The fecundity of Raja clavata in British waters

Abstract

The annual breeding cycle of Raja clavata in British waters is described. The ovarian cycle starts in December with the appearance of small, white ovarian eggs. Copulatory activity, based on the evidence of clasper wounds in the cloaca of one female, starts in February and reaches a peak in May-June, based on the occurrence of spermatophores in the nidamental glands. Egg laying starts in February, reaches a peak rate in June and ceases by September. Two methods of estimating the annual egg production are described, one based on the proportion of mature female rays with egg capsules and observed rates of egg laying and the other based on the decline in the number of ovarian eggs. Although the latter method is only valid for the period June to September, for these months it gives similar results to those of the alternative method. The estimated average production is 140 eggs a year. The number of ovarian eggs present in April and May equals the number of eggs laid after 1 June. Taking account of egg production before 1 June gives the relationship between annual egg production (y) and total maternal length in centimetres (x) as y = 1.19.x + 25.1 over the length range 77 to 105 cm. Although there is evidence that the duration of the breeding season may vary both annually and geographically, the limited data show that variations in annual egg production from year to year are small. It is suggested that length of daylight is the main factor controlling the ovarian cycle and that temperature controls the rate of egg laying. The length/fecundity relationship is linear and the reasons why it does not follow the more usual cubic relationship are discussed.