Substitution of live food by formulated diets in marine fish larvae

Digestive enzymes in fish larvae and juveniles—implications and applications to formulated diets

Publisher Summary This chapter defines important considerations in the use of condition indices as applied to marine fish larvae, and describes the present state of knowledge concerning each index. An appraisal of their relative merits is then attempted by comparing the strengths and limitations of each category. Measures of condition integrate feeding success over time, and give an indication of the probability of starvation and survival. This chapter explores that, the central purpose of measuring condition is to predict reliably survival probabilities under given food regimes in order that observed larval abundances can be used as early predictors of recruitment. The chapter illustrates that, future research on condition indices is likely to be more productive if it is devoted to the refinement of existing methods, rather than to search for the ideal condition index. The chapter concludes by providing suggestions for the use of condition indices in different situations and identifies topics requiring further research.

An Appraisal of Condition Measures for Marine Fish Larvae

Prospects in larval fish dietetics

Due to the importance of dietary lipid utilization for larval rearing success, increasing attention has been paid during the last years to different aspects of larval lipid nutrition such as digestion, absorption, transport and metabolism, which are frequently studied by different research groups. The present study reviews the published information on these aspects, including some recent results obtained in our laboratory, that contribute to a better understanding of larval lipid nutrition. Neutral lipase activity was found in the digesta of larval gilthead seabream as early as first feeding, followed by a significant increase which reached up 8 times the initial levels at day 15 and was clearly influenced by the fatty acid composition of dietary lipids. Accordingly, the capacity for lipid absorption by the intestinal epithelium has been also observed at the onset of exogenous feeding, although the specific location in the different digestive tract segments differ with species. Whereas the capacity to absorb lipid increases with development in live prey-fed larvae, this improvemment is delayed in larvae fed formulated diet. Increasing dietary phosphatidyl cholines levels enhanced lipid absorption regardless of whether it is of soybean or marine origin, but the latter improved hepatic lipid utilization. Enzymatic, histological and biochemical evidences suggest that marine fish larvae are able to effectively digest and absorb n-3 HUFA-rich triacylglycerols, but feeding with phosphoacylglycerols, particularly if they are rich in n-3 HUFA, would enhance phosphoacylglycerols digestion and specially lipid transport alowing a better n-3 HUFA incorporation into larval membrane lipids and promoting fish growth. Although the essentiality of n-3 HUFA for larval marine fish has been studied extensively, only recently has the importance of dietary arachidonic acid in the larvae of few species been recognised. Evidences for competitive interactions among these essential fatty acids suggest that besides a minimum dietary requirement for each essential fatty acid, their relative ratios must also be considered.

Recent advances in lipid nutrition in fish larvae.

The presence and localization of acid and alkaline phosphatase, non-specific proteases, aminopeptidase, amylase, non-specific esterase and lipase was investigated by histoenzymologic methods in fed and fasting turbot from day 1 to day 40 post-hatching and compared with published data. Alkaline phosphatase and aminopeptidase activities were delected at day 1 in the distal region of the developing digestive tube. At day 3 (opening of the mouth) aminopeptidase and alkaline phosphatase activities were found all along the intestine. Sites of non-specific esterase and protease activities became apparent in the digestive tract at days 2 and 3 respectively. Amylase was present in the exocrine pancreas at day 3 and in the lumen of the intestine at day 4. Acid phosphatase was active in the cellular structure surrounding the yolk stores and in the lipid droplets at day 1 and in the intestinal epithelium at day 3. Lipase was found at day 15 when the larvae metamorphose into juveniles.



All the investigated enzymes were detected in fasting animals, except for lipase. However, the intensities of the enzymatic activities were weaker in the fasting specimens relative to the fed specimens between days 7 and 10.

J.C.B. Cousin

Papers

Ontogeny of enzymatic activities in fed and fasting turbot, Scophthalmus maximus L.