Growth and population dynamics of 3 raja species batoidei in carmarthen bay british isles

Ryland J.S.; Ajayi T.O., 1984: Growth and population dynamics of 3 raja species batoidei in carmarthen bay british isles. Journal du Conseil Conseil International pour l'Exploration de la Mer 41(2): 111-120

Growth and population dynamics of a mixed population of R. clavata, R. microocellata, and R. montagui were studied in Carmarthen Bay, South Wales, from 1974-1976. All, but especially the 1st 2 spp., 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 myxopterygium (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 yr and 1047 mm, 9 yr and 875 mm, and 7 yr and 710 mm, but older, larger fish were caught by anglers. The values of the von Bertalanffy parameters L.infin. 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 3 spp. occurs mainly in the 5th yr. Total mortality based on research vessel catches for the 3 spp. 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 62-74 and 54-61 eggs, respectively. Most of the rays in Carmarthen Bay are young, with fish of age groups 0-2 comprising 87%, 71%, and 92% of the analyzed catch of R. clavata, R. microocellata, and R. montagui, respectively. Rays in the oldest age group of the 3 spp. represented by other than an occasional fish had mean measurements of 68%, 64%, and 73% of their L .infin. and estimated ages corresponding to 0.7 L .infin. are 16.0, 17.0, and 9.6 yr, respectively. The findings are discussed and, 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.