2007 - 2007 Centre for Environment, Fisheries & Aquaculture Science (Cefas) Skate and ray discard survey 2007/08 - Fisheries Science Partnership survey
FSP Skate and ray discard survey
To ascertain the likely survival rate of skates and rays discarded as a consequence of the proposed introduction of a maximum landing length, and also to develop on-board species identification guides to enhance the accuracy of data collected by species.
dataset
http://data.cefas.co.uk/#/View/632/
function: order
CEFASc5f6d12a-48b1-47a5-8f18-c62383112202
http://www.cefas.co.uk/
eng
OGP
urn:ogc:def:crs:EPSG::4326
biota
revision
2011-03-25
publication
2008-06-01
-7
-1.5
52.5
50.25
revision
2010-05-18
2007-07-01T00:00:00.000Z
2007-08-31T00:00:00.000Z
publication
2017-01-11
notPlanned
The short-term survival estimates were made using specially designed holding tanks to retain thornback rays on board the fishing vessel. These experiments were supplemented with a tagging programme in which 900 rays caught during the study were tagged and released. Information on tagged rays, when caught again in the future, will be used to estimate the long-term survival of rays and to analyse their movements and migrations. 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 study was conducted on board the commercial otter trawler ‘Our Olivia Bell’ BD 277 (length overall, 14.95 m; gr t 36; main engine, 227 kW), which has a track record of catching rays in the Bristol Channel. Three trips, each of four days, were made during July and August 2007. The vessel was rigged with a twin-trawl conventionally used in the fishery - two 70 ft (21.3 m) nets working 30-fathom (55m) bridals with 14-inch (36 cm) rock-hopper discs. The codend mesh was 85 mm in diameter and constructed from 4 mm single braided twine. To calculate estimates of short- and long-term survival of discarded rays, two haul durations were used: hauls of normal towing times and hauls of much shorter towing times. In all, 28 tows were conducted in areas where the vessel would normally fish for rays, 11 tows of a duration that is normal in commercial practice (3.58 – 4.25 h; referred to as commercial tows); 17 shorter tows (0.75 -2.00 h; referred to as short tows). All tows were conducted at tow speeds of 3-5 knots over the ground in water 30-60 m deep. Short-term survival: Rays caught in three commercial hauls, the first from each trip, were put in holding tanks and kept for approximately three days. This was for the duration of the trip and the maximum time possible, so these experiments only provided short-term survival estimates. If the rays died it could suggest catching and discarding was the cause of death, but it could also be that the rays died because they had been kept in the holding tanks (e.g. capture stress). Therefore, only half the tanks were filled with rays from commercial tows, the other half were filled with rays caught from short tows. It was assumed that rays from the short tows would be in better condition and were used as the control in the experiment. A comparison between the numbers surviving from the short tows and the commercial tows gave an indication of how being kept in holding tanks affected the rays’ chances of survival. For example, if all fish from the shorter tows had survived, then dead rays from the commercial tows probably died as a consequence of the catching process and not through being held in the tanks. The number of tanks that could be safely secured on board was 24. Consequently, the short-term survival experiments focused on a single species, the thornback ray. Thornback rays are generally considered to be one of the most robust ray species, therefore, the estimated short-term survival rates presented here are likely to be a maximum survival rate for the four species caught in the fishery. After hauling the short tows, thornback rays were taken immediately from the sorting pounds and placed in one of 12 holding tanks. From the three trips, 34 rays from nine short tows were kept in the holding tanks for up to 64 h. When sorting a commercial catch, there is a period of time before those that are unwanted are discarded to the sea. This is primarily due to the shooting of the gear for the next tow and processing of the catch. To account for this, for the commercial tows, rays were only put into holding tanks during the period between 10 and 20 min after the catch had been brought on board. From the three trips, 47 rays from three commercial tows were kept in the holding tanks for up to 64 h. No more than two fish were placed in a tank and, for the duration of the study, all fish were supplied with a constant flow of fresh seawater (15-20 l per min). Water temperature and oxygen saturation were monitored at regular intervals throughout; oxygen saturation did not fall below 85% during the experiment. No feeding was observed during the observation periods. The total body lengths of the retained thornback rays ranged from 20 cm to 90 cm. At intervals of approximately 0, 6, 24, 48 and 72 h, the fish in the holding tanks were checked. The fish were categorised as dead or alive, and dead rays were removed from the tanks. Death was adjudged by no movement of any muscle, gill or wing, and rigor mortis (upward curling of the wings). At the start and end of the experiment the fish were given a score to define their health status based on described criteria. Long-term survival: From the remaining eight commercial and eight short tows, 900 rays (139 thornback, 526 small-eyed, 82 spotted, 153 blonde) were tagged with dart tags and released. These were inserted into the muscle tissue of the dorsal wing - 379 tags were placed in rays from short tows and 521 from commercial tows. The health status of the tagged rays was assessed before release. Affect of codend weight on survival: The total catch weight for each haul was calculated. All species in the catch were measured, and length-weight conversion factors provided the total weight of each species. The estimated weight of invertebrates, benthos and marine debris was added to give a total weight for the haul. The total haul weight was halved to give an estimate for a single codend weight. To assess whether the codend weight could be affecting the survival of discarded rays, both the percentage of surviving rays retained in the holding tanks and the observed health status of the tagged and released rays were plotted against the codend weight. Summary: 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 longterm 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.
Public data (Crown Copyright) - Open Government Licence Terms and Conditions apply
Public data (Crown Copyright) - Open Government Licence Terms and Conditions apply
Data Manager
Centre for Environment, Fisheries & Aquaculture Science
Cefas Lowestoft Laboratory
Pakefield Road
Lowestoft
NR33 0HT
UK
+44 (0)1502 562244
originator
Data Manager
Centre for Environment, Fisheries & Aquaculture Science
Cefas Lowestoft Laboratory
Pakefield Road
Lowestoft
NR33 0HT
UK
+44 (0)1502 562244
custodian
Data Manager
Centre for Environment, Fisheries & Aquaculture Science
Cefas Lowestoft Laboratory
Pakefield Road
Lowestoft
NR33 0HT
UK
+44 (0)1502 562244
pointOfContact
2017-01-11T14:03:21