Recently, questions have been raised as to the appropriateness of stock boundaries in the Atlantic horse mackerel (<i>Trachurus trachurus</i>) and the herring (<i>Clupea harengus)</i> in European waters. To this end, 1919 horse mackerel were examined for parasitic infections from samples taken at nineteen sites around Europe. Forty-five species of parasites were recorded, eleven of which are new host records, and one may be a new species. A number of these are suitable for use as biological tags. Results showed a significant degree of separation between fish from the North Sea and Western stocks, based on the relative abundance of the nematodes <i>Aniaskis </i>spp. and <i>Hysterothylacium</i> sp. The endoparasitic fauna of 4033 herring at various life stages from sites to the west of the British Isles, the North Sea, Baltic Sea and northern Norway were examined to obtain information on stock identity, mixing and recruitment patterns in these areas. Results revealed that substantial stock mixing takes place to the west of the British Isles, with fish from a wide area being found together in non-spawning aggregations off the Hebrides. This area is also home at different times of year to two different spawning populations of fish which recruit from nursery grounds in the eastern North Sea and from the west of Scotland. Investigations were made of the usefulness of the ribosomal small sub-unit cytochrome oxidase (I) gene of the parasitic nematode worm, <i>Anisakis simplex s.s., </i>as a tag of host population biology. Results revealed the COI gene to be highly variable, with around 50% of worms sequenced having unique haplotypes. There were no population structures evident from analysis of genetic distances.

The work presented in this thesis examined the effects of dietary fish oil replacement on fish innate and adaptive immune function, disease resistance tissue histopathology and fatty acid composition of lipids in peripheral blood leukocytes. Dietary trials with Atlantic salmon (Salmo salar), Sea bass (Dicentrarchus labrax), Atlantic cod (Gadus morhua) and Arctic char (Salvelinus alpinus) were conducted in which fish oil was replaced by rapeseed oil, linseed oil, olive oil, palm oil, echium oil or a mixture of these oils. A significant reduction in respiratory burst activity was most pronounced in salmon and sea bass fed high levels of rapeseed oil-containing diets. In addition, rapeseed and olive oil inclusion in the diets of salmon and sea bass significantly reduced the head kidney macrophage phagocytic capacity to engulf yeast particles. A reduction in prostaglandin E2 levels was found to be related to a reduction in macrophage respiratory burst activity in salmon fed linseed oil diets and sea bass fed a dietary blend of linseed, palm and rapeseed oils. Changes in macrophage function may be a contributing factor causing a reduction in serum lysozyme activity observed in some trials. No significant differences were detected in cumulative mortality of Atlantic salmon fed an equal blend of linseed and rapeseed oils challenged with Aeromonas salmonicida. However, resistance to Vibrio anguillarium was significantly impaired in Atlantic salmon fed a blended oil diet containing linseed, rapeseed and palm oil. The major histological difference of fish fed vegetable oil diets was the accumulation of lipid droplets in their livers. Dietary fatty acid composition significantly affected the fatty acid composition of peripheral blood leukocytes. Generally, fish fed vegetable oil diets had increased levels of oleic acid, linoleic acid and a-linolenic acid and decreased levels of eicosapentaenoic acid, docosahexaenoic acid and a lower n-3/n-6 ratio than fish fed a FO diet. In conclusion, the results from these studies suggest that farmed fish species can be cultured on diets containing vegetable oils as the added oil source. However, feeding high levels of some vegetable oils may significantly alter some immune responses in the fish, especially head kidney macrophage function, disease resistance and, in addition, may cause an increase in tissue histopathology.