Food Webs in the Ocean Andrew W Trites Marine Mammal Research Unit University of British Columbia Who eats whom and how much?

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Presentation transcript:

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

Fatty acids Stable Isotopes

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 ( )  Coastal shelves ( )  Tropical estuaries ( ) 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) SpeciesConsumption (%) Production Efficiency (%) Residence Time Sea Birds y Mammals y Fish y Crabs & Shrimp m Squid m Zooplankton m

Food Web Patterns (Production Efficiency) SpeciesConsumption (%) Production Efficiency (%) Residence Time Sea Birds y Mammals y Fish y Crabs & Shrimp m Squid m Zooplankton m

Food Web Patterns (Residence Time) SpeciesConsumption (%) Production Efficiency (%) Residence Time Sea Birds y Mammals y Fish y Crabs & Shrimp m Squid m Zooplankton 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

Diet Studies

Single Species Modeling Low diversity Higher cost to young

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

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