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 inge stion and growth rates of rainbow smelt larvae Osmerus mordax. These 3 vari ables were selected because of their potential to substantially influence t he feeding success, growth, and the subsequent survival of smelt larvae. A laboratory experiment was first performed to evaluate, in turbulent conditi ons, the combined effects of turbidity and food density on the ingestion an d growth rates of smelt larvae. A field survey of the gut contents of larva l smelt was conducted to directly estimate ingestion rates in 2 different r egions of the St. Lawrence estuarine turbidity maximum (ETM) exhibiting dif ferent levels of turbidity but otherwise sharing similar environmental cond itions. This study demonstrated that lower energetic costs are incurred by larvae that exploit similar feeding conditions at higher turbidities. Larva l rainbow smelt in the ETM fed during the coincidence of daylight hours and flooding tide. Cestode parasites (genus Protocephalus) were found in the d igestive tract of 38% of the larvae collected in the ETM. Parasitised larva e ingested half as much food as non-parasitised larvae. The decrease in fee ding due to parasitism was associated with a reduced growth rate as-suggest ed by the significantly lower standard lengths observed in parasitised larv ae. Moreover, the size advantage of non-parasitised larvae is expected to b e amplified because larger larvae ingest proportionally more food than smal ler larvae. We suggest that the impact of parasitism on larval survival and subsequent recruitment in fishes merits far more attention than afforded t o date.