Distribution of Larval and Juvenile Fish in the Western Irish Sea: Relationship to Phytoplankton, Zooplankton Biomass and Reccurrent Physical Features

Abstract

In the western Irish Sea, differences in tidal currents and water depth create seasonal hydrographic regions. The summer stratified region has an associated cyclonic gyre of near-surface water. Each region has a distinct seasonal cycle of phytoplankton production, and the coastal region is one of the most important fish spawning areas in the Irish Sea. The abundances of larval fish and newly metamorphosed pelagic juveniles were negatively and positively correlated with depth, respectively, suggesting that fish that spawn in the coastal region do not spend all of their early life stages inshore. The distribution of larvae and 0-group pelagic fish reflects the close coupling between recurrent hydrographic features and biological production. The coastal region supports early and sustained primary production and generally a higher biomass of zooplankton in early spring than the offshore mixed and stratified regions. Later in the year, pelagic 0-group fish are associated with a greater biomass of zooplankton in the summer stratified region. The movement of pelagic 0-group fish into the summer stratified region may result from the entrainment of a southerly flow of coastal water into the western Irish Sea gyre. The seasonal gyre may physically retain these pelagic fish within the western Irish Sea during the summer.