Influence of turbidity, food density and parasites on the ingestion and growth of larval rainbow smelt Osmerus mordax in an estuarine turbidity maximum

We investigated the impact of turbidity, food density and parasites on ingestion and growth rates of rainbow smelt larvae Osmerus mordax. These 3 variables were selected because of their potential to substantially influence the feeding success, growth, and the subsequent survival of smelt larvae. A laboratory experiment was first performed to evaluate, in turbulent conditions, the combined effects of turbidity and food density on the ingestion and growth rates of smelt larvae. A field survey of the gut contents of larval smelt was conducted to directly estimate ingestion rates in 2 different regions of the St. Lawrence estuarine turbidity maximum (ETM) exhibiting different levels of turbidity but otherwise sharing similar environmental conditions. This study demonstrated that lower energetic costs are incurred by larvae that exploit similar feeding conditions at higher turbidities. Larval rainbow smelt in the ETM fed during the coincidence of daylight hours and flooding tide. Cestode parasites (genus Protocephalus) were found in the digestive tract of 38% of the larvae collected in the ETM. Parasitised larvae ingested half as much food as non-parasitised larvae. The decrease in feeding due to parasitism was associated with a reduced growth rate as-suggested by the significantly lower standard lengths observed in parasitised larvae. Moreover, the size advantage of non-parasitised larvae is expected to be amplified because larger larvae ingest proportionally more food than smaller larvae. We suggest that the impact of parasitism on larval survival and subsequent recruitment in fishes merits far more attention than afforded to date.