Meeting Abstract
As migratory animals move between environments, changes in selection pressures may impede their chance of success. However, in mass migrations, individuals that encounter these pressures at different times might differ in performance, and pressures may change in intensity. How closely matched are migrator performance and selection intensity through time? The life cycle of the Hawaiian goby, Sicyopterus stimpsoni, requires juveniles to migrate from marine, larval habitats to upstream, freshwater adult habitats. During migration, these fish face two major selection pressures: piscivorous predators, and waterfalls that block upstream reaches. Both pressures might decrease in intensity through time, as predators might become satiated later in migrations, and the flash flood pulses that stimulate migrations should decrease in strength, reducing the flow against which fish must climb. We tested whether the earliest fish to arrive during migrations might have the highest performance, enabling them to succeed against pressures at their greatest intensity. To test these predictions, we collected migrating juveniles each day for days 2 through 7 post-flood, and conducted both escape performance and climbing trials each day to compare performance across fish entering the stream at different times in the migration event. Our results showed that escape performance remained uniformly high throughout our sampling period, but that climbing performance peaked 5 days after the flooding event before again declining. These data indicate that early migrants are not necessarily the strongest individuals, and could improve understanding of how diverse morphologies and behaviors are maintained in upstream populations despite the strong selection imposed on these fish.
