Publication Abstract

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

M. Greer Walker, P.R. Witthames and I. Bautista De Los Santos

Current methods of stock assessment using plankton surveys andfecundity measurements are based on the assumption that the western mackerel (Scomberscombrus L.) has a determinate fecundity, that is, the number of oocytes destined to bematured in a season is decided prior to spawning and can be identified and counted beforerelease of the first egg batch. This assumption has been questioned in recent years andthe possibility of a serious miscalculation of fecundity has been raised. Evidencepresented here shows that:

A hiatus does not develop in the sizefrequency distribution between the previtellogenic and vitellogenic oocytes prior tospawning. The number of vitellogenic oocytes (potential annual fecundity) declines duringspawning. The agreement between the potential annual fecundity measured prior to spawningand the number of eggs shed during spawning calculated by estimating the size andfrequency of egg batches and the duration of the spawning season is somewhat ambiguous.The production of previtellogenic oocytes which replaces the potential annual fecundityoccurs during spawning and over a short time period and not over an extended time periodas is the case in fish with an indeterminate fecundity. Vitellogenic oocytes take about140-154 days to grow to full size. As the best estimates for the length of the spawningseason is 60-90 days it is unlikely that there would be sufficient time for any newvitellogenic oocytes to mature. The prevalence of atresia in the population varies between26 and 50 and the mean relative intensity between 4 and 12% during the spawningseason. The effect of these levels of atresia on the potential annual fecundity is toreduce it by 6-13% depending upon assumptions of the likely duration of alpha atreticoocytes in the ovary. Maximum levels of atresia occur three quarters of the way throughspawning and subsequently decline. This strategy would seem to be more characteristic of afish with a determinate fecundity. Fish with an indeterminate fecundity would be expectedto produce vitellogenic oocytes during spawning and accumulate large numbers of atreticoocytes at the end of the spawning season because there was insufficient time for them tomature.

It is concluded that for all practicalpurposes the mackerel should be considered as having a determinate fecundity.

Reference:

M. Greer Walker, P.R. Witthames and I. Bautista De Los Santos, 1994. Is thefecundity of the Atlantic mackerel (Scomber scombrus: Scombridae) determinate? Sarsia, 79:13-26.