Is the fecundity of the Atlantic mackerel (Scomber scombrus: Scombridae) determinate?

Current methods of stock assessment using plankton surveys and fecundity measurements are based on the assumption that the western mackerel (Scomber scombrus L.) has a determinate fecundity, that is, the number of oocytes destined to be matured in a season is decided prior to spawning and can be identified and counted before release of the first egg batch. This assumption has been questioned in recent years and the possibility of a serious miscalculation of fecundity has been raised. Evidence presented here shows that: A hiatus does not develop in the size frequency distribution between the previtellogenic and vitellogenic oocytes prior to spawning. The number of vitellogenic oocytes (potential annual fecundity) declines during spawning. The agreement between the potential annual fecundity measured prior to spawning and the number of eggs shed during spawning calculated by estimating the size and frequency of egg batches and the duration of the spawning season is somewhat ambiguous. The production of previtellogenic oocytes which replaces the potential annual fecundity occurs during spawning and over a short time period and not over an extended time period as is the case in fish with an indeterminate fecundity. Vitellogenic oocytes take about 140–154 days to grow to full size. As the best estimates for the length of the spawning season is 60–90 days it is unlikely that there would be sufficient time for any new vitellogenic oocytes to mature. The prevalence of atresia in the population varies between 26 and 50 % and the mean relative intensity between 4 and 12 % during the spawning season. The effect of these levels of atresia on the potential annual fecundity is to reduce it by 6–13 % depending upon assumptions of the likely duration of alpha atretic oocytes in the ovary. Maximum levels of atresia occur three quarters of the way through spawning and subsequently decline. This strategy would seem to be more characteristic of a fish with a determinate fecundity. Fish with an indeterminate fecundity would be expected to produce vitellogenic oocytes during spawning and accumulate large numbers of atretic oocytes at the end of the spawning season because there was insufficient time for them to mature. It is concluded that for all practical purposes the mackerel should be considered as having a determinate fecundity.