The relationship between seal abundance and cod worm (Phocanema decipiens) infestation in cod in Norwegian coastal waters

A sample of 52 cods, Gadus morhua, caught close to a grey seal, Halichoerus grypus, haul out site, and a total of 652 cods from 18 commercial catches (average sample size 36 cods) were examined with regard to cod worm infestation. The sampling was carried out from July 1978 to January 1981 in Norwegian coastal waters between 62° and 66° North. The presence of larval cod worm was recorded in 64% of the examined fishes, and the average infestation in all 704 fishes was 8.5 cod-worms per fish. However, the infestation decreased significantly with increasing distances from the haul out sites of grey seals and common seals, Phoca vitulina. The heaviest infestations were recorded in cod from the outern part of in shore waters, and slightly decreasing towards the fjords, fitting well to the observed distribution of seals. A substantial decrease was also recorded with increasing distance (and depth) off shore from the mentioned grey seal haul out site