Temporal and spatial patterns of spawning in jack mackerel, Trachurus declivis (Pisces:Carangidae), during 1988-91 in eastern Tasmanian waters

Abstract

Ichthyoplankton surveys conducted during the summer and autumn of 1988-89, 1989-90 and 1990-91 along the eastern coast of Tasmania were used to examine the spatial and temporal patterns of eggs and larvae of jack mackerel, Trachurus declivis. Results indicate that the species spawns along the entire eastern coast during summer. Trachurus declivis eggs were most abundant at shelf-break stations, indicating that spawning is concentrated in this region, although high concentrations of eggs were present on the inner shelf in 1988-89, which can be attributed to rapid onshore transport in that year. Larvae were evenly dispersed over the shelf, with the distribution of larval ages showing no indication of inshore recruitment. Considerable interannual differences in sea surface temperatures and vertical thermal structure were apparent, with the warmer waters and strong thermal stratification in the summer of 1988-89 resulting from the influx of subtropical East Australian Current (EAC) water onto the shelf, which corresponded with a major La Nina 'cold event' at that time. It is suggested that the distribution of spawning is unaffected by the interannual variations in oceanography as the mature population spawns in deeper water in the shelf-break region that is unaffected by the warming in surface waters.