The relationship between the presence of larval anisakine nematodes in cod and marine mammals in British home waters.
About 5,400 cod from off the Hebrides, the Irish Sea, the Bristol Channel, the Southern Bight, the central North Sea and off the north-east English coast were examined between July 1968 and June 1970. Their ages and lengths were recorded and the distribution, numbers and kinds of anisakine nematode larvae in their body muscles were noted. The importance of the grey seal (Halichoerus grypus) and the common seal (Phoca vitulina) as transmitters of these larvae to cod was investigated by an examination of their anisakine nematodes and also those of the cetaceans Balaenoptera acutorostrata, B. physalis, Physeter catodon, Globicephala melaena and Phocaena phocaena. Of all larvae recovered from the cod muscles, 60.2% were Terranova decipiens, the remainder belonging to the genus Anisakis. The former were found in all parts of the body musculature while the latter were restricted to the muscles of the abdominal wall. The mean number of T. decipiens larvae per cod increased with fish weight and length. The relationship was linear with the former and exponential with the latter. The increase in fish over 50 cm long was located in the abdominal wall. The mean number of Anisakis sp. larvae per cod increased then decreased with fish weight and length, the relationship being linear with the former and exponential with the latter. The weight and length at which most Anisakis sp. larvae were present varied between 2 and 5 kg and between 70 and 90 cm. Most T. decipiens were found in the cod populations from off the Hebrides, Irish Sea and Bristol Channel, and while many Anisakis sp. larvae were found in cod populations off the Hebrides, the north-east English coast, the central North Sea and Southern Bight, of these populations those off the north-east English coast contained most larvae. Only seals were significantly parasitized by adult T. decipiens and grey seals had significantly more of these than did common seals. More adults were present in grey seals in areas where larvae were more abundant in cod populations and it was concluded that grey seals are the only hosts of significance in transmitting larvae of T. decipiens to cod. Anisakis sp. adults were absent from common seals but were found in all species of Cetacea examined and also in grey seals. It was concluded that Cetacea are the most common hosts to species of Anisakis and that, as the distribution of larvae in the cod populations differs from that of grey seals, the latter are not hosts of significance to species of Anisakis. [AS].