Variation in abundance, diet, otolith zone patterns and black spot disease (Cryptocotyle lingua) of 0-group coastal cod (Gadus morhua L.) in northern Norway

Abstract

Knowledge of the early life-history and habitat requirements of coastal cod is very sparse. Young of the year cod juveniles from three different locations in northern Norway were collected monthly from August to November 2005 using beach seine. Diet analyses based on stomach contents showed that 0-group cod mainly ate copepods, gammarids, polychaetes, mysids, shrimps and isopods. The three locations differed with regard to prey composition, and copepods were generally most abundant, but benthic (polychaetes) and hyperbenthic species (decapods) had the highest weight proportion. To investigate when the first hyaline (winter) zone is formed, length and width of 122 pairs of otoliths were measured and assessed into four zone pattern categories. The proportion of cod with a hyaline otolith zone increased from 20.5% in October to 67.9% in November. There were clear differences between the locations with regard to the otolith-width fish length relation, and this relation was lowest at the location with the highest cod abundance (Jøvik). Abundance and seasonal infection of the digenean parasite Cryptocotyle lingua differed between the three samplings locations, most likely due to differences in abundance of the intermediate host (the snail Littorina littorea). Cod from Njosken exhibited the heaviest infections, whereas fish from Makkjosen had some and those from Jøvik had almost no parasites. It is suggested that infection of this parasite can be used as a habitat indicator of the cod during summer and autumn.