S.K. Tonheim

In vivo studies of digestion and nutrient assimilation in marine fish larvae

Abstract This paper describes a method for quantifying the functionality of the digestive system in fish larvae. The system described can provide data for the gut absorption, oxidation and retention (assimilation) of nutrients. A tube-feeding setup (originally described in Aquaculture 116 (1993) (341–352) using 14 C-labelled dietary nutrients formed the basis of a new incubation system. This permitted unabsorbed nutrients evacuated from the gut to be distinguished from molecules originating from catabolism of the absorbed nutrient, both of which are present in the incubation water. The system is based on the release, transfer and entrapment of metabolically produced 14 C–CO 2 through manipulation of the water pH. The efficiency of the trap has been validated and tested, and provides 100.0±1.3% (S.D.) recovery. The usefulness of the method is demonstrated in a study in which Atlantic halibut post-larvae (46 days post first feeding) were fed a 14 C-labelled protein diet. These data show that this protein has a digestibility of 42% for halibut post-larvae. If oxidation had not been measured through the use of the CO 2 trap, digestibility would have been greatly underestimated (at about 25%).

Metabolic budgets for lysine and glutamate in unfed herring (Clupea harengus) larvae

Abstract Fish larvae are thought to have little control of their amino acid (AA) metabolism, leading to higher AA requirements than adult fish and mammals. Therefore, it is important to know to what extent fish larvae have the capacity to spare indispensable amino acids (IAA) at the expense of dispensable amino acids (DAA). This study intended to estimate a metabolic budget both for an IAA and a DAA in fasting herring larvae. A mix of crystalline amino acids solubilised in 1/3 seawater and added as either l - 14 C-glutamate or l - 14 C-lysine, was tube-fed to first feeding and 47-day-old (pre-metamorphosis) herring larvae. Both when glutamate and lysine were given in high and low doses, the following components of the metabolic budget were estimated: total AA assimilation, retention of AA in the free AA and in the protein pools, AA conversion into lipid and AA oxidation. First feeding herring larvae absorbed around 93% of both lysine and glutamate. However, 23% of the tube-fed lysine was oxidised and 70% was retained in the body, while for glutamate oxidation was as high as 76% and retention is only 17%. A similar picture was observed for 47-day-old larvae. Here, oxidation was 22% and 62% and retention 63% and 32%, for lysine and glutamate, respectively. Therefore, herring larvae use glutamate preferentially to lysine as an energy substrate from first feeding onwards. These results suggest that fish larvae have better capacity of regulating AA catabolism than previously believed.

METABOLIC BUDGET OF AN ESSENTIAL AND A NON-ESSENTIAL DIETARY AMINO ACID IN HERRING (CLUPEA HARENGUS) LARVAE

In comparison to mammals, juvenile and adult fish have a higher amino acid (AA) requirement and a lower adaptability of AA metabolism. This means that fish are more sensitive to diets with low protein levels or with an imbalanced AA profile. Fish larvae are thought to have even less control of their AA metabolism, leading to higher catabolic losses of AA, and thereby to higher AA requirements (Dabrowski 1986). Therefore it is important to know to what extent fish larvae have the ‘ydpacity to spare essential AA (EAA) at the expenses of non-essential AA (NEAA). ‘This study intended to estimate a metabolic budget both for an EAA and a NEAA, in herring (Clupen harengus) larvae. A nlix of crystalline amino acids containing either ‘“C-Glutamate or 14C-Lysine was tube-feed to first feeding and 47 days-old Ipre-metamorphosis) in herring larvae. The following components of the metabolic budget were estimated: AA digestive absorption, AA conversion into lipidic molecules, AA oxidation, and conservation of AA in the FAA and in the protein pools. l-‘lrst feeding herring larvae absorbed around 92% of both lysine and glutamate. However, 23 % of the tube-feed lysine was ouidised and 69% was retained while for glutamate oxidation was as high as 76% and retention only 16%. A similar picture was observed for 47 days-old larvae. Here oxidation was 22% and 62% and retention 63% and 32%, for lysine and glutamate, respectively. Therefore, herring larvae use glutamate preferentially to lysine for energy production from first feeding onwards. These results suggest that fish larvae may have a better capacity of regulating AA catabolism than previously believed (Concei@o et al. 1997, 1998). Conce@o, L.E.C. et al. (I 997) Amino acid metabolism and protein turnover in larval turbot (Scophthalmus mmimus) fed natural zooplankton or Artemia. Mar. Biol.29. 25526.5. Concei@o, L.E.C et a1.(1998). Amino acid profiles and amino acid utilisation in larval African catfish (Clurias gariepinus): effects of ontogeny and temperature. Fish Physiol. Biochem 19: 43-47. Dabrowski, K.R. (1986) Comp. Biochem. Physiol. &5J: 639-655.