Growth and population dynamics of three Raja species (Batoidei) in Carmarthen Bay, British Isles

Abstract

Growth and population dynamics have been studied in a mixed population ofRaja clavata, R. microocellata, and R. montagui in Carmarthen Bay, South Wales, from 1974 to 1976. All, but especially the first two species, are fished commercially. An improved method for displaying the annuli in vertebrae is described, and all the data obtained are based on fish of established age. The maturation stages of the ovary and a previously unreported seasonal cycle of development and regression of mixopterygia (claspers) are described. For R. clavata, R. microocellata, and R. montagui respectively the maximum age and mean length at that age of trawled fish was 12 years and 1047 mm, 9 years and 875 mm, and 7 years and 710 mm, but older, larger fish were caught by anglers. The values of the von Bertalanffy parameters L ∞ and K were 1392 mm and 0·090, 1370 mm and 0·086, and 978 mm and 0·152 respectively. Females grew larger than males. First spawning in all three species occurs mainly in the fifth year. Total mortality based on research vessel catches for the three species was estimated as Z = 0·45, 0·54, and 0·46 respectively, but differences were found between the sexes, Z always being higher in males. For females and males respectively, estimates were Z = 0·27 and 0·56, 0·38 and 0·56, and 0·48 and 0·72. Using the multiple regression devised by Pauly (1980), natural mortality was estimated as M = 0·16 in R. clavata and M = 0·14 in R. microocellata, hence F = 0·29 and 0·40 respectively. For the smaller and less exploited R. montagui, M = 0·25 and F = 0·21. Fecundity in R. clavata and R. microocellata is probably less than 100 eggs annually, the counts indicating minima of 62 to 74 and 54 to 61 eggs respectively. Most of the rays in Carmarthen Bay are young, with fish of age groups 0 to 2 comprising 87 %, 71 %, and 92 % of the analysed catch of R. clavata, R. microocellata, and R. montagui respectively. Rays in the oldest age group of the three species represented by other than an occasional fish had mean measurements of 68 %, 64 %, and 73 % of their L ∞ and estimated ages corresponding to 0·7 L ∞ are 16·0, 17·0, and 9·6 years respectively. The findings are discussed and it is concluded that, while the stock is probably depleted, growth being insufficient to support the present level of exploitation, there should be no danger of recruitment failure so long as Z remains less than about 0·7. © 1984 Conseil International pour l'Exploration de la Mer.