Can liver histology provide valuable information on broodstock nutrition in marine fish species?

An appropriate management of the broodstock is necessary to successfully develop farming of marine species at a commercial scale. Considering the importance of nutrition in broodstock management, there has been a limited effort to develop a specific diet for Atlantic halibut. Now a day, with the recruitment of brood fish among the F1 and F2 generations, egg quality has been point out as a bottleneck by the juvenile producers and has intensified the need of an appropriate broodstock diet for this species. The large size and limited numbers of breeders limit the possibilities of conventional feeding experiments. Empirical observations indicate that brood fish maintained on diets containing above 20 % dietary lipids develop a hepatic steatosis and in consequence, most of the diets have dietary lipid content under this value. Yet, large PAS+ vacuoles were observed in the hepatocytes of farmed fish indicating important reserves of hepatic glycogen. The hepatocytes of wild caught fish presented macro and micro vacuoles of round and regular shape (lipid reserves) and limited amounts of PAS+ vacuoles in a well preserved cytoplasm. The aetiology of the hepatic steatosis in domesticated Atlantic halibut is poorly understood and might be the result of dietary fatty acid unbalance (18:1/22:6) as described by Spisni et al. 1998 in artificially fed sea bream and sea bass. Proximate analyses of liver composition together with liver histology of farmed fish fed a diet with low levels of monounsaturated fatty acids will give indication on hepatic steatosis is a consequence of fatty acid unbalance or if other factors like phospholipid content, carbohydrate content or total energy should be taken into consideration to develop a diet that preserves normal hepatic functions.