Food and habitat partitioning between young-of-year alewives and rainbow smelt in southeastern Lake Ontario

Synopsis Thermal and depth distributions, diets and time of feeding of young-of-year (YOY) alewives and YOY rainbow smelt were compared for evidence of resource partitioning in southeastern Lake Ontario. YOY alewives were largely epilimnial during August and September, but moved toward the bottom during fall turnover. Alewives were most abundant in the warmest available water. YOY rainbow smelt were concentrated at depths between 10 and 30 m in August and September, but moved into deeper water at fall turnover. Depth distribution of YOY smelt was correlated with temperatures of 8–12°C Both species fed predominantly during day on zooplankton during August and September. Cyclopoid copepods were the most common prey, but bosminids, eubosminids, and occasionally calanoid copepods were frequently eaten. As YOY rainbow smelt grew (> 60 mm), they consumed more Mysis relicta and amphipods, which became the major prey of rainbow smelt by November. YOY alewives consumed mostly zooplankton in all months. Diet overlap of the two species was greatest in warm water (> 12.0° during October (94.3% similarity) and August (80.0% similarity) and lowest in November (16.9% similarity). Positive size-selection on zooplankton was found in all months for YOY rainbow smelt, but only in late September through November for YOY alewives. Thus, during thermal stratification, the species were spatially segregated by water temperature but had a high degree of overlap in time of feeding and types of prey eaten. In contrast, after fall turnover there was a greater separation in diet but a higher overlap in habitat use.