mortality of Atlantic salmon smolts ranged from 0.3-5 % per kilometer for wild

smolts. The same lotic mortality was used for Atlantic salmon and brown trout as done by

Aldvén et al. (2015).

For each stream section smolt production was multiplied with the lotic mortality rate raised to

the distance of the specific stream section divided by 2 (Nilsson et al. 2013; se equation 1).

This gives the average distance smolts produced in the specific stream section has to migrate

before reaching next stream segment downstream and hence assumes that smolt production on

each stream section is evenly distributed. To calculate the total smolt production, the number

of smolts produced in the most upstream situated stream section was multiplied with the

specific mortality of the adjacent downstream stream section. Secondly the smolts produced

in adjacent stream section was added (equation 2). The process was repeated until the last

stream section was added.

7

Equation 1

smolt production stream sectionn = area of stream sectionn (m2) * average density of >0+/m2 * smoltification value * spawning habitat compensation * rearing habitat compensation * specific migration mortality stream sectionn(length of stream section n/2)

Equation 2

smolt production stream sectionn * specific migration mortality stream sectionn+1length of

stream section n+1 + smolt production stream sectionn+1

For smolts produced upstream the lake of Kärnsjön, three different lentic mortality spans were

incorporated. The high mortality scenario used a mortality per kilometer of 25-71 % (Nilsson

et al. 2013), the medium mortality scenario used a mortality rate of 15-20 % per kilometer (J.

Höjesjö, Gothenburg University, personal communication) and the low mortality scenario

used a mortality of 5 % per kilometer (E. Degerman, Swedish University of Agricultural

Science, personal communication). The total distance in lake Kärnsjön is approximately 10

kilometers.

To calculate potential increase in salmon smolt production, two different scenarios of mean

productivity in the upstream region were employed. The first one used an upstream mean

density of 30 % of the downstream mean density and the second scenario used the same mean

density as the mean density in the downstream section. For each scenario an average number

of smolt escapement was extracted from 100 simulations with random values within the range

of smoltification and migration mortality using Microsoft Excel®.

3. Results

3.1 Densities upstream and downstream the power station

Densities was significantly higher in the downstream area for both Atlantic salmon 0+ (young

of the year) (Mann-Whitney U test: U = 1380, n1 =129 n2 = 120, P < 0.001; Fig. 2) and brown

trout 0+ (Mann-Whitney U test: U = 5307.5, n1 =129 n2 = 120, P < 0.001; Fig. 3) when

compared with the area upstream the hydropower station. Densities was also significantly

lower for the upstream stretches when adjusted for compensatory release and fish

translocation (see section 2.2.1) (Atlantic salmon; Mann-Whitney U test: U = 466, n1 =126 n2

= 93, P < 0.001; Fig. 4; brown trout; Mann-Whitney U test: U = 4950, n1 =128 n2 = 115, P <

0.001; Fig. 5). Atlantic salmon had significantly higher densities of >0+ in the area

downstream the power station (Mann-Whitney U test: U = 1665.5, n1 =129 n2 = 120, P <

0.001; Fig. 2). On the contrary densities of >0+ of brown trout was higher upstream (Mann-

Whitney U test: U = 4406, P < 0.001; Fig. 3).

8

Figure 3. Boxplot showing 0+, >0+ and total density of brown trout in Örekilsälven downstream

and upstream the hydropower station in Torp respectively between 1979-2015. The box-and-

whisker plots show median values (black lines), the interquartile ranges (boxes; 25th and 75th

percentiles), and the 5th and 95th percentiles (whiskers). Circles represent outliers, located 1,5-3

interquartile ranges from the end of the box and asterisks represent extreme values, located more

than 3 interquartile ranges from the end of the boxes.

Figure 2. Boxplot showing 0+, >0+ and total density of Atlantic salmon in Örekilsälven

downstream and upstream the hydropower station in Torp respectively between 1979-2015. The

box-and-whisker plots show median values (black lines), the interquartile ranges (boxes; 25th and

75th percentiles), and the 5th and 95th percentiles (whiskers). Circles represent outliers, located

1,5-3 interquartile ranges from the end of the box and asterisks represent extreme values, located

more than 3 interquartile ranges from the end of the boxes.

9

Figure 5. Boxplot showing density of Atlantic salmon 0+ in Örekilsälven downstream and upstream the

hydropower station in Torp respectively between 1979-2015 after corrections for compensatory release

and translocationsBoxes represent downstream and upstream areas of the hydropower station in Torp

between 1979-2015. The box-and-whisker plots show median values (black lines), the interquartile

ranges (boxes; 25th and 75th percentiles), and the 5th and 95th percentiles (whiskers). Circles

represent outliers, located 1,5-3 interquartile ranges from the end of the box and asterisks represent

extreme values, located more than 3 interquartile ranges from the end of the boxes.

Figure 4. Boxplot showing density of Atlantic salmon 0+ in Örekilsälven downstream and upstream

the hydropower station in Torp respectively between 1979-2015 after corrections for compensatory

release and translocations. Boxes represent downstream and upstream areas of the hydropower

station in Torp between 1979-2015. The box-and-whisker plots show median values (black lines), the

interquartile ranges (boxes; 25th and 75th percentiles), and the 5th and 95th percentiles (whiskers).

Circles represent outliers, located 1,5-3 interquartile ranges from the end of the box and asterisks

represent extreme values, located more than 3 interquartile ranges from the end of the boxes.

10

3.3 Functionality of the fish ladder

Neither salmon 0+ nor brown trout of any age classes (0+, >0+, and total) showed an increase

in density when comparing densities upstream the hydropower station before and after

completion of the fish ladder. However, salmon >0+ had significantly higher density for the

after the fish ladder was built compared to before the fish ladder existence (Table 1; Fig. 6

and 7). Salmon >0+ and brown trout densities were significantly lower in the downstream

area after completion of the fish ladder (U test: P <0.05) whereas the was no difference for

salmon 0+ (Mann-Whitney U test: U = 1484, n1 =41 n2 = 88, P > 0.1).

Before/After

fish passage

N Mean

Rank

Mean

density

Standard

deviation

Mann-

Whitney

U

Asymp.

Sig (2-

tailed)

Salmon

0+

Before 21 65.7 17.8 57.8 930.5 0.28

After 99 59.4 5.4 16.9

Salmon

>0+

Before 21 46.4 1.0 3.6 743.0 0.02

After 99 63.5 2.7 6.6

Salmon

total

Before 21 57.1 18.8 57.7 968.0 0.59

After 99 61.2 8.2 19.6

Brown

trout 0+

Before 21 58.7 3.9 9.1 1003 0.78

After 99 60.9 3.4 8.8

Brown

trout >0+

Before 21 54.8 3.0 4.6 920.5 0.41

After 99 61.7 3.4 5.1

Brown

trout tot

Before 21 54.8 6.9 12.2 920.5 0.41

After 99 61.7 6.8 11.3

Corrections for compensatory release and translocation showed no significant difference in 0+

density for Atlantic salmon (Mann-Whitney U test: U = 412.5, n1 =11 n2 = 80, P > 0.10) or 0+

density for brown trout (Mann-Whitney U test: U = 852.5, n1 =18 n2 = 97, P > 0.10).

For the upstream area salmon 0+ were found in 20 % of the cases (n=120). When corrections

for compensatory release and translocations were performed salmon 0+ were only found in 5

% of the electrofishing samplings (n=100) and all these 5 occasions occurred after the

construction of the fish ladder (years 2002, 2007 and 2009). Brown trout 0+ were found in 45

% of the electrofishing samplings (n=120) and in 44.3 % of the samplings after corrections for

compensatory release and translocations (n=115).

Table 1. The table shows the result from a Mann-Whitney U test for Atlantic salmon and brown trout densities of

two different year classes (0+, >0+) and total density for the upstream section of Örekilsälven before and after

installation of the fish ladder.

11

Figure 6. Boxplot of Atlantic salmon 0+, >0+, and total density for the area upstream the

hydropower station in Torp before and after the construction of the fish ladder. The box-and-

whisker plots show median values (black lines), the interquartile ranges (boxes; 25th and 75th

percentiles), and the 5th and 95th percentiles (whiskers). Circles represent outliers, located

1,5-3 interquartile ranges from the end of the box and asterisks represent extreme values,

located more than 3 interquartile ranges from the end of the boxes.

Figure 7. Boxplot of brown trout 0+, >0+, and total density for the area upstream the

hydropower station in Torp before and after the construction of the fish ladder. The box-and-

whisker plots show median values (black lines), the interquartile ranges (boxes; 25th and 75th

percentiles), and the 5th and 95th percentiles (whiskers). Circles represent outliers, located

1,5-3 interquartile ranges from the end of the box and asterisks represent extreme values,

located more than 3 interquartile ranges from the end of the boxes.

12

3.4 Correlation with discharge

Overall, no linear trend between discharge and density could be detected when plotting the

mean autumn discharge against recruitment (density of 0+) the following year for Atlantic

salmon (Fig. 8) or brown trout (Fig. 9). Correction of 0+ densities for fish translocations did

not render a linear relationship either (Fig. 10, Fig. 11).

Figure 9. Mean monthly autumn discharge and brown

trout 0+ density for the following year in locations

situated upstream the hydropower station. Y = 5.04-

0.06x. Dashed lines represent mean confidence

interval (95 %).

Figure 8. Mean monthly autumn discharge and salmon 0+

density for the following year in locations situated upstream

the hydropower station. Y = 9.8-0.16x. Dashed lines

represent mean confidence interval (95 %). The dashed and

dotted line illustrates a quadratic model.

13

Figure 10. Mean monthly autumn discharge and

salmon 0+ density for the following year in

locations situated upstream the hydropower

station after correction of fish translocations.

Y =-0.48+0.05x. Dashed lines represent mean

confidence interval (95 %).

Figure 11. Mean monthly autumn discharge and

brown trout 0+ density for the following year in

locations situated upstream the hydropower

station after correction of fish translocations.

Y = 4.66+-0.05x. Dashed lines represent mean

confidence interval (95 %).

14

3.5 Smolt models

The scenarios for upstream production, not including migration mortality, and the present

production of the downstream area for Atlantic salmon are shown in Fig. 12. The mean

number of salmon smolts produced today in the upstream area was calculated to 1556.

Increasing the upstream mean >0+ density to 30 % of the overall downstream mean density of

>0+ (future 1) increased this production to 2229 and using the same mean density upstream as

found downstream (future 2) gave a smolt production of 7406 (Fig. 12). Downstream

production had a mean value of 4123 and was constant since no increase in densities of >0+

was made. Production values are based on a mean of 100 simulations each.

The number of smolts produced upstream and downstream (Fig. 12) are reduced by lotic and

lentic migration mortality before reaching the ocean, resulting in the so called smolt

escapement. The smolt escapement for the upstream region varied with lentic mortality, lotic

mortality and upstream density of >0+ (Fig. 13 and 14). Downstream smolt escapement,

however, was only affected by lotic mortality as the lake is located upstream this area and no

additional density scenarios were made in this area. The percentage of smolts produced

upstream and downstream reaching the ocean under different lotic and lentic mortality

scenarios is shown in table 2. Total smolt escapment is derived by adding downstream

escapement with any of the upstream escapement scenarios (Fig. 13 and 14). Hence, the

estimated range of total present salmon escapement under lotic mortality 1 ranged between

2428-2547 (Fig. 13) and between 3469-3896 for lotic mortality 2 (Fig 14.). Future smolt

escapement under lotic mortality 1 was estimated to 2428-3015 (Fig. 13) and to 3470-5409

for lotic mortality 2 (Fig 14.).

Figure 12. Number of salmon smolt produced in the area downstream the hydropower

station (blue bar) and the upstream area today and two possible future scenarios (green

bars). Note that the production presented here does not account for any migration

mortality. The values represent the mean of 100 simulations each. Error bars denote

standard deviation.

0

1000

2000

3000

4000

5000

6000

7000

8000

Downstream Today Future 1 Future 2

Nu

mb

er o

f sa

lmo

n s

mo

lt p

rod

uce

d

15

Figure 14. Salmon smolt escapement (measured as the number of smolts that were estimated to

reach the sea) is affected by the mortality rate in lakes (lentic) and stream sections (lotic). In

lotic mortality scenario 2, mortality rates from Thorstad et al. (2012) were used. Three different

lentic mortalities (low, medium, high) are shown for today and two possible future scenarios for

smolt produced in the upstream area. The length of the lake equals 10 kilometers. Downstream

bar (blue) represents the number of salmon smolts produced downstream of the hydropower

station that reach the ocean, thus unaffected by lentic mortality. Mean ± standard deviation.

Figure 13. Salmon smolt escapement (measured as the number of smolts that were estimated to

reach the sea) is affected by the mortality rate in lakes (lentic) and stream sections (lotic). In lotic

mortality scenario 1, mortality rates from Nilsson et al. (2013) were used. Three different lentic

mortalities (low, medium, high) are shown for today and two possible future scenarios for smolts

produced in the upstream area. The length of the lake equals 10 kilometers. Downstream bar

(blue) represents the number of salmon smolt produced downstream of the hydropower station

that reach the ocean, thus unaffected by lentic mortality. Mean ± standard deviation.

16

Present smolt escapement for brown trout ranged between 86-186 for lotic mortality scenario

1 (Fig. 15) and 124-470 for lotic mortality scenario 2 (Fig. 16). The range of estimated smolt

escapement is affected by the lentic mortality scenario applied in the model (low, medium,

high). Percentage of smolts produced in the downstream and upstream section under different

lentic mortalities are shown in table 2.

Stream section Lentic mortality Percentage of smolts

produced reaching the

ocean

Lotic

mortality 1

Lotic

mortality 2

Downstream

58.9 %

84.1 %

Upstream Low 7.67 % 27.5 %

Medium 1.93 % 6.67 %

High 0.02 % 0.05 %

Figure 15. Present smolt escapement for brown trout (measured as the number of

smolts that were estimated to reach the sea) is affected by the mortality rate in lakes

(lentic) and stream sections (lotic). In lotic mortality scenario 1, mortality rates from

Nilsson et al. (2013) were used. Three different lentic mortalities (low, medium, high)

are shown for smolts produced in the upstream area. The length of the lake equals 10

kilometers. Downstream bar (blue) represents the number of brown trout smolt

produced downstream of the hydropower station that reach the ocean, thus unaffected

by lentic mortality. Mean ± standard deviation.

Table 2. The percentage of smolts produced in the downstream and upstream area of Örekilsälven reaching the

ocean under the two different lotic mortality scenarios used. Smolt escapement in the upstream area is also

reduced by the three different lentic mortalities (low, medium, high) used. Percentage values were calculated

from the mean of 100 simulations of the production of smolts (Fig. 12) and the smolt escapement for 100

simulations for each of the lake mortality scenarios (Fig. 13 and 14).

17

4. Discussion Recruitment, indicated as density of 0+, of both Atlantic salmon and brown trout was higher

in the area downstream of the hydropower station compared to the area upstream. This was in

accordance with the hypothesis and hence not surprising. The stream section downstream the

hydropower station provides large areas of suitable spawning and rearing habitats whereas the

upstream area stresses a longer migration for the fish with several migration obstacles.

Although densities of 0+ were lower upstream for both salmon and brown trout, the densities

of >0+ showed a contrasting result between the two species, where salmon >0+ density was

higher downstream and brown trout >0+ was lower downstream. Habitat choice of both

Atlantic salmon and brown trout usually leads to spatial overlap to some degree (Heggenes et

al. 1999, Klemetsen et al. 2003) and interspecific competition in juveniles of both species,

when co-occurring, is common (Harwood et al. 2002, Stradmeyer et al. 2008). Traditionally,

juvenile brown trout has been considered competitively superior to juvenile Atlantic salmon

(Stradmeyer et al. 2008, Van Zwol et al. 2012). However, emerging studies suggest that this

might not always be the case (Berg et al. 2014). As 0+ Atlantic salmon are typically more

often found further away from the stream banks (Bremset and Berg 1999) and brown trout

typically prefer shallower waters (Heggenes 1996, Linnansaari et al. 2010, Berg et al. 2014),

it is possible that juvenile salmon have a higher competitive ability compared to brown trout

in the lower stretches of Örekilsälven where the depth and distances from the stream banks

are higher. In the more upstream parts of Örekilsälven where discharge is lower, brown trout

may instead have the competitive advantage resulting in a higher density of >0+ trout. In

addition, brown trout in the upstream area is considered resident to a high degree and would

be less affected by a poor passage at the hydropower station in Torp, resulting in reduced

competition between brown trout and salmon in this area. Resident trout have been

Figure 16. Present smolt escapement for brown trout (measured as the number of

smolts that were estimated to reach the sea) is affected by the mortality rate in lakes

(lentic) and stream sections (lotic). In lotic mortality scenario 2, mortality rates from

Thorstad et al. (2012) were used. Three different lentic mortalities (low, medium,

high) are shown for smolts produced in the upstream area. The length of the lake

equals 10 kilometers. Downstream bar (blue) represents the number of brown trout

smolt produced downstream of the hydropower station that reach the ocean, thus

unaffected by lentic mortality. Mean ± standard deviation.

18

recognized to produce fewer juveniles than anadromous trout (Bohlin et al. 2001) which also

could explain why 0+ density is higher downstream the hydropower station compared to the

upstream area where resident life form of brown trout is more extensive.

4.1 Fish ladder effectiveness and efficiency

No increase in densities of brown trout (total) or salmon 0+ upstream the hydropower station

could be detected for the period after completion of the fish ladder, indicating low efficiency.

However, densities of brown trout (total) were significantly lower during this time in the

downstream area compared to the period before the fish ladder. This could indicate that a

lower amount of brown trout found their way to Örekilsälven during the second time period.

Hence fewer fish was possibly available for ascension to the upstream area and therefore

could conceal any potential effect of the fish ladder. Although, no overall trend in decreasing

brown trout stocks has been recognized (Höjesjö et al. in press.A), marine survival seems to

be site specific, where decreases in survival have been observed in some rivers (Jonsson and

Jonsson 2009) and not in others (Jensen et al. 2015).

The increase in salmon >0+ upstream after the construction of the fish ladder contrasts the

fact that no increase in salmon 0+ were observed. Fish translocations that have been extensive

after construction of fish ladder could explain this result as no correction of >0+ density was

possible. Still, the fact that translocations, when included, did not affect the outcome of

salmon 0+, confounds the observed increase in >0+ (Table 1). Parr typically move within the

river system between seasons (McCormick et al. 1998, Stickler et al. 2008) but it is highly

unlikely that the increase in >0+ density upstream the hydropower station is a result of parr

produced downstream ascending the fish ladder due to their limited swimming capacity.

Instead, the increase could be due to an increased survival rate for 0+ in the upstream area

resulting in a significantly higher density of salmon >0+ but not for salmon 0+.

Median density for salmon 0+ and >0+ for the upstream area after construction equalled zero,

but several outliers of higher densities were found (Fig. 6). When corrected for fish

translocation for this period, most of these outliers were discarded and only five occasions

with salmon 0+ remained. These occasions were electro fishing samples where the presence

of salmon 0+ could not be directly associated with a translocation according to the correction

criteria used in this study. Despite carefully performed it cannot be completely ruled out that

the five remaining occasions with 0+ are not a result of spontaneous ascent through the fish

ladder. It is possible that translocated adult fish move from the release spot in the river and

spawns somewhere else and hence biasing the correction for translocations. Salmon (0+) have

been observed to drift downstream, probably as a strategy adopted when individuals are

displaced from suitable habitats by intraspecific competition, (Johnston 1997, Bujold et al.

2004). In a river system such as Örekilsälven, with very low densities of 0+ and vast suitable

habitat upstream, it is highly unlikely that 0+ released in the upstream area would have to drift

any longer distances before finding a suitable habitat without being displaced. For instance,

one locality occupied by salmon 0+ is located 10 kilometres downstream of the release spot of

adult fish previous year and 15 kilometres downstream the closest release point of 0+. The

second time salmon 0+ were found in the same location, no translocations of adult fish had

been performed the year before and no translocation of 0+ were made the concerned year. In

other word, these occasions strongly imply that the fish ladder has successfully allowed ascent

of Atlantic salmon during certain years indicating that there is a motivation for adult salmon

to migrate higher in the system. However, these occasions are rare and represent only 5 %

(n=100) of the electro fishing samplings when corrected for fish translocations. This, along,

with low densities, indicates that although allowing salmon to migrate upstream occasionally,

19

the fish ladder has an overall poor function for salmon and cannot be said to be efficient

(Lucas and Baras 2001, Fergusson et al. 2002, as cited in Noonan et al. 2012).

Densities of brown trout did not increase in the upstream area following the construction of

the fish ladder. Fish translocations of brown trout in Örekilsälven are scarce, and are not

believed to have had a major impact on the observed densities and occurrence of brown trout

upstream the hydropower station. Compared to Atlantic salmon, spontaneous recruitment was

found in a much higher extent in brown trout (in 44.3 % of the samplings for brown trout

compared to 5 % for the Atlantic salmon). However, this does not necessarily prove a higher

efficiency of the fish ladder for brown trout but is more likely explained by the high degree of

resident trout in the upstream area (SERS 2016). Partial anadromy in brown trout populations

is common (Jonsson and Jonsson 1993, Klemetsen et al. 2003, del Villar-Guerra et al. 2014)

and this in most likely also the case in Örekilsälven. Truly, anthropogenic migration barriers

have existed in the location of the contemporary hydropower station for centuries. If

connectivity has been impaired for such a long time, it is likely that brown trout have adopted

a more resident life history, as this would be advantageous in a system with several migration

barriers downstream (Jonsson and Jonsson 1993). Judging from the low densities of brown

trout and the absence of increase in density after construction of the fish ladder, the efficiency

is most likely deficient also for brown trout. It cannot be ruled out that the fish ladder has

allowed ascent for some individuals in certain years as for salmon, but the recommendation of

90-100 % of the migrating adults to ascend safely and rapidly to mitigate fragmentation from

anthropogenic barriers (Lucas and Baras 2001, Fergusson et al. 2002, as cited in Noonan et al.

2012) is most probably violated. The high degree of resident trout also implies a limited

function of the fish ladder as an efficient fish ladder would offer high connectivity and allow

brown trout to migrate to the ocean, growing bigger and hence increase their egg clutch

(Bohlin et al. 2001). As the migratory behavior seems to be plastic, with resident parents

giving birth to both migratory and resident offspring (Jonsson and Jonsson 1993), a system

with high connectivity would be expected to favor anadromy over residency if migration

distances are not too far.

A fish ladder with a low efficiency does not only fail in mitigating the barrier effect of dams,

but may also affect individual spawning performance by spending resources in trying to

ascend (Gowans et al. 2003, Castro-Santos et al. 2009). This is of special importance for

anadromous Atlantic salmon and brown trout that completely cease or dramatically decrease

their feeding when migrating upstream to spawn (Bardonnet and Baglinière 2000, Degerman

et al. 2001, Klemetsen et al. 2003). If ascent is managed nevertheless, not only is the energy

storage for spawning reduced (Gowans et a. 2003), but migration is delayed (Castro-Santos et

al. 2009, Roscoe and Hinch 2010, Marschall et al. 2011) and thus less time can be spent on

spawning. It is therefore crucial that a fish ladder works efficiently, not just for the number of

fish ascending, but also for the time and energy being invested in ascent.

4.2 Influence of discharge

Discharge is often of high importance for initiating and stimulating upstream migration in

salmonids (Aarestrup et al. 2003, Arnekleiv and Rønning 2004, Jonsson and Jonsson 2002,

Mitchell and Cunjak 2007). However, in this study, no linear trend between discharge

(defined as mean per month for the period September-November) and recruitment upstream

the hydropower station was found for either brown trout or salmon (Fig. 8-11). Still, it is

possible that discharge had an effect at a smaller temporal scale than autumn monthly mean as

used in the study. Data on daily discharge showed relatively large fluctuations over few days

and potentially the few confirmed spontaneous ascent of salmon coincided with a specific

20

discharge not identified here. Four of the five proposed spontaneous salmon ascents had a

relatively high discharge previous year (Fig. 10) but this might only be circumstantial due to

scarce data. Both lower and higher discharge were associated with increased densities of

brown trout 0+, although the high proportion of resident trout in the upstream area

complicates any conclusions. A higher discharge could be beneficial for upstream migration

as seen in Arnekleiv and Kraabøl (1996), Gowans et al. (1999) and Thorstad et al. (2005). In

many cases an increase in discharge increases the attraction flow of the fish passage which is

a crucial part for successful ascent (Lundqvist et al. 2008, Calles and Greenberg 2009, Bunt et

al. 2012). The fish ladder in Torp, however, is only designed for a discharge of 0.25-0.5 m3/s,

meaning that a high elevation in diversion of water from the turbines to the fish ladder

potentially would increase the attraction but also, most likely, impede passage efficiency. In

aspect of the proposed low efficiency of the fish ladder in this study, the observed fluctuations

in 0+ density are more likely to be contingent or explained by other factors such as

environmental circumstances affecting the spawning success.

4.3 Smolt production

The present smolt escapement of Atlantic salmon smolts, as calculated in the smolt models,

ranged between 2428-3896 depending on lotic and lentic mortality (Fig. 13 and 14). However,

the habitat survey for the upstream area was made in 1985 and changes after this would affect

the accuracy of the estimation which is important to consider. Most of today’s escapement of

salmon smolts originate from spawning sites located downstream the hydropower station,

ranging between 87.66-99.99 % of the total contemporary production (Fig. 13 and 14).

Considering the large areas of suitable spawning and rearing habitat upstream these values

might appear to be low. Still, estimation of upstream escapement of salmon smolts originate

from mean values upstream (median values were 0.00), hence including all outliers (Fig. 6)

and not accounting for the artificially increased densities due to fish translocations. Moreover,

the model does not account for the presence of hydropower stations during downstream

migration. Hydropower stations often induce an elevated mortality if smolts pass through the

turbines (Larinier 2008, Greenberg et al. 2012, Norrgård et al. 2013) or by increasing the risk

of predation when smolts aggregate upstream the dam construction (Aarestrup and Koed

2003, Schilt 2007, Castro-Santos et al. 2009). In other words, the large range of estimated

escapement of 0.31-428 for salmon smolts and 0.26-347 for brown trout smolts from the

upstream area is more likely an overestimation than the opposite.

Future production of salmon smolt in the upstream area, not including migration (lotic and

lentic) mortality, is likely to increase with improved connectivity (Fig. 12). Comparable

systems are rare but Rolfsån in the county of Halland shares the feature with large lakes in the

system and has recently gained increased access to upstream areas for salmonids. At present,

results from electro fishing in Rolfsån shows that densities of salmon >0+ upstream the lakes

are 30 % of the densities downstream the lakes (future 1 scenario on this study). However, it

is possible that there is a lag phase in colonization of these new accessible habitats and that

densities of salmon could be equal upstream to the present densities downstream downstream

(E. Degerman, Swedish University of Agricultural Science, personal communication).

Upstream areas in Örekilsälven provide suitable habitats and the natural migration mortality

for adult salmonids travelling upstream in this area is thought to be limited (Bohlin et al.

2001). Therefore, an equal density of salmon upstream the hydropower station and

downstream is not unrealistic but speculated to occur over time with restored hydrological

connectivity in Örekilsälven. Restoring the hydrological connectivity would probably also

increase the density of brown trout in the upstream area, but the actual magnitude of this is

hard to assess and therefore no estimation of future smolt escapement was performed.

21

Facilitating migration in the river system will most likely increase the anadromous proportion

of the brown trout population as this will be evolutionary advantageous when the migration

distance is not too far (Jonsson and Jonsson 1993, Bohlin et al. 2001, Brenkman et al. 2008).

An increase in anadromy will likely increase the densities of brown trout due to the beneficial

feeding grounds at sea (Bohlin et al. 2001). Future densities of salmon and brown trout in

Örekilsälven will also depend on interspecific interactions and competition.

Colonization of newly accessible habitat has been documented for salmonid species (e.g.

Bryant et al. 1999, Gardner et al. 2013, Hogg et al. 2015). Time and success of the

colonization is considered to depend on four factors: (1) accessibility, (2) proximity to donor

stock, (3) productivity and condition of donor stock, and (4) habitat suitability for the species

and life history variant (Pess et al. 2014). In Örekilsälven, the relatively long distance from

the presently accessed habitat to suitable areas upstream could halt colonization of the

upstream area. In addition, the strong natal homing displayed in Atlantic salmon (Stabell

1984, Hansen et al. 1993, Keefer et al. 2014) would also impede fast exploitation of new

habitats (Pess et al. 2014). However, the fact that spontaneous upstream migration has

occurred indicates a motivation to ascend within the salmon population and implicates a

probable colonization if connectivity is improved. Brown trout typically has higher straying

rates than Atlantic salmon (Frank et al. 2012, Degerman et al. 2012a, Östergren et al. 2012)

and is also more widespread in the upstream area at present. Thus, colonization of the

upstream area for brown trout is likely to be more rapid compared to Atlantic salmon.

The smolt model predicts that apart from upstream density of salmon and brown trout, the rate

of lotic and lentic mortality is paramount for the actual escapement of smolts (Table 2).

Tagging and tracking smolt migration downstream in other river systems have given a wide

range of migration mortality per kilometre in lotic sections (Jepsen et al. 2000, Olsson et al.

2001, Calles and Greenberg 2009, Thorstad et al. 2012) as well as in lentic migration

pathways (Jepsen et al. 1998, Jepsen et al. 2000, Olsson et al. 2001). These discrepancies

indicate that migration mortality (lotic and lentic) varies both spatially and temporally (Olsson

et al. 2001).

In lotic mortality 1 (Fig. 13 and 15), mortality rates were based on the different ranks of

rearing quality, as done in Nilsson et al. (2013). To evaluate the estimated smolt escapement

from the model, Nilsson et al. (2013) used a smolt trap to estimate actual brown trout smolt

escapement in two Swedish streams. Results from this evaluation showed that the model

tended to estimate a lower production than observed for several years in Kävlingeån, a river

of comparable size to Örekilsälven. Potentially, the underestimations were derived by using

exaggerated migration mortality in the lotic section. In another tagging and tracking study,

smolt migrating downstream in Högvadsån, a tributary to Ätran, showed an average migration

mortality of 5 % per kilometre in slow flowing sections (E. Degerman, Swedish University of

Agricultural Science, personal communication). In addition, the six studies reviewed in

Thorstad et al. (2012), investigating migration mortality for wild salmon smolt, all fitted

within the range of 0.3-5 %. These results could indicate that mortality rates in Örekilsälven

are closer to the lower mortality rate in scenario 2 (Fig. 14 and 16). Still, higher mortality

rates per kilometre have been found (Jepsen et al. 1998, Olsson et al. 2001, Calles and

Greenberg 2009). Additional data collection, using tagging and tracking of down migrating

smolts in Örekilsälven, are necessary for more accurate estimations. Moreover, performing

new habitat surveys of the upstream area of Örekilsälven is crucial for a more precise picture

of the contemporary production in this area.

22

Data on mortality rates experienced by smolts migrating through lentic systems are scarce but

typically show an elevated mortality compared to stream sections (Jepsen et al. 1998, Jepsen

et al. 2000, Olsson et al. 2001). The three different mortality rates used in the model rendered

large differences in smolt escapement (Fig. 13-15). Applying the high mortality rate (25-71 %

mortality per kilometre) virtually prevented any smolts from the upstream region to reach the

sea, whereas incorporating a medium or a low lentic mortality allowed some smolt

escapement from the upstream area (Table 2). Hence, the mortality rate in the lake could act

as a natural bottleneck for the escapement of smolts in Örekilsälven. Although studies

assessing migration mortality of smolts in lentic systems demonstrated mortality rates within

the high mortality scenario (Jepsen et al. 2000, Olsson et al. 2001), there is evidence that

mortality rates can be lower and within the medium mortality scenario used here (Jepsen et al.

1998). It is crucial to bear in mind that the studies identifying a high migration mortality in

lentic systems observed mortality rates in human made reservoirs, where mortality rates are

expected to be higher compared to natural lakes (Jepsen et al. 2000). In fact, in their study in

the Bygholm reservoir, Jepsen et al. (2000) recorded an average survival time of brown trout

smolts of 4 days and concluded that difficulties in finding the outlet probably had a major

impact on mortality rates. Thus, if migrating smolts in Örekilsälven are able to find the outlet

in lake Kärnsjön and proceed the migration, mortality rates in the lake are likely to be

reduced. In addition, lentic mortality of smolts descending trough lake Kärnsjön will be

determined by the swimming speed of migrating smolts as well as the presence and amount of

smolt predators e.g. pikeperch and pike. Based on the natural features of lake Kärnsjön and

assumed that downstream passage is made optimal for salmonid smolts, I speculate that lentic

mortality in lake Kärnsjön is closer to the medium and low mortality scenarios than the high.

Despite a high potential in salmon smolt production for the upstream area (Fig. 12), it is

important to remember that features such as the presence of a lake and long migration

pathways entail natural constraints on the possible smolt escapement from the upstream area

in Örekilsälven even if high connectivity is achieved in the future. Still, improving the

efficiency of the fish ladder and thus increasing the connectivity, smolt escapement of both

Atlantic salmon and brown trout from the upstream area is likely to increase substantially and

aid in the restoration of this historically and extensively modified stream.

5. Conclusions Electro fishing samples revealed low densities of foremost Atlantic salmon in the area

upstream of the hydropower station of Torp despite extensive suitable habitats. Densities of

0+ brown trout were higher downstream whereas densities of brown trout >0+ was higher

upstream presumably due to high proportion of upstream residency and lower competition

with salmon in the upstream area. No increase in brown trout density or salmon 0+ density

was apparent in the upstream area after construction of the fish ladder, whereas salmon >0+

increased after the fish ladder was completed. Extensive translocations of adult and 0+ salmon

during this period complicates the picture and is probably the explanation for the increase in

salmon >0+. When corrected for translocations, salmon 0+ were only found in 5 % of the

electro fishing occasions after construction of fish ladder and 0+ brown trout in 44.3 %. This

indicates that salmon has successfully ascended the fish ladder but only occasionally. The few

spontaneous ascents along with overall low densities of salmon upstream point to a low and

not satisfactory efficiency of the fish ladder. The absent increase in brown trout density

together with a high proportion of residency also indicates a low efficiency of the ladder for

brown trout. No effect of discharge was apparent but due to the low number of verified

spontaneous ascents for salmon and the high degree of residency in brown trout, discharge

probably influenced on a much finer time scale than the one used in the study. In order to

minimize the barrier effects of the hydropower station and provide high longitudinal

23

connectivity, a new fish passage, alternatively decommission of the dam is needed. Dam

removal would also benefit other migrating species in the system such as sea lamprey and

European eel.

The smolt model used revealed that only a fraction of the present smolt escapement originates

from the upstream area. Improved hydrological connectivity will probably increase the

escapement of both Atlantic salmon and brown trout in the upstream area, but the actual

numbers depend on multiple factors. The colonization extent and time is affected by straying

rates and interspecific interactions between brown trout and Atlantic salmon and other fish

species. Densities of Atlantic salmon upstream the hydropower station are expected to be

equal to densities in the downstream area, whereas brown trout is assumed to adopt anadromy

to a higher degree once connectivity is improved. The actual number of smolts escaping the

system is likely to vary temporally and be dependent on site specific mortality in both lotic

and lentic sections. Referring to current knowledge and features of Örekilsälven, I speculate

that migration mortality in lotic sections will be in the lower range and in the low-medium

range for the lentic sections. Based on these speculations, future smolt escapement of both

Atlantic salmon and brown trout from the upstream area will increase substantially and

contribute to a considerable part of the total smolt escapement if high connectivity is

achieved. Still, the major part of the smolt escapement will also in the future originate from

the downstream area. This is a is a result of natural constraints in the upstream area including

the presence of a lake and long migrations distances. Downstream migration pass the

hydropower dam is not assessed in this study but is of equal importance as upstream

migration to ensure high connectivity. Future efforts in Örekilsälven should focus on

downstream migration, contemporizing the habitat survey in the upstream part for more

accurate estimation of smolt production, monitoring and emphasizing additional parts of the

native fish fauna.

24

Acknowledgement Many people have been involved in the work of this essay and contributed to its present

content. Without your input and support, the writing of this paper would not have been

possible. Firstly, I want to thank Lars-Olof Ramnelid and Daniel Johansson at the county

administration board in Västra Götaland for giving me the opportunity to work with this

project. Your feedback, help, knowledge and open attitude has been invaluable and inspiring.

The county administration board in Vänersborg with personnel were always welcoming and

helpful whenever I needed something. I owe special thanks to Lars Thorsson who helped with

electro fishing protocol and shared with his knowledge of Örekilsälven. There are many other

persons to thank and some of them are Lars-Åke Winbladh, Martin Dellien, Ingvar Lagenfelt,

Key Höglind, Emil Larsson, Badreddine Bererhi, Johanna Ek, Magnus Lovén Wallerius, Ida

Hedén, the staff at the Department of Biological and Environmental Sciences at University of

Gothenburg and the municipality of Södertälje. Erik Degerman contributed with ideas and

input about the calculations of smolt production. Lastly, I want give my deepest gratitude to

my supervisor Johan Höjesjö for guidance and feedback on the manuscript and for inspiration

and encouragement in the field of limnology.

25

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