Effects of anthropogenic feeding on the migratory behaviour of coastal cod ( Gadus morhua) in Northwest Iceland

Abstract

To study the effects of anthropogenic feeding on the behaviour of cod ( Gadus morhua), a 30 km 2 area was reserved in Arnarfjördur, a fjord in Northwest Iceland. At four feed stations, frozen trash-fish was dispensed 2–3 times per week from April 2005 to December 2006. Four sizable ‘herds’ of coastal cod were rapidly formed. Cod were tagged outside the herds in June 2005 and July 2006 (unconditioned fish) and within the herds in July 2006 (conditioned fish). From August to October the conditioned cod were only 1/3 as prone to fishing by the commercial fleet as the unconditioned cod. The increased food availability resulted in high growth rates, high condition factors and high liver indices of cod tagged and recaptured within herds. The conditioned cod showed high fidelity to a given herd; for all cod recaptured in the herds, 82% of them had been tagged in the same herd, 12% in the nearest herd and 6% in two distant herds. After the termination of the anthropogenic feeding, a higher percentage of conditioned than unconditioned cod were recaptured in a distant spawning area (Breidafjördur), a difference explained by a size related threshold to migratory activity.