Evaluating the use of the Daily Egg Production Method for stock assessment of blue mackerel, Scomber australasicus

The present study evaluates the suitability of the Daily Egg Production Method (DEPM) for stock assessment of blue mackerel, Scomber australasicus and assesses methodological options for future applications. In southern Australia, estimates of mean daily egg production were higher for Californian Vertical Egg Tow(CalVET) nets than bongo nets, and in eastern Australia, were higher in October 2003 than July 2004. Estimates of spawning area for southern Australia were three times higher for bongo nets than CalVET nets. Similar estimates of spawning area were obtained using standard (manual) gridding and natural neighbour methods. Large samples and reliable estimates of all adult parameters were obtained for southern Australia. Relatively few spawning adults were collected off eastern Australia. Preliminary best estimates of spawning biomass for southern and eastern Australia were 56 228 t and 29 578 t, respectively, with most estimates within the ranges of 45 000-68 000 t and 20 000-40 000 t respectively. The DEPM is suitable for stock assessment of S. australasicus. Several technical refinements are required to enhance future applications, including: genetic techniques for identifying early stage eggs; a temperature-egg development key; improved methods for sampling adults off eastern Australia; and measurements of the degeneration rates of post-ovulatory follicles at several temperatures.