Determinacy of fecundity and oocyte atresia in sole (Solea solea) from the Channel, the North Sea and the Irish Sea

Abstract

Changes in the morphology of the ovary are described during maturation and it is established that it is a homogeneous structure for sampling purposes at maturity stages four and five. Several aspects of vitellogenic oocyte growth and recruitement were examined as criteria to ascertain whether the annual fecundity can be determined in Solea solea just prior to spawning. The first criterion was whether a hiatus develops in the size frequency distribution between the previtellogenic and vitellogenic oocytes just prior to spawning. We were unable to demonstrate this feature in sole from division VIId (N = 5) or IVc (N = 5) but it was present in two out of five fish from division VIIe and all fish examined from division VIIa (N = 5) and IVb west (N = 5). The hiates occurred between 175 and 250 µm (cohort size 25 m) in the size frequency distribution and measured between 25 and 75 µm. Oocyte size frequency distribution from mid spawning and late spawning-spent sole from division IVc showed a hiatus in 9 out of 10 fish which became wider as the residual annual potential fecundity declined. Next the rate of recruitment and growth of vitellogenic oocytes was studied in sole sampled at monthly intervals (July 1993 to February 1994) from division VIId in relation to the start and duration of the annual spawning season. The growth rate of the leading oocyte cohort was 2.73 × 10⁻⁵ mm³ per day on 20 September and increased to 3.91 × 10⁻³ at a mean diameter of 839 µm by 10 February just prior to spawning. However, the observed maximum growth rate, even when adjusted for the positive influence of temperature, allowed little scope for previtellogenic oocytes to complete maturation during a spawning period of 60 days. The recruitment of viltellogenic oocytes was 3013 oocytes per day on 3 August and declined rapidly as spawning approached. In the three months prior to spawning no significant increase was found in the annual potential fecundity. An analysis of a sample of spawning fish from IVc demonstrated that 88% had less than 50% of their predicted potential fecundity remaining in the ovary. The mean batch fecundity was 8 400 (SE ± 13 363) and in conjunction with existing data on spawning frequency and duration gave a realised fecundity close to the potential fecundity. It is concluded that for all practical purposes the sole can be regarded as having a determinate fecundity in all the areas studied.

The regulation of potential fecundity by follicular atresia was assessed in pre-spawning and spawning sole. In ICES areas IV and VII the prevalence of atretic oocytes in pre-spawning fish varied between 0.04 and 0.69 and the relative intensity varied between 0.017 and 0.076. During spawning in division IVc the prevalence of atresia varied between 0.45 and 0.57 and the relative intensity between 0.022 and 0.058. If we assume a turnover rate of 9 days the loss from the potential annual fecundity, during spawning, in division IVc would be 12.4%.