Do Thermoclines Explain the Vertical Distributions of Larval Fishes in the Dynamic Coastal Waters of South-eastern Australia?

Abstract

This study examined whether the position and strength of the thermocline influenced the vertical distributions of larval fishes in the coastal waters off Sydney. Depth-stratified sampling of larval assemblages was done in stratified and non-stratified conditions. Larval assemblages were depth- stratified, and individual taxa displayed specific depth distributions in all types of water columns. In stratified and non-stratified water columns some species were characteristic of shallow, mid and deep depth strata. The presence of a thermocline did not appear to influence broad-scale patterns of vertical distribution of larval fishes, and it was not considered a good predictor of larval vertical distribution. Larval behaviour appeared to be the major influence determining vertical distributions of larval fishes. It is argued that thermoclines may not be important to larval fishes in waters characterized by dynamic oceanography, such as those off Sydney, where the position and integrity of the thermocline can vary greatly over small temporal and spatial scales. Greater emphasis needs to be placed on teasing out the importance of various interacting biotic and abiotic features of the water column in structuring vertical distributions of larval fishes.