Food Limitation in Larval Fish: Ontogenetic Variation in Feeding Scope and Its Potential Effect on Survival

We used the radiated shanny Ulvaria subbifurcata as a model species to explore the relative gut fullness from hatch to metamorphosis of wild larvae, and compared these values with those of laboratory-reared larvae fed at maximum rates. Ingestion rates of most wild larvae were above starvation levels but below the maximum feeding levels of laboratory-reared larvae. Twenty-six percent of freshly-hatched wild larvae and 11% of large, pre-settlement wild larvae had insufficient food in their stomach to satisfy metabolic requirements. These results, taken on their own, are consistent with the much hypothesized increased foraging performance and survival of larger larvae relative to smaller conspecifics. However, feeding scope—defined as the difference between the minimum daily food required for maintenance and the maximum set by gut capacity—increased disproportionately with size during ontogeny. As a result, none of the larger (>500 μg C) wild larvae fed at saturating levels. We conclude that environmental factors operating to increase encounters between larvae and prey (e.g. prey densities, turbulence, patch encounter) and ingestion rates will affect smaller and larger larvae differently. As a consequence, increased encounter rates should lead to increased survival in smaller larvae, and (primarily) to increased growth in larger larvae.