Historical daytime vertical structure of larval fish assemblages in southeast Australian coastal waters: A benchmark for examining regional ecosystem change

Abstract

Historical data are often used as benchmarks or reference points for assessing regional changes in ecosystem structure and function. For that purpose, we provide historical (1991–1992) data concerning the spatial and temporal consistency of vertical structuring of a diverse assemblage of larval fishes in inner continental shelf waters adjacent to Sydney, Australia. Daytime vertically stratified sampling at seven depths (0–65 m) across four stations and five sampling seasons yielded 35,772 individuals of 94 identifiable taxa from 81 families. Assemblages displayed consistent vertical stratification between surface (0 and 5 m) and subsurface (15 m and deeper) waters, with further spatio-temporal sub-structuring of subsurface assemblages between upper (15–30 m) and lower (45 m and deeper) water column. Differences in assemblage structure between surface and subsurface waters were primarily driven by several species that predominantly occurred in one depth zone and not the other. In contrast, differentiation between subsurface assemblages was dynamic and driven by taxa common across upper and lower subsurface depths but occurring in differing densities in certain depth strata that was spatially and temporally variable and not related to thermal stratification of the water column. Despite significant small-scale spatio-temporal variability, larval taxonomic diversity and total abundance was most often greatest in the upper and mid water column (15–30 m), potentially a response to light levels and prey concentrations. Nevertheless, the data show that all depths in the water column provide important habitat for larval fishes that need to be considered in ecosystem functioning and climate change projections.