Knowledge of the summer spawning patterns of Jack Mackerel and Australian Sardine is needed to underpin future assessment of these stocks and to underpin the ecologically sustainable development of pelagic fish resources off the East Coast of Australia.
Methods for estimating the population size of Jack Mackerel and Australian Sardine need be established to address community concerns regarding the potential ecological and social impacts of large scale fishing for small pelagic fishes off the East Coast.
This study was undertaken collaboratively by fisheries scientists from the South Australian Research and Development Institute (SARDI) and the University of Tasmania. It was the first dedicated application of the Daily Egg Production Method (DEPM) to Jack Mackerel, Trachurus declivis. It successfully collected large numbers of samples of eggs and adults concurrently from the key spawning area off eastern Australia during what has been previously identified as the main spawning period. The study established an effective method for sampling adult Jack Mackerel and provides the first estimates for this species of the adult reproductive parameters required for application of the DEPM. The spawning biomass of Jack Mackerel off eastern Australia during January 2014 was estimated to be approximately 157,805 t (95% CI = 59,570 358,731). Most of the estimates of spawning biomass obtained in sensitivity analyses were between approximately 95,000 t and 215,000 t. Plausible values for only two parameters provide estimates of spawning biomass that were outside that range; both of these parameters were estimated with a high degree of confidence in the present study.
This was also the first study to investigate the spawning habitat of Australian Sardine Sardinops sagax off eastern Australia during summer. It showed that during January 2014 spawning occurred between northern Tasmania and southern Victoria. The spawning biomass at this location during this period was approximately 10,962 t. This estimate should be treated with caution as adult samples were not collected during the study. It also is important to note that this not an estimate of the total adult biomass of Australian Sardine off eastern Australia. It is only an estimate of the portion of the population that was spawning in this southern part of the range during that period. The main spawning area of Australian Sardine off eastern Australia occurs off southern Queensland and northern NSW during late winter and early spring.
Keywords: Jack Mackerel, Trachurus declivis, Australian Sardine, Sardinops sagax, Daily Egg Production Method, Spawning Biomass, Small Pelagic Fishery, eastern Australia, Tasmania, Bass Strait.