Embed Size (px)
Citation preview
The Severn Estuary Data Set
Our Severn Estuary Data Set is based on regular sampling at power station intakes.
Monthly sampling commenced at Hinkley Point B in 1980, and is still continuing.
Fish and macro-crustaceans are monitored on the power station filter screens and plankton nets are placed in the intake.
The Habitat
Hinkley Point B is situated on the edge of Bridgwater Bay.
The maximum tidal range is about 15 m and there are extensive areas of inter-tidal mud.
This macrotidal system has suspended sediment loads as high as 3 g per litre.
Salinity ranges between 18 and 32 parts per thousand.
Very little primary production
The main energy input is detritus and dissolved organic carbon, mostly of terrestrial origin.
Within Bridgwater Bay there is little planktonic or benthic primary production, because of the turbidity of the water and the instability of the substrate.
For the Bristol Channel including Bridgwater Bay, Joint & Pomroy (1981) estimated annual primary production to be only 6.8 g C m-2 y-1.
In comparison their estimate for the outer Bristol Channel in the vicinity of Lundy Island was 164.9 g C m-2 y-1
.
The larger mobile species
About 80 species of fish and 15 macro-crustaceans have been recorded.
Species acquisition curves, historical records and published reports all suggest that our Hinkley sampling records all the resident and most of the migratory fish and macro-crustaceans present between 1980 and now.
The almost linear increase in species after 100 samples is due to the capture of occasional migrants.
Fish Species Accumulation Curve100 ransomisations of sample order
Sample35030025020015010050
Spe
cies
Num
ber
80757065605550454035302520151050
Fish species accumulation curve100 randomisations of sample order
What is happening to fish species in the Severn?
Fish populations are generally less stable in estuaries than in the open sea.This is because the populations usually comprise the younger age classes, and
species that only utilise the estuary for a proportion of their life. Shown here are two common species, whiting and flounder, fish populations that
are observed to be fluctuating around reasonably constant levels. These examples lend support to the view that density-dependent control is
operating.
Long-term stability in fish populations
The monthly abundance of whiting Merlangius merlangus between the years 1980 and 2011 in the Severn Estuary. The trend line is a 12-month moving average.
The monthly abundance of flounder Platichthys flesus between the years 1980 and 2011 in the Severn Estuary. The trend line is a 12-month moving average.
Whiting and flounder dynamics
The monthly abundance of eel, Anguilla anguilla, between the years 1980 and 2011 in the Severn Estuary.
An example of exponential decline - the eel
A 30-year study of the estuarine population of yellow eel, Anguilla anguilla, abundance in Bridgwater Bay, Somerset, UK, shows that the population number has collapsed. Since 1980, the decline has averaged 15% per year. The abundance of eel in 2009 is estimated at only 1% of that in 1980.
Henderson, P., Plenty, S., Newton, L. and Bird, D. (2011) Evidence for a population collapse of European eel (Anguilla anguilla) in the Bristol Channel. Journal of the Marine Biological Association of the United Kingdom . pp. 1-9. ISSN 0025-3154
An example of exponential decline - the eel
An example of exponential increase - the sole
Note that abundance is a log scale, so an exponential gives a straight line – the increase over the last 32 years is approximately exponential.
An example of exponential increase - the sole
Dangers of short-term sampling
1980 1985 1990 1995 2000 2005 20100
100
200
300
400
500
600
Annual captures of snake pipefish in Bridgwater Bay, Somerset 1981-2013
Year
An
nu
al n
um
be
r c
au
gh
t
The snake pipefish – a short population explosion – hardly present for 2 decades
Threats to marine life
There are many threats to the marine life in the Severn.
It is an important estuary that has been developed and industrialized over a long period, and this development is unlikely to stop in the foreseeable future.
Now I will briefly outline some of the major impacts and the threats they produce to the fish of the Severn
Threats to marine life
Obstructions to movement.These have had great effects
e.g. on Salmon, Shad and Lamprey• Anadromous fish move from the sea
up rivers to spawn. Obstructions to their movement have been particularly disastrous.
• For example: In the River Severn “Lampreys too, which were formerly considered of more importance than salmon, and were caught in the upper Severn, have altogether ceased to visit it since the erection of the first weir in 1843”.
A particular estuarine problem
Threats to marine life – fishing
Threats to marine life cooling water intakes
Threats to marine life cooling water outfalls
Threats to marine life dredging
Threats to marine life ports
Threats to marine life loss of wetlands and salt marsh die-back
Future threats to marine lifetidal generators
A changing world
With all the changes that are occurring in the Severn estuary, it is important to remember how complex the interactions are between the many species of fish in the estuary and the environment in which they live.
Changes in fish species richness
Species richness has been increasing from about 35 to greater than 40 per year.
Over the same period average temperature has also increased.
Temporal variation in species number and average seawater temperature
Year
1980 1990 2000 2010
Fis
h sp
ecie
s re
cord
ed J
anua
ry-D
ecem
ber
30
35
40
45
50
55
60
Ave
rage
sea
wat
er te
mp
erat
ure
Janu
ary-
Dec
embe
r
9
10
11
12
13
14
15
Year vs Total species number Year vs Total species number: 1992 Year vs Ave temp
The change in the 15 most abundant species
The change in next 15 most abundant species
In part these changes can be
related to physical change in
temperature, salinity and NAO
Animal populations can behave in surprising ways.
We need to continue collecting and recording if we are to create the data sets that will lead to the predictive science we desire, that allow us to predict the impacts we have on the aquatic environment.
A conclusion
