Nitrogen balance in marine fish larvae: influence of developmental stage and prey density

The utilization and fate of nitrogen in larvae of plaice (Pleuronectes platessa), blenny (Blennius pavo) and herring (Clupea harengus), from the stage of first-feeding to metamorphosis, was examined under laboratory conditions. Rates of ammonia excretion, primary amine defaecation, and growth in terms of protein-nitrogen were monitored throughout larval life. Data were used to calculate daily ration, the coefficient of nitrogen utilization (absorption efficiency), and gross and net growth efficiencies. The developmental pattern of nitrogen balance was similar for plaice and blenny larvae. These species showed increasing growth efficiency (k₁: 55 to 80%) with decreasing weight-specific waste nitrogen losses with age. Absorption efficiencies. were high (83 to 98%) in plaice and blenny larvae, and tended to increase with development in the former species. Ration relative to body weight decreased with growth in both species. Herring larval development, although at a slower rate than blenny and plaice, appeared normal up to 33 d, after which high mortality occurred. Absorption efficiency in this species tended to decline (83 to 43%) with age, until metabolic costs exceeded the absorbed ration and growth ceased. Artemia sp. nauplii proved a suitable food source for the rearing of plaice and blenny larvae, but this diet may have long-term toxicity or deficiency effects on herring. Availability and density of food affected nitrogen balance in the larvae of all three species. Feeding stimulated the output of wastes in excretion and defaecation by a factor of up to ten times the 12-h non-feeding basal rates. Waste nitrogen output reached a peak some 2 to 3 h after commencement of feeding and returned slowly to the baseline in 5 to 10 h after cessation of feeding. There was an asymptotic increase in ration, ammonia output and growth of larvae as prey density increased. Ration saturated at a higher prey density (>4 prey ml^-1) than either growth or excretion rate (1 prey ml^-1). Thus the efficiency with which food is absorbed and utilized for growth must eventually decline in response to high prey density. The idea that larval fish are adapted to maximize ingestion and growth rate, rather than optimize growth efficiency and thus to respond to prey occurring in either low density or in occasional patches, is supported by these results.