Hur mycket torsk konsumerar mellanskarv i byfjorden?
Foto: Patrik Eld, Lysekil
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We have measured the length of otoliths from cod in 76 pellets regurgitated by cormorants (Phalacrocorax carbo sinensis), collected from the small island Ringburen in the By Fjord which is part of the fjord system between the Orust Island and the mainland. Using a mathematical formula developed in the present paper, we determined the individual weight of the cods consumed by the cormorants. If an annual mean of 10 – 25 cormorants feed in the By Fjord, we estimate that their annual catches of cod from the By Fjord are in the interval 1400 – 3 400 kg distributed on 18 000 – 44 000 individuals of which 90 – 220 are mature. This might affect the local recruitment of cod and jeopardize the expected positive effect of recently established artificial reefs. There are no estimates of the mass of the cod stock and its partition into different size classes, the annual recruitment, and rates of growth. At the present, it is therefore not possible to assess the effect of the predation by cormorants upon the cod stock. A mathematical model of the cod stock in the fjord system should help structuring the problem and may also deliver prognoses under different assumptions.
Författare Lars-Olof Axelsson, Anders Stigebrandt
Nyckelord Cormorant, Cod consumption, Byfjorden