Research report

​A survey of parasitic nematodes in maricultured finfish in Scotland - except salmon

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University of Stirling
Bron, J

The primary goal of the work was to improve knowledge of the prevalence and intensity of anisakine nematodes in marine farmed fish (Atlantic halibut and rainbow trout) in Scottish waters. A secondary goal was to ensure product quality and safety for the consumer.

To safeguard consumers against Anisakiasis, the EU implemented freezing as the method used to kill any nematodes in fishery products eaten raw or cold smoked (EC 853/2004, Section VIII, Annex III, Section D). However, the introduction of freezing to the farmed salmon industry had major implications for the smoked fish product. Work previously funded by the Food Standards Agency in Scotland showed that farmed Atlantic salmon were free from Anisakis. These data have been considered by the European Food Safety Authority (EFSA) and lead to an amendment of Annex III that farmed salmon need not be treated. However, numerous other fish species are farmed in Scotland for which no such data exists.

This study has investigated two other commercially produced fish species that, at some point during their life cycle, are farmed in seawater in Scotland: Atlantic halibut and rainbow trout. Anisakid nematodes are widely reported from wild Atlantic halibut, in contrast little data exists for wild marine rainbow trout.

Samples were taken of 225 rainbow trout from four fish farms on the West coast of Scotland and 150 Atlantic halibut from two farms. Twelve further Atlantic halibut were obtained from wild fisheries. Fish flesh was examined for nematode larvae and no evidence was found for the presence of anisakid nematodes in any of the farmed fish sampled. In addition, no food items, other than pelleted feed, were found in the stomach and intestines of any farmed fish. Wild halibut showed a prevalence of 75% and intensity of 12.75 ± 25.81 of Anisakis spp., although no Pseudoterranova decipiens were found.

It can therefore be assumed that under current farming practices, Scottish farmed halibut and rainbow trout are wholly free from anisakid infection or that such infection is extremely rare and therefore that these farmed species do not pose a significant risk to consumers in terms of the ingestion of these parasites.

An analysis of the current farm cycles for Atlantic halibut and rainbow trout in Scotland indicated that the risk of infection with anisakids is extremely low. To become infected, a fish must consume an infected prey item. As commercial aquaculture in Scotland relies on processed pelleted feed, the risk of wild infected prey being consumed is very low, particularly since fish are generally fed to satiation.

The only exception is the early stage of halibut rearing, where live planktonic organisms are employed for early feeding of juveniles. However, as long as cultured plankton species are employed rather than wild plankton, the risk of infection with nematodes is eliminated in this phase. Where fish are raised throughout their entire growth cycle in tanks that use treated or filtered water only, the risk of infection with anisakids is eliminated. The risk of infection, while remaining extremely low, is nominally increased where farms are located close to anisakid host populations such as seal colonies.

Project Code: FS241054

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