Ichthyoplankton Assemblages at Seamounts and Oceanic Islands

Seamounts and oceanic islands support demersal or neritic fish populations and are localized sources of eggs and larvae added to a background of oceanic ichthyoplankton. Associated larval fish assemblages, however, have been identified in few studies, and most have been based on subjective classifications of species by reproductive characteristics. At seamounts, little evidence exists that ichthyoplankton assemblages differ from the background field despite physical mechanisms proposed to maintain planktonic forms. Larvae of bottom-associated species are notably rare in samples taken in these regions. In contrast, characteristic ichthyoplankton assemblages have been described at oceanic islands. Unique assemblages may exist in embayments and lagoons, but they are not well described. Nearshore assemblages (0 to 0.5 km offshore) are dominated by larvae of small species with demersal eggs; neritic assemblages (0.5 to 5 km offshore) include inshore species with demersal and planktonic eggs mixed with larvae of certain species that are usually oceanic as adults. Although larvae of some inshore species with pelagic eggs are found offshore in oceanic assemblages (beyond 3 to 5 km), assemblages there are poorly described. Spatial and seasonal spawning behavior of adults plays the key role in formation of ichthyoplankton assemblages. Mechanisms that may maintain ichthyoplankton assemblages at islands include boundary layers, small scale frontal dynamics, tidal currents, topographically produced eddies, seasonally reduced or variable currents, and regions of no or returning flow. Behavior of larvae, particularly that affecting vertical distribution, can modify the influences of these mechanisms. The integrity of assemblages can be disrupted by both biotic and abiotic factors.