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Deliverable: results of transplantation of cod on their classification

The project was designed to determine if CODTRACE techniques were able to provide legally indisputable evidence for the harvest location of unknown cod. This was tested through transplants of eggs, and also through double blind testing of cod samples. With the transplantation experiments, eggs from Irish Sea cod were successfully transplanted to Iceland and reared in common conditions with Icelandic cod. Siblings from the same batch of Irish Sea eggs were reared in Irish Sea water in the Isle of Man concurrently with the fish in Iceland. All three groups of fish were sampled at five months of age; Irish Sea fish reared in Irish Sea water, Irish Sea fish reared in Icelandic water, and Icelandic fish reared in Icelandic water. Two of the CODTRACE techniques, the bacterial and the parasite assemblages, produced no useful results because the fish were too small and because of the water treatment used in a hatchery-rearing environment. Otolith microchemistry results indicated that many elements (Sr, Na, Pb, Ba, and Cu) were influenced by environmental conditions, but some elements (Mg and Mn) reflected the stock origin regardless of local conditions. Microsatellite characterisation revealed significant divergence between the Icelandic fish and the Irish Sea fish, whether transplanted or not.

The otolith morphometry characterisations revealed that most of the transplanted fish were classified as Icelandic rather than Irish Sea fish; otolith shape was more strongly influenced by environmental factors (rearing conditions) than previously assumed. In summary, only genetic characteristics and some selected isotopes in the otoliths can be considered as invariant markers of the cod source population considered here. Body and otolith morphology, and most otolith microchemistry were affected by local conditions, thus reflecting harvest location, therefore these latter analyses are more likely to determine where a cod was harvested independently from the location of its natal population (important for separating where a migrating cod was actually caught).

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