Midwater Fish Assemblages and Seamounts

Meso- and bathypelagic fishes are conspicuous components of the 'deep scattering layers' (DSL) of the world oceans. These ichthyofauna interact with resident demersal fauna at seamounts in several ways: (1) horizontal impingement by non- or weak vertical migrators (mainly deep meso- and bathypelagic forms); (2) impingement of mesopelagic vertical migrators during migration, including topographic trapping; (3) adoption of a benthopelagic lifestyle over seamounts by large, adult pelagic fishes; and (4) adoption of a pseudoceanic lifestyle by species from primarily pelagic families, including endemism and active aggregation, the latter being corroborated by acoustic evidence. Bio-physical coupling mechanisms are highly variable and site dependent, but some physical oceanographic features such as Taylor caps may serve to retain pelagic populations over seamounts; the most important physical feature is water depth over the seamount summit. There is evidence that pelagic fauna provide a crucial trophic link in seamount ecosystems and may be responsible for the large biomass of demersal fishes found at these locations. There is little evidence for direct bottom-up enhancement, whereas two behaviour-based explanations are supported by field studies: (1) the 'food-rest' hypothesis that predatory fishes use the seamount to hold station (rest) while taking advantage of the horizontal advection of prey (food); and (2) the 'topographic trapping/interception' hypothesis that predatory fishes rely on seamounts to concentrate the density of vertically migrating pelagic prey. The exact mechanism may vary, but the overall effect is to convert mid-trophic level biomass (pelagic prey) to higher trophic level biomass (seamount-associated fishes) with increased efficiency.