Final Report Discard survival rates of commercially caught ray 2007 Cefas Lowestoft, Exeter Prepared by: T.L. Catchpole R. Enever
Summary 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 objective of this study was to estimate the short-term discard survival rate of rays. The short-term survival estimates were made using specially designed holding tanks to retain rays on board during four three-day trips on the FV Cerulean (Figure 2, Plate 2). These holding tank experiments were complemented with a control experiment conducted ashore. The short-term survival experiments showed that for every two rays caught and subsequently discarded to sea in the Bristol Channel ray fishery it is estimated that one will survive for at least two days. It is estimated that the percentage of thornback, painted and spotted rays surviving in the short-tem (48 hr) after being discarded to the sea is within the range of 55-59%. Plate 1: Biological parameters being taken prior to fish entering holding tanks 2
Introduction This is a study of the survival of discarded rays caught in the Bristol Channel; Seafish, through the Technology and Innovation Primer Award, funded the study. 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. These species are considered relatively vulnerable to exploitation due to them being long-lived, slow growing and having low fecundity. This means that stocks of these species may only be able to replenish slowly and so could be quickly over-fished. In a recent scientific investigation conducted by IUCN-Shark Specialist Group, the thornback ray, blonde ray, and small-eyed ray (also known locally as painted or sandy ray) were categorized as near threatened (www.iucnredlist.org). Though many rays are landed in mixed demersal fisheries, target fisheries also operate in certain areas and at certain times of the year (Figure 1). The Bristol Channel fishery is one of the UK s most important target fisheries for rays with annual landings (by weight) representing 20% of the English and Welsh total. Rays caught in the Bristol Channel are annually valued at ~ 1 million. In the English Channel, Western approaches, Celtic and Irish seas (ICES sub-area VII) it is estimated that 3.8 million rays are caught by commercial fishing vessels every year. Of these, 2.2 million (60%) are subsequently discarded (data from Cefas discard database for method see Enever et al 2007). This project was designed to provide data that estimated the survival rates of these discarded rays. Objective: To estimate the short-term survival of rays in the discarded catch component of a ray target fishery. 3
Figure 1: Haul locations around England and Wales where commercial fishers catch rays. Source: Cefas catch and discard database. The Bristol Channel is highlighted as an important ray fishery. Methods The short-term survival estimates were made by using specially designed holding tanks to retain rays on board a commercial fishing vessel (Figure 2, Plate 2). The study was conducted on board the commercial otter trawler Cerulean BD 1 (length overall, 14.98 m; gr t, 48; main engine, 227 kw), which has a track record of catching rays in the Bristol Channel. Four trips, ranging from 3-5 days in length, were made during May and August 2007. The vessel was rigged with a twin-trawl conventionally used in this fishery - two 65 ft (20.1 m) nets working 30 fa (55m) bridals with 14-inch (36 cm) rock hopper discs. The codend mesh was 80mm in diameter and constructed from 4mm single braided twine. From the four trips, four hauls were conducted in areas where the vessel would normally fish for rays. Fish from these hauls were placed into the on board holding tanks for observation. The hauls from which rays were selected for the survival experiments reflected normal commercial practice; tow durations varied between 2.7 and 4.5 hrs. All tows were conducted at tow speeds of 3-5 knots over the ground in waters 40-70 m deep. 4
Short-term survival Rays caught in four commercial hauls, the first from each trip, were put in holding tanks and kept for two days (48 hours). This was the maximum time possible on some of the trips; therefore these experiments only provided short-term survival estimates. These experiments were conducted on board during commercial fishing operations and so the number of tanks that could be safely secured on board was restricted to ensure the experimental equipment did not impede the fishing activities of the vessel. On trips one and two six holding tanks were used; on trips three and four, following discussions with the skipper, 12 holding tanks were used. Seventy-seven rays from four species where used in these experiments - Cuckoo ray (raja naevus; n=6), small-eyed ray (raja microocellata; n=39), spotted ray (raja montagui; n=11) and thornback ray (raja clavata; n=21). 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, rays were only put into holding tanks during the period between 10 and 20 minutes after the catch had come on board. In the four trips a total of 77 rays from four commercial tows were kept in the holding tanks for up to 72 hours. No more than three fish were placed in a tank and fish were supplied with a constant flow of fresh seawater (15-20 litres per minute) for the duration of the study. No feeding took place during the observation period. The body lengths of the retained rays ranged from 26-60 cm. At intervals of 0, 6, 24 and 48 hours, the fish in the holding tanks were checked (Plate 3). The fish were categorised as dead or alive and any dead rays were removed from the tanks. Death was adjudged by no movement of any muscle, spiracle or wing, and rigour mortise (upward curling of wings). At the start and end of the experiment the fish were given a score to define their health status based on the following criteria whereby the higher the score the healthier the fish: 1: No body movement, minor movement of spiracles 2: Limp body / wing movement and spiracle movement 3: Vigorous wing / body movement and rapid spiracle movement. 5
Figure 2. Schematic of the survival tanks showing two stacks of 6 compartments. i.e. 12 compartments in total. 6
Plate 2: Two stacks of holding tanks (12 compartments) situated port-side and aft underneath the vessels shelter deck aboard FV Cerulean. 7
Control If the rays died whilst in the holding tanks on board it could be for one of two reasons: 1) The stresses induced upon them through the catching and discard process and 2) the stresses of being held in the holding tanks. In order to evaluate the effect of the holding tanks on the health of the rays a control experiment was set up on land. Thornback rays, captured in the wild were held in aquaria facilities ashore for one month and fed daily to ensure they were healthy prior to the experiment. Five thornback rays, 62-70 cm total length, were kept in the holding tanks for two days to act as a control for the experiment. Each fish was placed in a separate compartment, and, as with the experiments at sea, all fish were supplied with a constant flow of fresh seawater (15-20 litres per minute), no feeding took place during observation period. Plate 3. A thornback ray being assessed for a 24 hr survival check 8
Results Short-term survival In the four trips, 43 of the 77 rays (56%) survived in the holding tanks for at least 48 hours after being caught. The survival of rays from each haul ranged from 45-70% (Table 1). Table 1. Summary of short-term survival experiments using holding tanks to retain rays. Haul number Time in holding tank (hr) Tow duration (hr) Observed rays Mean length (cm) % Alive at end 1 48 4.5 10 50.1 70 2 48 4.3 14 41.4 64 3 48 2.7 29 42.8 45 4 48 3.0 24 42.5 58 Total Mean = 48 Mean = 3.6 77 43.4 56 Control 48 n/a 5 66.2 100 The average length of rays surviving in the tanks was 46cm (31-56cm). The average length of rays dying while in the holding tanks was 41 cm (26-60cm). Table 2. Percentage short-term survival of discarded rays by species and mean health score (lower score = healthier fish) of the rays that survived after 48 hr Species Number % Surviving in tanks Mean health score of fish that survived Start End Thornback ray 21 57 1.7 2.4 Small-eyed ray 39 59 1.8 2.3 Spotted ray 11 55 2.2 2.3 Cuckoo ray 6 33 1.5 2 The survival rates differed between species with the estimate for cuckoo rays less than for the other species (Table 2). The numbers of cuckoo rays investigated was lower than for other species and so the confidence in this estimate is less for this species. The short-term discard survival rate of thornback, small-eyed and spotted ray is estimated at 55-59%. The total health score of those rays that survived after 48 hr was lower at the start of the experiment than at the end for all species (Table 2). This means that the health of the rays that survived improved between the start and end of the experiment. It suggests that those fish that survived actually recovered from the catch and discard process while being retained. 9
Control All five of the thornback rays held in holding tanks ashore for 48 hr survived; the holding tanks had no discernable effect on the health of the captive rays (Table 1). 10
Conclusion For every two rays discarded to sea in the Bristol Channel ray fishery it is estimated that one survives for at least 2 days. The percentage of thornback, small-eyed and spotted rays surviving in the short-tem (48 hr) after being discarded to the sea was within the range of 55-59%. This supports a recent study (Catchpole and Enever, 2007) that estimated the short-term survival (72 hr) of thornback rays at 67%. However, the deterioration in health of the rays that survived after 48 hr suggests that overall discard mortality may be higher in the longer-term. A tagging programme currently being done at Cefas is hoped will provide some information on long-term survival. The health of rays may also have deteriorated owing to them being held in tanks. The control experiment, carried out ashore, showed that 100% of the thornback rays (n=5) survived the 48 hr period in the holding tanks. However, the control rays were not subjected to the movement that the experimental rays experienced while being held in tanks at sea. Catchpole and Enever (2007) used a sea-based control, using rays caught in non-commercial short tows, to demonstrate a survival rate in the holding tanks for thornback rays of 91%. In this experiment there was some indication that the health of those rays that survived improved while being retained. Therefore, these data would suggest that the holding tanks had little effect on the survival of rays. There are other factors that may affect survivability that were not assessed in this study. For example, Catchpole and Enever (2007) showed that codend weight had an effect on thornback ray short-term survival and on their health status. The main factors affecting the survival of discarded ray are likely to include, the species of ray, initial health of the rays, the time spent on deck before being returned to the sea and the weight and catch composition in the codend. Fishermen and scientists alike consider discarding as an unnecessary waste of resources. At present there is a considerable drive to reduce discarding in commercial fisheries. The results presented here support other studies, which have shown that around half the discarded rays actually survive, at least in the short-term. This survival rate is higher than for many whitefish species, most of which do not survive being discarded, and should therefore be taken into account when analysing discard data for these species. References T. L. Catchpole, R. Enever, S. Doran. Bristol Channel ray survival, Final Report, Fisheries Science Partnership: 2007/08, Cefas. R. Enever, A. Revill and A. Grant (2007). Discarding in the English Channel, Western approaches, Celtic and Irish seas (ICES subarea VII). Fisheries Research, 86 (2-3), pp 43-152. Acknowledgments Many thanks to Jasper (skipper), Jason, Rob and Dom, the crew of the F.V Cerulean, for their help and enthusiasm in collecting this data. Also thanks to Dick Talbot, the part owner of Cerulean, for allowing Cefas access for sampling. 11
Project costs and staffing The project was completed on time and within budget (Table 3) Table 3 Project budget Grant awarded 4500 Actual expenditure 9000 Staff input 4500 T&S and equipment 4500 Cefas contribution 4500 12