Food Webs in the Ocean
Andrew W Trites
Marine Mammal Research Unit
University of British Columbia
Who eats whom and how much?
Steller Sea Lions
Diet: Stomachs
Diet: Scats
Diet: Scats
Diet: Scats
Fatty acidsStable Isotopes
Bering Sea Food Web
Bering Sea Food Web
Food Web Patterns (Species)
Sizes larger at higher trophic levels
Numbers more at the bottom of food webs
Proportions constant at each trophic level
Diets most are restricted – sizes & levels humans can eat all
Food Web Patterns (Chain Lengths)
Typically short (2-5 linkages) Oceanic upwelling (2.2-2.8) Coastal shelves (2.8-4.0) Tropical estuaries (3.0-5.0)
Function of Environmental stability Energy transfer efficiency
Consumption
Estimating Energy Requirements
Stomach contents Feeding rates Metabolism Mathematical models
single species multispecies (ecosystem)
Studies
Captive Field Models
Metabolism
Digestive Efficiencies
Foraging Energetics
Activity Budgets
Growth Curves
Food Web Patterns (Consumption)
Species Consumption
(%)
Production
Efficiency (%)
Residence
Time
Sea Birds 15-20 80 5 y
Mammals 4-8 2-6 15-50 y
Fish 1-4 40-80 1.5 y
Crabs & Shrimp 2-3 130-150 8 m
Squid 8-12 300 4 m
Zooplankton 20-30 500-600 2 m
Food Web Patterns (Production Efficiency)
Species Consumption
(%)
Production
Efficiency (%)
Residence
Time
Sea Birds 15-20 80 5 y
Mammals 4-8 2-6 15-50 y
Fish 1-4 40-80 1.5 y
Crabs & Shrimp 2-3 130-150 8 m
Squid 8-12 300 4 m
Zooplankton 20-30 500-600 2 m
Food Web Patterns (Residence Time)
Species Consumption
(%)
Production
Efficiency (%)
Residence
Time
Sea Birds 15-20 80 5 y
Mammals 4-8 2-6 15-50 y
Fish 1-4 40-80 1.5 y
Crabs & Shrimp 2-3 130-150 8 m
Squid 8-12 300 4 m
Zooplankton 20-30 500-600 2 m
Food Web Patterns
10% trophic transfer efficiency
Consumption young > old not constant over time quality of prey changes
Food Web Patterns(Fisheries)
High fish catches associated with high primary production fishing at lower trophic levels
Potential concerns may break long food chains may affect ecosystem stability
Food Web Patterns(Steller sea lions)
Diet studies Single species modeling Ecosystem modeling Captive feeding trials Predation
Diet Studies 1950s 1990s Shifted from fatty
fishes to low fat fishes
S qu id /O c to p usF la tfishH e rrin g /S an d la nc e /e tc .P o lloc k/C od /H ak eA tka m ac ke re lO th erS a lm o n
DietCom position (1990s)
Diet Studies
13
24
59
41 74
97 M ean = 140
U nits : thousands of m etric tons
S qu id /O c to p usF la tfishH e rrin g /S an d la nc e /e tc .P o lloc k/C od /H ak eA tka m ac ke re lO th erS a lm o n
Annual Prey Requirem ents (1998)
Single Species Modeling Low diversity Higher cost to
young
Be a ke d Wha le s
Sp e rm Wha le s
Ad ult Po llo c k
La rg e Fla tfish
Pe la g ic sJ uve nilePo llo c k
Ep ifa una
Sm a llFla tfish
La rg eZo o p la nkto n
J e lly Fish
Wa lrus&Be a rd e d
Ste lle r Se a Lio ns
Se a ls
To o the d Wha le s
Ba le e n Wha le s
He rb ivo ro usPla nkto n
Phyto p la nkto nDe tritus
In fa una
Be nthic P.Fe e d e rs
O the rDe m e rsa lFishe s
C e p ha lo p o d s
De e p wa te r FishPisc .
Bird s
5
4
3
2
1
Tro
phi
c L
eve
lRe la tive Ab und a nc e s 1950s 1980s
Ecosystem Modeling
Captive Feeding Trials
Require 35-80% more pollock than herring
Predation Studies
Food Web Analysis (Steller Sea Lions)
Composition of North Pacific has changed Diet has changed Diet and ecosystem composition are consistent Health consequences of eating too many gadids Carrying capacity of pinnipeds is lower Recovery linked to a more diverse diet & reduced predation Environment appears to be the driving force
Conclusions
Food webs & consumption estimates essential for fisheries management & understanding ecosystem dynamics
Require a combination of field studies, captive studies and models
Key to understanding what our marine ecosystems once were, what they are currently, and what they might be in the future