Download Establishing the structure of aquatic food webs in managed

Establishing the structure of aquatic food webs
in managed extensive ponds
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• Monoculture
- Single manufactured food source –
expensive + ethical arguments of
fishmeal
- Water quality more unstable due to
nutrient inputs from food
- Water quality controlled by water
exchange (expensive)
- Nutrients in effluent can exceed legal
limits (fines)
- Disease more likely to spread in monoculture as all fish are the same.
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Extensive eco-aquaculture
• No feeding – cheaper
• Good water quality – plants and
microalgae absorbing nutrients
• Water quality more stable →
predators eating algae
• Less stress on fish etc.
• Less chance of disease
• High fish/shrimp quality
• Better prices for products
• No nutrients in effluents
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Importance of the food web
1. Need to know the diet of your target fish/shrimp in case of
problems.
2. Important is the % reliance on single prey source – can cause.
instability1.
3. If you are targeting more than one organism – there maybe
interactions.
4. May want to improve quality of fish – by manipulating diet.
5. Might want to relieve predation pressure on a target species
1McCann,
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K.S. (2000) The diversity–stability debate. Nature 405, 228-233
We used chemical tracers
– the isotopes of C and N
Generally an animal’s isotope
signal reflects the prey that it
has eaten
C increases 0-1‰
In this example we assume
equal consumption and C
increase is zero
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We used chemical tracers
– the isotopes of C and N
Generally an animal’s isotope
signal reflects the prey that it
has eaten
C increases 0-1‰
In this example we assume
equal consumption and C
increase is zero
N normally increases ~ 3‰ at
each trophic level
So N can be used to indicate
whether animal is carnivore,
herbivore etc.
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Isotopes can:
1. Identify dietary sources
2. Identify trophic level
3. When used inconjunction with isotopic models like
• Isosource or SIAR can identify percentage contribution
of each dietary source
• Isoweb – can dietary contributions for the entire food
web
4. Trophic niche width
5. When used with stomach contents analysis or other tracers
such as fatty acids, uncertainty can be reduced.
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Phragmites
australis
Spartina
densiflora
Ruppia
maritima
Phytoplankton
Biofilm +
Sediments
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Polychaetes
Isopods
Amphipods
Chironomids
Pol
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Ostracod
Zooplankton
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Isotopes can help us understand the links
The full food web has many other connections
(avifauna, reptiles, …)
Top Consumers are human
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Examples of isotope use
Spartina vs Phragmites
Invasion of Spartina means that it has now colonised 90% of pond verges that used be occupie
chiefly by Phragmites.
Contribution of the 2
species approx. equal
despite Spartina
dominance – suggests
not much Spartina
productivity enters the
pond ecosystem.
Average of 8
Species
Averaged contribution of
primary producers
Dicentrarchus spp. P. macrodactylus
(L=5,TL=3.47)
(L=4,TL=2.64)
Using isotopes we trace where the C and N in our commercial species came from
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Prey
POM
Sediment
Phragmites
Biofilm
Spartina
Ruppia
POM
Sediment
Phragmites
Biofilm
Spartina
Ruppia
POM
Sediment
Biofilm
Phragmites
Spartina
Ruppia
Freq. Signif. Low95% High95%
100
5.6
41.8
100
0.4
35.1
100
0.6
31.5
50
0.04
31.9
100
0.3
26.4
0
0
21.6
100
7.5
38.6
100
2.5
37.6
100
2.3
31.9
100
0.8
31.8
40
0.1
21.9
20
0.03
20.6
6.2
37.8
1.3
35.2
1.0
32.6
1.2
29.5
1.4
26.7
0.1
23.3
Mode
24.4
21.0
19.9
16.9
14.9
3.4
24.5
21.8
19.0
18.4
9.9
7.0
23.0
20.0
17.5
15.4
13.3
7.8
Mean
24.3
18.8
17.2
16.3
13.9
9.5
23.7
21.1
17.9
17.0
10.9
9.4
22.6
19.1
17.3
15.7
14.2
11.1
Importance of shrimp to the food web
Mullet diet
 21% shrimp
Mugilidae (L=5, n=25)
Bass diet
 19% shrimp
 5% mullet
Dicentrarchus spp. (L=5,
n=38)
95 tonnes/year harvested using nasa traps but:-
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Prey
Freq. Signif.
Pomatoschistus spp.
100
G. affinis
100
F. heteroclitus
60
P. macrodactylus
33
Amphipods
40
C. carpio
20
P. varians
20
Mugilidae
20
P. varians
60
Ostracods
80
Amphipods
20
Polychaetes
0
T. verticalis
0
Chironomids
0
Mysids
20
Cladocerans
20
P. macrodactylus
75
Biofilm
0
Copepods
33
Low95%
3.1
0.6
0.6
0.06
0.8
0.01
0.01
0.06
0.2
0.5
0.002
0
0
0
0
0.01
0.3
0
0.005
High95%
32.1
32.8
28.2
25.8
22.9
23.9
22.1
20.9
20.7
20.2
19.3
19.0
18.6
18.7
18.6
18.0
15.6
17.5
17.3
Mode
17.8
18.5
14.0
13.0
8.0
8.2
5.9
4.9
12.0
11.3
9.9
8.0
7.5
7.8
6.9
5.2
9.3
4.2
4.6
Mean
18.0
17.7
14.8
13.1
11.2
11.2
9.9
9.4
10.9
10.9
9.6
9.4
9.1
9.1
9.1
8.6
8.6
8.2
8.1
Conclusions
Food web knowledge can be used to:
1. Enable informed decision making regarding harvest
rates
2. Help correct productivity problems
3. Improve quality of harvested fish/shrimp
4. Assess benefits of reducing predation pressure on a
harvested species
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Thank you for your
attention!
Email: [email protected]
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