Spawning biomass of jack mackerel (Trachurus declivis) off eastern Australia: Critical knowledge for managing a controversial fishery

Jack mackerel (Trachurus declivis) is a key target species of the Australian Small Pelagic Fishery. The fishery was subject to extensive public scrutiny following an attempt to introduce a large factory-trawler in 2012. One of the concerns raised related to the reliability and age of estimates of the spawning biomass of jack mackerel. This study reports on the first dedicated application of the Daily Egg Production Method (DEPM) to jack mackerel. Large numbers of samples of eggs and adults were collected concurrently from the key spawning area off eastern Australia during what has been previously identified as the main spawning period. This study established an effective adult sampling method and provides the first estimates for this species of adult reproductive parameters required for application of the DEPM. Estimates of all DEPM parameters, except batch fecundity, were based on relatively large sample sizes. 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 t). Sensitivity analyses indicate this estimate is robust to likely uncertainty in estimates of key parameters. Future research should focus on elucidating the size-fecundity relationship and spatial, temporal and size-related variations in spawning fraction.