Naturally-occurring stable isotopes as direct measures of larval feeding efficiency, nutrient incorporation and turnover

Stable isotopes are non-hazardous markers that have been widely-used in assessing energy flow within aquatic ecosystems. Hatchery systems are also highly amenable to this approach, as they represent controlled mesocosms with a limited number of food sources and short planktonic food chains with rapid and measurable bioaccumulation of the heavier stable isotopes of carbon and nitrogen at each trophic step. Differences in the natural isotopic composition of dietary components may be used to provide direct integrated measures of ingestion, nutrient incorporation and growth through development under normal feeding and environmental conditions, in either the laboratory or the hatchery. Simple isotopic mixing models allow estimation of relative utilisation of inert diets and live feeds, and individual components of compound feeds. Such experiments have investigated the effectiveness of co-feeding regimes, optimal timing of live food transitions (eg from rotifers to Artemia), presentation of inert diets, optimal size/age for weaning and incorporation of specific dietary components. Furthermore, time-series measurement of changes in tissue isotopic signature (δ15N, δ13C) enables modelling of growth dilution and tissue turnover components of isotopic change driven by nutritional sources. These measures need to take into account the difference in isotope values that is typically observed between the diet and consumer (isotopic discrimination factor, Δ). In marine larvae and early post-larvae, Δ13C and Δ15N have been found to range widely, from 0.4-4.1‰ and 0.1-5.3‰ respectively. The observation of such a high level of variation within species and life stages indicates a strong effect of diet quality on isotopic discrimination. Elucidating mechanisms underlying such observations, and much greater resolution in larval nutritional studies, can be achieved by application of rapidly-developing techniques for compound specific stable isotope analysis in tracing the transfer of dietary sources of carbon and nitrogen into tissue components. Fast growing aquatic larvae represent excellent model organisms exhibiting rapid transitions in isotopic composition in response to diet, rapidly changing feeding behaviour and transitions in trophic level with ready ingestion of modifiable experimental diets in short and controlled food chains. Thus results of studies of the effects of diet composition, developmental stage, growth rates or environmental conditions on stable isotope incorporation will be of broad relevance not only in terms of larval nutrition but can also more broadly inform the design and interpretation of ecological studies.