Publication Abstract

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

P.R. Witthames and M. Greer Walker

Changes in themorphology of the ovary are described during maturation and it is established that it is ahomogeneous structure for sampling purposes at maturity stages four and five. Severalaspects of vitellogenic oocyte growth and recruitement were examined as criteria toascertain whether the annual fecundity can be determined in Solea solea just prior tospawning. The first criterion was whether a hiatus develops in the size frequencydistribution 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 examinedfrom division VIIa (N = 5) and IVb west (N = 5). The hiates occurred between 175 and 250µm (cohort size 25 µm) in the size frequency distribution and measured between 25 and 75µm. Oocyte size frequency distribution from mid spawning and late spawning-spent solefrom division IVc showed a hiatus in 9 out of 10 fish which became wider as the residualannual potential fecundity declined. Next the rate of recruitment and growth ofvitellogenic oocytes was studied in sole sampled at monthly intervals (July 1993 toFebruary 1994) from division Vlld in relation to the start and duration of the annualspawning season. The growth rate of the leading oocyte cohort was 2.73 x 10^-5mm³ per day on 20 September and increased to 3.91 x 10^-3 at a meandiameter of 839 µm by 10 February just prior to spawning. However, the observed maximumgrowth rate, even when adjusted for the positive influence of temperature, allowed littlescope for previtellogenic oocytes to complete maturation during a spawning period of 60days. The recruitment of viltello,,enic oocytes was 3 013 oocytes per day on 3 August anddeclined rapidly as spawning approached. In the three months prior to spawning nosignificant increase was found in the annual potential fecundity. An analysis of a sampleof spawning fish from We demonstrated that 88% had less than 5070 of their predictedpotential fecundity remaining in the ovary. The mean batch fecundity was 8400 (SE ±13363) and in conjunction with existing data on spawning frequency and duration gave arealised fecundity close to the potential fecundity. It is concluded that for allpractical purposes the sole can be regarded as having a determinate fecundity in all theareas studied.

The regulation of potential fecundity byfollicular atresia was assessed in pre-spawning and spawning sole. In ICES areas IV andVII the prevalence of atretic oocytes in pre-spawning fish varied between 0.04 and 0.69and the relative intensity varied between 0.017 and 0.076. During spawning in division 1Vcthe prevalence of atresia varied between 0.45 and 0.57 and the relative intensity between0.02? and 0.058. If we assume a turnover rate of 9 days the loss from the potential annualfecundity, during spawning, in division IVc would be 12.4%.

Reference:

P.R. Witthames and M. Greer Walker, 1995. Determinacy of fecundity and atresia in sole(Solea solea) from the Channel, the North Sea and the Irish Sea. Aquatic Living Resources,8(1): 91-109.