Direct and indirect effects of minke whale abundance on cod and herring fisheries: A scenario experiment for the Greater Barents Sea

Tore Schweder,Einar Hatlebakk,Gro S Hagen

doi:10.7557/3.2976

Tore Schweder, Einar Hatlebakk + Show 1 more

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https://doi.org/10.7557/3.2976

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Abstract

To study the pattern of interaction between minke whale (Balaenoptera acutorostrata) abundance and the main fisheries in the Greater Barents Sea, a simulation experiment was carried out. The population model involves 4 species interconnected in a food web: cod (Gadus morhua), capelin (Mallotus villosus), herring (Clupea harengus) and minke whales. Minke whales are preying on cod, capelin andherring; cod are preying on (young) cod, capelin and herring; herring in the Barents Sea are preying on capelin; while capelin is a bottom prey in the model. The consumption function for minke whales is non-linear in available prey abundance, and is estimated from stomach content data and prey abundance data. The model is dynamic, with a time step of one month, and there are two areas: the BarentsSea and the Norwegian Sea. Minke whale abundances are kept on fixed levels, while recruitment in fish is stochastic.Cod and herring fisheries are managed by quotas targeting fixed fishing mortalities, while capelin is managed with a view to allow the cod to have enough food and leaving a sufficient spawning stock of capelin. The model is simulated over a period of 100 years for a number of fixed levels of minke whaleabundance, and simulated catches of cod, herring and capelin are recorded.The experiment showed interactions between whale abundance and fish catches to be mainly linear. For cod catches, both the direct effect of whales consuming cod, and the indirect effect due to whales competing with cod for food and otherwise altering the ecosystem, are linear and of equal importance. The net effect on the herring fishery is of the same magnitude as the net effect on the cod fishery, witheach extra whale reducing the catches of both species by some 5 tonnes. These conclusions are conditional on the model and its parameterisation.

Highlights

Minke whale (Balaenoptera acutorostrata) the natural mortality due to minke whale predaabundance has been estimated at around 85,000 tion on cod when minke whales were removed in the Greater Barents Sea and the Vestfjorden from the system
Will the fishermen be able to take more cod (Gadus morhua), capelin (Mallotus villosus) and herring if the stocks of marine mammals are reduced? Will the fisheries. Their tentative conclusion is that the effect of an increasing stock of minke whales on the important fish stocks is that the herring stock will be most heavily affected
Respond linearly to changes in whale abundance, or are there perhaps levels of whale abundance where an increase can have a positive effect, at least on some fisheries? How much of the effect on the fisheries can be ascribed to minke whale predation on the fish stock in question, and how much is due to the indirect effect of minke whales being competitors of cod and herring and otherwise influencing the ecosystem?

Summary

Introduction

Minke whale (Balaenoptera acutorostrata) the natural mortality due to minke whale predaabundance has been estimated at around 85 ,000 tion on cod when minke whales were removed in the Greater Barents Sea and the Vestfjorden from the system. Tjelmeland and Bogstad (1998) call for studies of the effects of varying minke whale abundance on fish stocks and catches, when fisheries are managed by adaptive strategies. How much of the effect on the fisheries can be ascribed to minke whale predation on the fish stock in question, and how much is due to the indirect effect of minke whales being competitors of cod and herring and otherwise influencing the ecosystem?. The species in the model are minke whales, cod, herring and capelin, all distributed by area, time and age. Minke whales prey on cod, We wi ll investigate these questions by rulming a scenario experiment in a multi-species and herring and capelin. Herring prey on multi-fleet model roughly tailored to the fisheries and ecosystem of the Barents and the Norwegian Sea. The multi-fleet aspect of the model is of no consequence in the present exer-

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