Assessment of spat collector ropes in Galician mussel farming

Ramon Filgueira, Laura G. Peteiro, Uxıo Labarta, Marıa Jose Fernandez-Reiriz *

Instituto de Investigaciones Marinas CSIC, Eduardo Cabello 6, 36208 Vigo, Spain

Received 14 February 2007; accepted 5 June 2007

Abstract

The large amount of mussel seed required to support the present mussel farming production levels in Galicia (NW Spain) forces

the development of new designs in artificial spat collectors for continual improvement of mussel seed gathering. In the present study,

we have assessed both settlement and recruitment of Mytilus galloprovincialis on four different collector ropes in the Rıa de Ares-

Betanzos (Galicia). Besides the traditional collector ropes (lacing without loops and non-filamentous structure; NF-NL), three new

rope designs with different lacing and structures were evaluated; ropes with a filamentous loop complement (F-L), ropes with a non-

filamentous loop complement (NF-L) and filamentous ropes without loops (F-NL). Ropes with loops showed higher settlement

densities (53,925 � 4625 and 42,433 � 5525 indiv./m for F-L and NF-L, respectively) than ropes without loops (26,475 � 3875

and 13,033 � 1136 indiv./m for F-NL and NF-NL, respectively). This may be explained by the increase in available surface area

provided by the loops. Several studies recognized the importance of filamentous substrata for mussel spat settlement, which may

help to explain greater settlement densities on filamentous structures between ropes with the same lacing. In recruitment evaluation,

ropes with filamentous loops showed the highest densities expressed in indiv./m (5493 � 587) as was the case of settlement.

However, when density was expressed in kg/m, the ropes with non-filamentous loops had a higher yield (8.48 � 0.22 kg/m), that

could be a result of differences in adjusted shell length between ropes. Intra-specific competition and predation were identified as

important factors affecting post-settlement mortality. The latter factors could also influence population length distribution. Ropes

with rigid loops (NF-L) may supply refuges for spat from predators and therefore, enhance the recruitment of larger individuals,

although other factors like size selective settlement could play a significant role in this result.

# 2007 Elsevier B.V. All rights reserved.

Keywords: Mytilus galloprovincialis; Artificial spat collector; Mussel farming; Settlement pattern; Recruitment pattern

www.elsevier.com/locate/aqua-online

Aquacultural Engineering 37 (2007) 195–201

1. Introduction

The supply of mussel seed is critical for the

development of industrial mussel cultivation (Fuentes

and Molares, 1994). Mussel farming (Mytilus gallopro-

vincialis) in Galicia (NW Spain) requires, according to

Perez-Camacho et al. (1995), approximately 9000 Tm of

mussel seed per year to support the current mussel

production rate (250,000 Tm per year; Labarta, 2004).

* Corresponding author. Tel.: +34 986 231930;

fax: +34 986 292762.

E-mail address: mjreiriz@iim.csic.es (M.J. Fernandez-Reiriz).

0144-8609/$ – see front matter # 2007 Elsevier B.V. All rights reserved.

doi:10.1016/j.aquaeng.2007.06.001

66% of mussel seed used in mussel cultivation is obtained

by scraping directly from intertidal exposed rocky shores

where mussel seed is attached, although gathering of

mussel seed from artificial collector ropes has increased

in recent years (Perez-Camacho and Labarta, 2004),

principally due to its higher growth rate when cultivated

on the raft (Perez-Camacho et al., 1995; Babarro et al.,

2000, 2003). Since Galician legislature restricts the

number of ropes per raft (maximum 600 ropes of 12 m

length during the larval settlement season), it is crucial to

increase the seed yield obtained on artificial collectors by

the development of new designs.

Composition and structure of settlement materials

are important factors affecting the amount of collected

R. Filgueira et al. / Aquacultural Engineering 37 (2007) 195–201196

larvae. Settlement is defined as the point when an

individual first takes up permanent residence on the

substratum (Connell, 1985), although the mussel

settlement process is dynamic and may involve various

settlement substrates and transitions to new substrates

(Alfaro, 2006). With regard to composition, the

following properties are highlighted: polarity (Hansen

and Waite, 1991), surface free energy (Nishida et al.,

2003), wettability (Alfred et al., 2005) and associated

fouling (Alfaro et al., 2006). With regard to structural

properties, the available surface area (Walter and

Liebezeit, 2003), and the thickness of filaments (King

et al., 1990; Pulfrich, 1996; Alfaro and Jeffs, 2002;

Lekang et al., 2003; Walter and Liebezeit, 2003) are

important factors in larval settlement. Nonetheless, as

consequence of post-settlement mortality and emigra-

tion processes, settlement densities might not be

directly related to the amount of viable individuals

available for cultivation. Several factors may contribute

to the variability of post-settlement mortality and,

therefore, recruitment, defined as the recently settled

juveniles that have survived for a period of time after

settlement (Connell, 1985). Hunt and Scheibling (1997)

identified a number of causes of post-settlement

mortality which may be affected by the characteristics

of collector ropes: hydrodynamic disturbance (Eckman,

1987; McShane and Nylor, 1995), competition for space

and food (Guinez and Castilla, 1999; Guinez, 2005) and

predation (Schiel, 2004; Morrisey et al., 2006). The

physicochemical characteristics of collector ropes can

impact on the strength of seed attachment, thereby

Fig. 1. Map of the Rıa de Ares-Betanzo

modifying the probability of detachment by physical

disturbance (Lekang et al., 2003). The available surface

area on the different collector ropes determines spatial

competition and associated mortality. In addition,

structural complexity may promote protection from

predators, thus reducing post-settlement mortality

(Moreno, 1995; Walters and Wethey, 1996; Frandsen

and Dolmer, 2002).

In the present study the settlement and recruitment of

Mytilus galloprovincialis on four different collector

ropes in the Rıa de Ares-Betanzos (Galicia, NW Spain)

are assessed. In addition to the traditional collectors,

three new rope designs with different lacing and

structure are tested for their potential to improve the

mussel seed gathering in the rıa.

2. Material and methods

2.1. Experimental design

Experimental collector ropes were deployed in

Arnela, a location commonly used as a mussel seed

collection area at the inner southern shore of the Rıa de

Ares-Betanzos (Fig. 1). Four different collector ropes

with different lacing (with or without a loop comple-

ment) and structural designs (filamentous or non-

filamentous) were tested. Structural complexity of the

collectors does not allow the surface area to be

measured exactly, although lacing with the loop design

provides a greater surface area than lacing without

loops. Collector ropes NF-NL correspond to the

s, showing the Arnela study area.

R. Filgueira et al. / Aquacultural Engineering 37 (2007) 195–201 197

traditional nylon ropes employed for mussel seed

collection in industrial cultivation, whereas the other

experimental ropes are polypropylene.

Three ropes of each type were hung randomly from

the prow of a raft over the period 23rd March to 3rd

October 2006. Distance between ropes was about

50 cm. An initial sampling was carried out on 19th July

2006 to evaluate larval settlement when the seed length

was manageable. A final sampling took place on 3rd

October 2006 to evaluate the recruitment when ‘‘early

thinning-out’’ was performed. ‘‘Early thinning-out’’ is

an industrial procedure in which mussel seed is

detached and placed again in the culture at lower

densities.

2.2. Mussel sampling

For each sampling (settlement and recruitment)

and rope (three ropes of each type: F-L, NF-L, F-NL

and NF-NL), three samples were taken at 0.97–1.03 m

depth. The initial sampling to evaluate larval

settlement involved the removal by scraping of all

individuals from a 2 cm section of each rope. Samples

were preserved in 70% alcohol until laboratory

processing. Since scraping involved the mussels

attached to fibers from the collectors, the mussels

were detached using a bleach dilution (Davies, 1974)

and ultrasonic bath treatment for 5 min. The mussels

were classified into six length classes by washing the

material through a series of successively finer mesh

sieves. Each sieve was previously correlated with a

mussel length range that determines the following size

classes: <0.5, 0.5–1.5, 1.3–1.9, 1.7–2.1, 2.0–5.0 and

>5.0 mm. Each sieved fraction was dried at 80 8C and

counted using a binocular microscope. The adjusted

shell length was calculated with the formula: L ¼PðCL� FÞ � N�1 (Box et al., 1989), where L is the

mean shell length, CL is the individual length class, F

is the frequency, and N is the total number of

individuals.

Sampling for mussel recruitment evaluation

involved the collection of 1 kg (wet weight) of total

mussel detached from each rope in the ‘‘early thinning-

out’’ process. The density of the mussels was estimated

by counting, and the individual mussel length

determined using calipers (Mitutoyo1). The length

was defined as the maximum measurement to the

nearest 0.1 mm along the anterior–posterior axis. The

samples were then separated into 5 mm length classes

for the length frequency distribution calculation and the

adjusted shell length was calculated as described

above.

2.3. Data analysis

The effect of collector type on density expressed as

indiv./m and adjusted length (mm) of mussel seed was

tested for the settlement and recruitment samplings

using one-way ANOVA and Tukey’s test as a post-hoc

test. In the recruitment sampling the density expressed

as kg/m was tested in the same way.

For each sampling, the length frequency distribution

was tested between collector types using contingency

tables and x2 statistics. For the settlement sampling, the

effect of collector type on density (indiv./m) was also

tested in length classes corresponding to both modes

detected in length frequency distributions using one-

way ANOVAs and Tukey’s test as a post-hoc test.

Due to the fact that mortality and emigration cannot

be distinguished, it has been used the term instanta-

neous mortality coefficient (Z) to evaluate the net loss of

individuals during the sampling time interval, using the

expression: Nt = N0 e�zt where N0 and Nt are the number

of mussels per sample at the beginning and at the end of

sampling interval (t) expressed in days. One-way

ANOVA was used to compare mortality coefficients

of mussel seed collectors and Turkey’s test was used as

post-hoc test. Levene’s test and normality of residuals

were carried out to check the ANOVA assumptions. All

data analysis was carried out using the statistical

package SPSS 13.0.

3. Results

3.1. Settlement

One-way ANOVA results (Table 1) show a sig-

nificant effect ( p < 0.001) of collector type (F-L, NF-L,

F-NL, NF-NL) on settlement density. The post-hoc test

shows differences between settlement densities for each

collector type. Ropes with filamentous loops record the

highest density values (F-L; 53,925 � 4625 indiv./m),

followed by ropes with non-filamentous loops (NF-L;

42,433 � 5525 indiv./m), filamentous ropes without

loops (F-NL; 26,475 � 3875 indiv./m) and non-fila-

mentous ropes without loops (NF-NL; 13,033 � 1136

indiv./m).

With regard to the settlement adjusted length, one-

way ANOVA results (Table 1) show a significant effect

( p < 0.01) of collector type. Mussel seed from ropes

with non-filamentous loops (NF-L) shows a signifi-

cantly higher adjusted length (2.23 � 0.04 mm) than for

the other collector ropes (1.80 � 0.20, 1.60 � 0.19 and

1.53 � 0.08 mm for F-NL, NF-NL and F-L, respec-

tively).

R. Filgueira et al. / Aquacultural Engineering 37 (2007) 195–201198

Table 2

Pair-wise comparisons (x2) of settlement length frequency distribu-

tions between different collectors tested

Collector ropes NF-NL F-NL F-L NF-L

NF-NL

x2 – 13.81* 24.59* 46.9*

p – <0.05 <0.001 <0.001

F-NL

x2 – – 17.28* 23.96*

p – – <0.05 <0.001

F-L

x2 – – – 69.68*

p – – – <0.001

NF-L

x2 – – – –

p – – – –

The asterisks indicate significant differences.

Table 1

One-way ANOVA tests to determine the effect of collector type on (i)

total settlement density, (ii) settlement density of individuals from the

<0.5 mm length class, (iii) settlement density of individuals from the

2–5 mm length class, and (iv) settlement adjusted length

Settlement

Sources of variation d.f. SS MS F-value p

Total density (indiv./m)

Collector 3 2.89 � 109 9.6 � 108 56.54 <0.001

Density (indiv. < 0.5 mm/m)

Collector 3 2.83 � 108 9.44 � 107 12.55 <0.01

Density (indiv. 2–5 mm/m)

Collector 3 3.68 � 108 1.22 � 108 38.02 <0.001

Adjusted length (mm)

Collector 3 0.878 0.293 13.86 <0.01

Fig. 2 shows settlement length frequency distribu-

tions for each collector type. All length frequency

distributions are different between collectors (Table 2).

The settlement density on the main length classes

(<0.5 mm and 2–5 mm) of each of these length classes

was compared between collector types. One-way

ANOVA results show a significant effect of the collector

type on density of both length classes (Table 1). With

regard to the <0.5 mm length class, ropes with

filamentous loops (F-L) show significantly higher

densities (16,915 � 2831 indiv. <0.5 mm/m) than the

other collectors (9125 � 750, 6337 � 4413 and 4028

� 1436 indiv. <0.5 mm/m for NF-L, F-NL and NF-

NL, respectively). For the 2–5 mm length class, the

post-hoc test shows a significantly higher density

(18,537 � 3113 indiv. 2–5 mm/m) on ropes with non-

filamentous loops (NF-L) than for the other collectors

(Table 1). Similarly, the density is higher on ropes

with filamentous loops than non-filamentous ropes

without loops (10,583 � 1248 and 3636 � 942 indiv.

2–5 mm/m, F-L and NF-NL, respectively). Neither of

these latter collectors show density differences with the

Fig. 2. Settlement length class frequency distribution for the different

collector ropes tested.

filamentous ropes without loops (F-NL; 6987 � 888

indiv. 2–5 mm/m).

3.2. Recruitment

Recruitment could not be evaluated in non-filamen-

tous ropes without loops (NF-NL) because of low

densities in combination with irregular distribution of

mussels on the rope plus significant Chthamalus sp.

barnacle colonization. With regard to the other

collectors tested, one-way ANOVA results (Table 3)

show a significant effect of the collector type on

recruitment density (indiv./m and kg/m). The post-hoc

test on density expressed as indiv./m reveals that ropes

with filamentous loops (F-L) have significantly higher

densities (5493 � 587 indiv./m) than the other collec-

tors (4178 � 108 and 3895 � 246 indiv./m for NF-L

and F-NL, respectively). Conversely, the post-hoc test

on density expressed as kg/m highlights that ropes with

non-filamentous loops (NF-L) record significantly

higher values (8.48 � 0.22 kg/m) than the other ropes

(3.11 � 0.44 kg/m and 2.83 � 0.13 kg/m for F-L and F-

NL, respectively).

Differences in density related to the units of

measurement are associated to differences in adjusted

shell length from each collector (Table 3). Post-hoc test

shows significantly higher adjusted lengths (25.95 �0.21 mm) from ropes with non-filamentous loops (NF-L)

than from the other collectors (17.71 � 0.16 and

16.56 � 0.32 mm for F-NL and F-L, respectively).

Length differences are observed between all collectors

R. Filgueira et al. / Aquacultural Engineering 37 (2007) 195–201 199

Table 3

One-way ANOVA tests to determine the effect of collector type on recruitment adjusted shell length, total density and mortality coefficient

Recruitment

Sources of variation d.f. SS MS F-value p

Density (indiv./m)

Collector 2 4.37 � 106 2.18 � 106 15.73 <0.01

Density (kg/m)

Collector 2 60.84 30.42 363.46 <0.001

Adjusted length (mm)

Collector 2 157.47 78.73 1385.10 <0.001

Mortality coefficient (day�1)

Collector 2 5.24 � 10�5 2.62 � 10�5 7.59 <0.05

and are related to the differences in length frequency

distributions (Fig. 3; Table 4).

Differences in mortality coefficients (ANOVA;

Table 3) between collectors give rise to a different

recruitment density pattern than observed during

settlement. Pair-wise analysis of mortality coefficients

Table 4

Pair-wise comparisons (x2) of recruitment length frequency distribu-

tions between different collectors tested

Collector ropes F-NL F-L NF-L

F-NL

x2 – 28.25* 182.58*

p – <0.001 <0.001

F-L

x2 – – 260.53*

p – – <0.001

NF-L

x2 – – –

p – – –

The asterisks indicate significant differences.

Fig. 3. Recruitment length class frequency distribution for the dif-

ferent collector ropes.

between collectors reveals significantly lower mortality

coefficients (0.025 � 0.0025 day�1) on filamentous

ropes without loops than for the other collectors

(0.030 � 0.0021 day�1 and 0.030 � 0.0003 day�1 for

NF-L and F-L, respectively) which show similar values

between them (ANOVA, Table 3). Analysis of the

Pearson’s coefficients reveals a significant and positive

correlation between settlement density and mortality

coefficients (r = 0.83, p = 0.005, n = 3).

4. Discussion

One of the main factors influencing settlement

density on collector ropes is the available surface area

(Walter and Liebezeit, 2003). In the present study, ropes

with a loop complement had an increased surface,

which may explain the greater settlement densities

compared to ropes without loops. Rope structure is

another important factor. Preferable settlement on

filamentous substrata has been observed by several

authors (see review by Lutz and Kennish, 1992) and

could explain the greater settlement densities observed

on filamentous structures in ropes with the same lacing.

Ropes with non-filamentous loops showed signifi-

cantly higher adjusted lengths than the other collectors

(ANOVA; Table 1), which is related to differences in

length frequency distributions. The density pattern

recorded between collectors in the main size classes was

different. With regard to <0.5 mm length class, greater

settlement density was recorded on ropes with

filamentous loops (F-L) while the highest settlement

density in the 2–5 mm length class was recorded on

ropes with non-filamentous loops (NF-L). Pulfrich

(1996), Buchanan and Babcock (1997) and Alfaro and

Jeffs (2002) reported size-specific settlement on

morphologically distinct substrata, which could explain

the differences between length classes in settlement

density. In addition, the possibility of differential

R. Filgueira et al. / Aquacultural Engineering 37 (2007) 195–201200

mortality with regard to size between collectors cannot

be excluded (Buchanan and Babcock, 1997).

The interaction between larval settlement, emigra-

tion and early post-settlement mortality determines the

extent of larval recruitment. The natural phenomenon of

self-thinning during high-density growth is one of the

main causes of post-settlement mortality (Kautsky,

1982; Connell, 1985; Hunt and Scheibling, 1997;

Guinez and Castilla, 1999; Alunno-Bruscia et al., 2000;

Guinez, 2005). The negative relationship between

biomass and the number of individuals per area is

related to space and food limitation, both of which

regulate recruitment (Guinez and Castilla, 1999;

Guinez, 2005). In the present study these limitations

were reflected by the positive correlation ( p < 0.01)

between mortality coefficient and settlement density,

which involves the recruitment density approximation

between collectors. Nonetheless, ropes with filamen-

tous loops (F-L) maintained the greatest densities in

both recruitment and settlement evaluation. On the

other hand the self-thinning processes could modify the

length frequency distribution.

An additional important factor regulating post-

settlement mortality is predation. Predators can also

directly affect the length frequency distribution by

selective feeding pressure on larger individuals. Fish

predation on mollusc seed has been extensively

documented (Crooks, 2002; Bartsch et al., 2005; Rilov

and Schiel, 2006) and is a major cause of mussel seed

mortality in industrial cultivation (Schiel, 2004;

Morrisey et al., 2006). This is especially important in

the Rıa de Ares-Betanzos where Peteiro et al. (2007)

reported that predation is the main limiting factor on

mussel seed supply especially by the fish (Spondylio-

soma cantharus). Rope lacing complexity could lead to

the creation of refuges for spat, with the potential to

reduce post-settlement mortality from environmental

physical disturbances (Shanks and Wright, 1986) or

predation (Walters and Wethey, 1996; Frandsen and

Dolmer, 2002). In the present study, non-filamentous

loops (NF-L) are quite rigid and could provide better

protection from predators as shown by the highest

adjusted length recorded for these ropes. Although the

hypothesis of differential predation between collectors

could explain the lack of larger individuals on ropes

with less protection from predators, differences in

length could be attributed to differential settlement

substrata with regard to mussel size.

In addition, rope structure complexity could enhance

mussel attachment (Curiel and Caceres-Martınez, 1999)

by diminishing detachment and post-settlement mor-

tality. This is reflected in the present study by the low

recruitment on non-filamentous ropes without loops

(NF-NL) compared to filamentous ropes without loops

(F-NL). Nonetheless the effect of the chemical

composition of ropes cannot be disqualified since

non-filamentous ropes without loops (NF-NL) are the

only ones constructed from nylon. Differences in

chemical composition could imply differences in the

fouling and biofilm of the rope, which have been

established as a cue to the settlement process (Bao et al.,

2007).

5. Conclusions

Higher mussel settlement densities are obtained on

ropes with greater available surface area, although a

filamentous structure enhances the amount of settled

individuals.

The relationship between settlement and recruit-

ment densities is influenced by both inta-specific

competition and predation as it has been observed in the

present study. Nevertheless, the highly mobile beha-

viour of mussels at this early stage could modify this

relation.

Mussels from ropes with non-filamentous loops

showed higher adjusted shell length in recruitment. It

could be related to different selection of settlement

substrata with regard to size or differential mortality

related to intra-specific competition or predation.

Maybe the supply of refuges for spat from predation

(rigid loops) might enhance the recruitment of larger

individuals. Nevertheless, a proper predation experi-

ment should be conducted to test this effect.

Acknowledgements

We wish to thank PROINSA mussel farm and their

employees, especially H. Regueiro, M. Garcıa, C. Brea

and O. Fernandez-Rosende for technical assistance. We

also greatly appreciate the contribution of Servimar

norte, especially S. Ordonez for experimental ropes

supply. This study was supported by the contract-

project PROINSA, Code CSIC 20061089, Galicia

PGIDIT06RMA018E.

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