Characterising the spawning patterns of Jack Mackerel (Trachurus declivis) off eastern Australia to optimise future survey design

Estimates of spawning biomass obtained using the daily egg production method (DEPM) are used to establish catch limits for Jack Mackerel (Trachurus declivis) off eastern Australia. Information from concurrent ichthyoplankton and adult surveys conducted between Port Stephens, New South Wales and South East Cape, Tasmania during January 2014 was used to assess the environmental factors that determine the spawning patterns of Jack Mackerel. Adults were collected using a modified demersal trawl net deployed during daylight hours. Samples were unbiased with respect to sex, spawning activity and size. Large fish were collected from both the inner shelf and shelf break; spawning fractions and egg densities were high inshore and low offshore. These findings suggest complex spatio-temporal patterns of spawning, different to previous studies suggesting that most spawning occurred at the shelf break (∼200 m). Eggs were most abundant in sea surface temperatures (SSTs) of 15–20 °C and at depths of <130 m. Future ichthyoplankton surveys should target waters with SSTs of 14–23 °C and depths of 30–250 m. Future adult surveys should sample the same range of depths and latitudes as the ichthyoplankton surveys and be structured as systematically as permitted by the availability of habitats suitable for demersal trawling. The DEPM does not provide information about the abundance of non-spawning adults outside the main spawning area. Extending future adult surveys beyond the spawning area would address this limitation by providing estimates of the distribution and relative abundance of adults across the entire range of the population. Findings of this study will help to improve the design of future DEPM surveys.