The Severn Estuary Forum is a key annual event in its eighth year and hosted by the Severn Estuary Partnership: an independent, estuary-wide initiative, involving all those interested in the management of the estuary, from planners to port authorities, fishermen to farmers. This year’s Forum was opened by the Lord Mayor of Gloucester and supported by CIWEM. It focussed on a number of diverse topics, including the upper estuary; renewable energy possibilities; a review of the Severn Estuary Flood Risk Management Strategy; an overview of the operations and maintenance of the Severn River Crossings; Local Enterprise Partnerships; The Bristol Deep Sea Container Terminal; proposals for a Severnside Airport and Fisheries amongst others. These engaging and exciting events are intended for all interested in learning about the latest research and policy developments dealing with the Severn Estuary and its future, and always guarantee a lively and informative day of presentations and talks. They offer a unique opportunity to learn from others, share ideas and participate in the management of the Severn Estuary.

1. Fish in the
Severn Estuary
Dr Richard Seaby
Pisces Conservation Ltd.
richard@pisces-conservation.com

2. 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 Severn Estuary Data Set

3. 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.
The Habitat

4. 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.
Very little primary production

5. Fish species accumulation curve
100 randomisations of sample order
About 80 species of fish and
15 macro-crustaceans have
been recorded.
Fish Species Accumulation Curve
Species Number
100 ransomisations of sample order
80
75
70
65
60
55
50
45
40
35
30
25
20
15
10
5
0
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.
50
100
150
200
Sample
250
300
350
The almost linear increase in
species after 100 samples is
due to the capture of
occasional migrants.
The larger mobile species

6. What is happening to fish species in
the Severn?

7. 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

8. 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

9. The monthly
abundance of eel,
Anguilla anguilla,
between the years
1980 and 2011 in the
Severn Estuary.
An example of exponential decline - the eel

10. 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

11. An example of exponential increase - the sole

12. 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

13. Annual captures of snake pipefish in
Bridgwater Bay, Somerset 1981-2013
600
The snake pipefish – a short population
explosion – hardly present for 2 decades
Annual number caught
500
400
300
200
100
0
1980
1990
2000
Year
Dangers of short-term sampling
2010

14. 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

15. 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

16. Threats to marine life – fishing

17. Threats to marine life
cooling water intakes

18. Threats to marine life
cooling water outfalls

19. Threats to marine life
dredging

20. Threats to marine life
ports

21. Threats to marine life
loss of wetlands and salt marsh die-back

22. Future threats to marine life
tidal generators

23. 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.
A changing world

24. Fish species recorded January-December
60
55
15
Year vs Total species number
Year vs Total species number: 1992
Year vs Ave temp
14
50
13
45
12
40
11
35
10
30
1980
1990
2000
9
2010
Average seawater temperature January-December
Temporal variation in species number and average seawater temperature
Species richness has
been increasing from
about 35 to greater
than 40 per year.
Over the same period
average temperature
has also increased.
Year
Changes in fish species richness

25. The change in the 15 most abundant species

26. The change in next 15 most abundant species

27. In part these
changes can be
related to physical
change in
temperature, salinit
y and NAO

28. A conclusion
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.