Size Distribution and Food of Thornback Rays (Raja clavata L) Caught on Rod and Line on the Mayo Coast

Abstract

A total of 732 Thornback Rays (R. clavata) were examined for length, wingspan, weight, sex and food in two bays on the west coast of Ireland. The samples were taken by rod and line during angling festivals in Clew Bay in June 1971 and in Broadhaven Bay in September 1971. The length/wingspan relationship for both sexes in both areas was found to be linear and the wingspan/weight relationship was the same for both sexes. The males become mature between 15.0 and 17.0 inches (38 to 43 cm) wingspan, while the females become mature between 18.0 and 20.0 inches (45.5 to 50.5 cm) wingspan. The sex ratio of males to females in each of the two samples was 1:1 in Clew Bay and 1.4:1 in Broadhaven Bay. The main food items in the stomachs of the rays from Clew Bay were Macropipus spp. (44 per cent), Crangon (23 per cent), Carcinus (21 per cent) and Lamellibranchs (10 per cent). In Broadhavcn Bay Ammodytidae (21.5 per cent), other fishes (19.5 per cent) and Macropipus spp. (14 per cent) formed the bulk of food items. A total of 71 R. clavata were tagged in Broadhaven Bay in the summer of 1971 and to date there have been 8 recaptures 7 within Broadhaven Bay. The days at liberty varied from 0 to 775 and the greatest distance travelled was 14 miles.