Nematodes in the Fillets of Cod and other Fishes in Newfoundland and Neighbouring Areas

Abstract

Inspection of the fillets of cod and other fish species for nematodes was carried out mainly in the years 1947 to 1953. Fillets were examined on a candling table. Two varieties of nematodes were found: one, 94% in cod and 86% of the total in American plaice, witch flounder, haddock and redfish, was Porrocaecum, ranging between 15–16 and 57–58 mm. in length; the other, 6% in cod and 14% of the total for the other 4 fish species mentioned above, did not possess a caecum and had a longer pharynx than Porrocaecum relative to the total length and to the ventriculus length. The latter nematode was 17–18 to 33–34 mm. in length and resembled Anisakis but may possibly have been Eustoma.The infection level per 100 lb. (45 kg.) of cod fillets was lowest, 1 to 6 nematodes in ascending order, on Flemish Cap, the Grand Bank, the east coast of Newfoundland, off Labrador and southern St. Pierre Bank. It was highest, 223 nematodes, in the southern part of the Gulf of St. Lawrence. Intermediate values, 10 to 65 nematodes, were present in other areas. The nematode infection level in the larger American plaice was similar to that in cod, and infection per 100 lb. of fillets was especially heavy at the smaller sizes of plaice. Witch flounder, redfish and haddock showed low infection levels. Smelts living in the immediate neighbourhood of seal colonies were sometimes highly infected in terms of nematodes per 100 lb. of fillets. Nematodes were found in the fillets of the angler, Greenland cod, Greenland halibut, pollock, yellowtail flounder, longhorn sculpin and tomcod. It was concluded that the site of original infection of the fish was close to the site of deposition of nematode eggs by the seal host, and that the first intermediate hosts, invertebrates or small fish, were not very migratory.In the fillet, nematodes were most numerous in the thicker parts dorsal and posterior to the body cavity and less numerous in the thinner tail portion. The distribution fits the idea that nematodes emerging from the stomach of the fish enter the neighbouring body muscles.Of the seals which are the final hosts for Porrocaecum, harbour seals are common in Newfoundland areas where nematode infection of cod is high, as on the west coast of the island, but they are also especially numerous in southern Labrador where nematode infection of cod is low. The usual intermediate invertebrate or fish hosts may be scarce or lacking in Labrador and on the northeast coast of Newfoundland.It is concluded that the larger, browner, and more numerous Porrocaecum presents the problem of commercial importance, rather than the smaller, whiter Anisakis. The only practical biological attack in view at the moment is the reduction in numbers of harbour and grey seals, in the hope that the more numerous harp seal is not so important a final host as it at first sight appears to be in the Gulf of St. Lawrence. In many areas where harp seals are not present, or, where present, do not carry Porrocaecum, the harbour and grey seals appear to be the only final hosts of Porrocaecum. St. Mary's Bay, Newfoundland has many advantages as a site for a practical long-term experiment on reduction of nematode levels in cod flesh.