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