1) Long-term research on fish populations in Lake Windermere using archived samples has allowed reconstruction of the food web using stable isotope analysis. 2) Isotope analysis shows roach rely mostly on littoral prey while perch have shifted to pelagic and profundal resources due to trophic overlap with roach. Arctic charr also appear to now rely more on profundal resources. 3) Pike diet has shifted from Arctic charr to roach, changing the routing of energy in the food web from pelagic to littoral zones. There has also been a subtle increase in trophic efficiency within the web.

1. Archives and isotopes: changes afoot in the
food web of Windermere
Jonathan Grey, P Smyntek, S Maberly & I Winfield et al
j.grey@qmul.ac.uk

2. Long term research on the fish of Windermere
1940+ Arctic charr gill net survey
1943+ Perch traps
1944+ Pike gill nets
1966+ Arctic charr fishery record scheme
1990+ hydroacoustic survey
1995+ Roach gill nets

3. Long term fish abundance
General increase since late 1990s, especially in the south basin
data from Winfield et al
6000
5000
Abundance (fish ha-1)
4000
3000
2000
1000
0
1990 1995 2000 2005 2010
Year

4. Decrease in Arctic charr
Marked decline since 2000; 2010 CPUE lowest recorded
data from Winfield et al
6
5
CPUE (fish h-1)
4
3
2
1
0
1965 1970 1975 1980 1985 1990 1995 2000 2005 2010
Year

5. Increase in roach
data from Winfield et al
50
CPUE (Fish 100 m 2 net-1 day-1)
40
30
20
10
0
1995 2000 2005 2010
Year

6. Fishy Conflict
English Lake
District

7. Stable isotope ‘mapping’

8. Isotopes and archives
-12
-14 A
-16
C (‰)
-18
13
-20
Acidified scale
-22
-24
-26
y = 0.99x + 0.64; r2 = 0.92; P<0.0001
-28
-30
-30 -28 -26 -24 -22 -20 -18 -16 -14 -12
13
Defatted muscle C (‰)
Grey et al 2009

9. Hypotheses testable using SIA
1. Roach populations have increased as a result of warming surface water
2. The recent reduction in summer zooplankton is causally linked to increased
predation pressure by planktivorous fish
3. The increased abundance of summer phytoplankton, and associated
changes in the lake such as increased ratio of phytoplankton chlorophyll a to
total phosphorus, is causally linked to the reduction in grazing pressure
4. The decline in Arctic charr in the lake is causally linked to competition for food
with other planktivorous fish, particularly the introduced roach in recent years
5. Invasion by a generalist consumer (roach) has also affected the top predator
in the lake (the pike) by shortening the food chain, further influencing food-
web interactions.

10. Windermere isotopic food web…. then
Trophic Level Indicator
?
15N (‰)
15N
Offshore (Pelagic) 13C (‰) Near shore (Littoral)

11. Isotope mixing model of dietary contribution
Importance of littoral prey; greater importance of
primary consumers
1
Cyclops
0.9
0.8
Proportion in Diet
0.7
Daphnia
0.6
0.5
0.4
Macroinverts
0.3
0.2
0.1
Chironomids
0
2000 2005 2009 2010

12. Isotope mixing model of dietary contribution
Increase in pelagic and possibly profundal prey
1
0.9 Cyclops
0.8
Proportion in Diet
0.7
0.6 Daphnia
0.5
0.4
Macroinverts
0.3
0.2
0.1 Chironomids
0
2000 2005 2009 2010

13. Isotope mixing model of dietary contribution
Slight shift from littoral toward profundal prey
1
Cyclops
0.9
0.8
Proportion in Diet
0.7
Daphnia
0.6
0.5
0.4
Macroinverts
0.3
0.2
0.1
Chironomids
0
1985-1987 1992-1994 2000-2002 2003-2005 2009-2010

14. Isotope mixing model of dietary contribution
Shift from charr to roach
1
0.9 Roach
0.8
Proportion in Diet
0.7
0.6
0.5 Perch
0.4
0.3
0.2
Charr
0.1
0
2000 2005 2009 2010

15. Windermere isotopic food web….. now
Trophic Level Indicator
15N (‰)
15N
Offshore (Pelagic) 13C (‰) Near shore (Littoral)

16. Change in trophic position?
Shift from charr to roach
1.4
N data
1.2
Pike
15
Charr Pike
Change in trophic position from
1.0
0.8
P-zoop Roach
0.6
H-zoop M-inverts
0.4
Basal Res
Pike - Roach
Pike - Charr
0.2
2000 2010
2000 2002 2004 2006 2008 2010
Year

17. Summary
• Long term collections and archived tissues / samples from the FBA / CEH has
provided material for food web reconstruction
• Stable isotope analysis of carbon and nitrogen can be conducted on such
material with confidence
• Roach diet comprises mostly littoral prey; trophic overlap with perch appears
to have pushed perch toward reliance on pelagic and profundal resources
• Knock on effect to charr pushing them toward reliance on more profundal
resource. Cautionary note: we do not know what YOY fish are doing
• Shift in pike diet from charr to roach and consequent shift in ‘routing’ of energy
through the foodweb from the pelagic to the littoral zone
• Subtle shift in trophic position – increasing efficiency within food web?
http://www.windermere-science.org.uk/home j.grey@qmul.ac.uk