A population study of cod, Gadus morhua L., in northern Norway using otolith structure and parasite tags

Abstract

A total of 502 cod, Gadus morhua L., was taken during spring and autumn sampling from three different localities in northern Norway: the Barents Sea, a silled fjord (Balsfjord) and an open fjord (Malangen). Each cod was allocated, according to its otolith structure, to one of two types, coastal or Arcto-Norwegian, and was then examined for infections of four species of parasite: the myxosporeans Myxidium oviforme and Zschokkella hildae, the digenean Hemiurus levinseni, and the copepod Lernaeocera branchialis. Both types of cod were present at each locality, with coastal cod dominating in the fjords in both seasons and in the Barents Sea in spring, but with Arcto-Norwegian cod dominating in the Barents Sea in autumn. Differences in the proportions of the two types of cod and in parasite prevalences between seasons and localities were interpreted as indicating a migration of coastal cod from the Barents Sea sampling area into the fjords between March and October. We found no evidence that Arcto-Norwegian cod from the Barents Sea migrate into the fjords, but our results suggest that the fjords contain local resident populations of Arcto-Norwegian cod. It is concluded that local parasite faunas are not greatly influenced by genetic differences between the two types of cod, but that their compositions are determined largely by variations in the abundance of intermediate hosts.