Estimating larval fish ingestion rates: can laboratory derived values be reliably extrapolated to the wild?'

We used literature data to compare the relative influences of body size, water temperature, food density and experimental method on laboratory derived estimates of the ingestion rates of marine fish larvae. We subsequently used these results to evaluate whether larval feeding rates in nature are likely to be food-limited. Larval dry weight, temperature and food density explained 85 % of the variance in the laboratory derived ingestion rates of 11 species. After removing the effects of larval size and water temperature on ingestion rates, larval functional response was steepest at food densities < 185 pg 1 l ; beyond this level, ingestion rates were independent of food density. A conlparison of the laboratory functional response with natural microzooplankton densities shows that (1) larvae are unlikely to feed at maximal rates in the sea; (2) larval feeding rates are most sensitive to changes in food abundance across the range of food densities that are most likely to occur in nature (median = 31 pg 1-'; 90th percentile = 148 pg I-'; N = 46). However, in situ ingestion rate estimates for 8 species of marine fish larvae indicate that these larvae fed at rates independent of food density and near-maximally, despite relatively low food densities. We conclude that this difference between in situ and laboratory estimates of ingestion rates as a function of prey density results primarily from the failure of most integrated census estimates of prey density to adequately represent the real contact rate of larvae with their prey and the failure of most laboratory experimental designs to incorporate relevant variables known to influence prey encounter rates and selection. Integrated measures of in situ prey abundance and most experimental designs fail to account for small-scale temporal and spatial patchiness in the dstributions of larvae and their prey, for various aspects of larval behaviour, and for water-column turbulence.