Role of early life-history constraints and resource polymorphism in the segregation of sympatric populations of an estuarine fish

In marine ecosystems, evolutionary studies of reduced gene flow among fish populations in the absence of strong physical barriers have tended to focus on mechanisms to increase survival of the critical early life-history stages. In contrast, ecological resource partitioning among later life-history stages has received considerable attention in studies of freshwater ecosystems. Few studies have considered both mechanisms simultaneously and none have documented their influence on sympatric co-existence in an estuarine environment. Despite the fact that they are highly mobile and exploit a dispersive environment, the two sympatric populations of rainbow smelt (Osmerus mordax) found in the St. Lawrence Estuary, Canada, appear to be spatially segregated along the north (north-shore population) and south (south-shore population) shores of the estuary. The existence of population-specific spawning sites and times as well as nursery areas would indicate selective pressures acting on the early life-history stages. If resource partitioning contributes to segregation, we would also expect to identify population-specific morphological adaptations correlated with distinct feeding niches. Spawning of the two populations was temporally and spatially segregated. The north-shore population spawns approximately 2 weeks before the south-shore population directly on shallow shoals in the fluvial estuary, whereas the south-shore population spawns in small tributaries along the south shore of the middle estuary. Larval populations are also clearly segregated among discrete nursery habitats, representing the first documented case of larval segregation between sympatric populations of a fish with a pelagic larval stage. Larvae of the north-shore population exploited all channel habitats within the estuarine turbidity maximum and to a minor degree adjacent shoals and small bays. In contrast, larvae of the south-shore population were largely confined to the south shore shoals and large shallow bays at the downstream limit of the estuarine turbidity maximum. Adult smelt populations are morphologically distinct and reflect a benthic/pelagic dichotomy in morphology. However, the phenotype–habitat association is not paralleled by the expected benthivorous/planktivorous dichotomy in diet and feeding apparatus morphology. These population-specific adaptations may serve to maximize foraging efficiency within distinct environmental settings (deep channels vs shoals) rather than favouring

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