The Prevalence, Abundance, and Density of Pseudoterranova sp. (p) Larvae in the Flesh of Cod (Gadus morhua) Relative to Proximity of Grey Seal (Halichoerus grypus) Colonies on the Coast off Drangar, Northwest Iceland
About 300 cod (Gadus morhua) were sampled on three different sites of different proximity to grey seals (Halichoerus grypus), which are the most important final host for Pseudoterranova krabbei, off Drangar, Northwest Iceland, in the summer of 2004. Cod caught at each station were grouped into four sizes: (1) 40–54？cm, (2) 55–59？cm, (3) 60–69？cm and, (4) >70？cm. Difference in prevalence between catch-sites were analyzed by a binary logistic regression model. Abundance, and density were investigated with multiple regressions. As expected prevalence, abundance and density of Pseudoterranova larvae were highest in the fish caught closest to shore, which was also in closest proximity to grey seal colonies and in the shallowest waters. In the closest proximity to grey seal colonies and in the shallowest water, the prevalence and abundance of sealworm larvae increased with increased length of fish. Finally, the density of Pseudoterranova larvae in cod declined exponentially with depth: 50% at 65 meters and almost zero at 210 meters. In the paper these observations were discussed in relation to sealworm eggs dispersal, depth gradient off the coast, temperature tolerance of the sealworm ovum, and behavioral types of Icelandic cod. 1. Introduction Nematode infestation of cod (Gadus morhua) causes losses to fisherman and to fish merchants alike and, in consequence, necessitated special precautions, which may add considerably to the production or processing costs [1]. In Iceland the codworm (sealworm) Pseudoterranova sp. (p) is the main culprit of parasite infestation in marine fish and has to be removed from fish fillets at an annual cost of at least 10 million？US$ [2]. Similar situation is observed in other countries of the North Atlantic [3]. Grey seal (Halichoerus grypus) is considered to be the most important final host for Pseudoterranova krabbei and common seals (Phoca vitulina) for P. decipiens sensu stricto [4]. The larvae of P. krabbei and P. decipiens however are structurally indistinguishable and have only been identified by allozyme markers. In Icelandic waters grey seal is recognized as the main final host for Pseudoterranova larvae and common seals are believed to contribute not as much, as the grey seal, to the infestation of cod and other commercially important fish [5, 6]. Grey seals are far more infected with Pseudoterranova sp. (p) nematodes than common seals in Icelandic waters [7, 8]. Even though the Icelandic common seal population is likely twice as big in numbers as the grey seal population [9, 10], the average Pseudoterranova burden was
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