Final Report Programme 21 : Bristol Channel ray survival

This Fisheries Science Partnership project was developed to estimate the survival of rays, which are caught commercially in a target trawl fishery in the Bristol Channel, and subsequently discarded. The study was proposed by the NFFO and categorized by Defra and Cefas as priority for support. All ray and skate species are commonly marketed as skate. The status of several skate and ray stocks in the Northeast Atlantic is of concern, with some species classified as depleted while others are locally extirpated. The objectives of this study were to (1) estimate the short-term discard survival rates of thornback rays; (2) estimate the long-term discard survival rates of the most commercially important ray species; and (3) investigate the effect of codend weight on ray discard survival. The short-term survival estimates were made using specially designed holding tanks to retain thornback rays on board during three 4-day trips on the FV 'Our Olivia Belle'. These holding tank experiments were supplemented with a tagging programme in which 900 thornback, small-eyed, blonde and spotted rays were caught, tagged and released. Information on tagged rays, when caught again in the future, will be used to estimate the longer-term survival of discarded rays. Consequently, the results from the tagging programme can only be presented once sufficient tags have been returned, and will be included in a future report. The short-term survival experiments showed that for every three thornback rays returned to sea in the Bristol Channel ray fishery, an estimated two survive for at least 3 days. The few tag returns received so far show that discarded rays are surviving for at least 45 days. The study indicated that as total catch weight and tow duration increases, the survival rate for discarded rays decreases; also, that large thornback rays do not have greater rates of survival than small rays. The implications of these results for the management of ray fisheries are discussed.