The Relationship Between the Presence of Larval Anisakine Nematodes in Cod and Marine Mammals in British Home Waters

Abstract

The infestation of cod, Gadus morhua (L.), from Scottish waters with larvae of parasitic anisakine nematodes was studied by Rae (1960, 1963). The final hosts of marine species of this subfamily are normally fish-eating vertebrates. In fish, their larvae are commonly found in the internal organs, but two of those occurring in cod, Terranova decipiens (Krabbe) and an undetermined species of Anisakis, are also found in the body musculature where their presence is of concern to the fish food industry and consumers. Previous authors have shown that these two nematodes have, as their final hosts, a wide variety of fish-eating mammals, thus Terranova decipiens is reported from at least eighteen species of Pinnipedia, five species of Cetacea and one species of Mustelidae, whilst species of Anisakis have been recorded from at least twenty-five species of Cetacea and eight species of Pinnipedia. Scott & Martin (1957, 1959) and Templeman, Squires & Flemming (1957) examined the muscles of cod off Eastern Canada for larval nematodes and concluded that the larvae mostly belonged to the species Porrocaecum decipiens (= Terranova decipiens) but between 3 and 6% were Anisakis sp. The distribution of larvae within the cod populations was related to the known distribution of harp seals (Pagophilus groenlandicus), harbour seals (= common seals, Phoca vitulina) and grey seals (Halichoerus grypus). Scott & Fisher (1958a, b) examined these three seal species, common porpoises (Phocaena phocaena (L.) and one white whale (Delphinapterus leucas) for adult nematodes. They considered that only common and grey seals were significant in transmitting larval Terranova decipiens to cod. Mansfield (1968) examined the same three species of seal from the Gulf of St Lawrence and calculated that in this area the relative importance of common seals, grey seals and harp seals to the infestation of cod was 2%, 45% and 53% respectively. In British home waters, Rae (1960) examined cod from the northern areas and related the numbers of larvae that he found to the known breeding sites of grey seals but although he suggested (1963) that all the nematodes he found in a sample of seventeen grey seals from Scotland were Porrocaecum decipiens (= Terranova decipiens), Van Theil (1966) found Porrocaecum decipiens and also Anisakis sp. and Contracaecum sp. in a sample of eighteen seals from the same area. The present study was started because of the marked disparity between the reported number of adult nematodes in seals from Canada and the United Kingdom and to extend the survey done by Rae (1960). In it the distribution of anisakine larvae in British home water stocks of cod was examined further, and the normal populations of gut nematodes