Abstract
Despite the rapid growth in popularity of flyshoot commercial fishing in the English Channel and the North Sea, discard survival estimates for rays and skates caught with this gear have yet to be published. This research was conducted on request of the producer organization FROM Nord, to specifically examine the discard survival of thornback rays caught by flyshooters in the Eastern English Channel (7.d), to provide survival estimates for fisheries management (i.e. landing obligation exemption) and within the framework of the FIP (Fishery Improvement Project) on thornback rays in 7d. In the period from June to September 2022, from four commercial sea trips fishing with flyshoot, 460 thornback rays of the catch were measured, sexed, and scored for injuries, reflex impairments (RAMP), and vitality. Only two rays (0.43%) were scored as being in an excellent condition (vitality class “A”), while the majority were found to be in “poor” conditions (vitality class “C”, 70.43%) or “good” conditions (vitality class “B”, 24.57%). Finally, 21 (4.57%) thornback rays were found dead when landed onboard, resulting in an immediate survival of 95.43%. This value falls close to the estimates for active and passive gears (93.56-100%) reported during the SUMARiS project (van Bogaert et al. 2020).
In addition, from these 460 rays, 80 were sampled (representing each vitality class) and selected to be monitored onboard until the end of the fishing trip, after which the rays were brought to captive holding facilities (Nausicaá) for delayed survival monitoring (n=67) for 21 days. A total of 24 rays died during the monitoring period, of which 13 died onboard of the vessel before reaching the aquaria, whilst 11 died throughout the 21 days of monitoring. Due to the high onboard mortality, estimations of delayed and total survival were performed following two methodologies: (1) including all 24 mortality events or (2) censoring the mortality events occurring at sea. Considering this, delayed survival was found to vary significantly between trips, ranging from 47.33 to 87.49% when considering all mortality events, and between 77.78 and 100% when censoring onboard mortalities. The third and fourth trips were found to register comparatively lower survival. This variation between trips was likely to be influenced to some degree by the temperature differences to which rays were exposed when transported from their natural habitat to the aquaria. The vitality class was found to significantly affect the rays’ probability to survive, with rays in worse conditions (i.e. vitality class “C”) being significantly more likely to face delayed mortality. Considering these factors, the delayed survival was estimated using a generalized linear mixed-effect model. Including all 24 mortality events, the delayed survival was estimated at 73.06% (CI: 56.5-85%), which situates flyshoot discard survival for thornbacks below the estimates reported for trammel netters (93.5%), above beam trawl (56.9%) and very close to otter trawls (76.5%) (van Bogaert et al. 2020). However, when censoring the 13 onboard mortalities, delayed survival for flyshoot finds itself closer to trammel netters than other active gears, with an estimated 91.57% (CI: 69.1-98.14) delayed survival. Considering the calculated immediate and delayed survival presented above, the total survival is estimated at 69.73% (CI: 53.92-81.12%). or 87.40% (CI: 65.95- 93.66) when including all mortalities or censoring onboard mortalities respectively.
Finally technical (i.e., exposure time, landed weight), environmental (i.e., water depth, water temperature, sea state, and substrate type), and individual variables (i.e., length, sex, and fish condition) were analysed for significance as predictors of the immediate and delayed survival status. The probability of immediate survival was found to be significantly affected by the length and injury score of each fish, as well as by the total landed weight and sea state of the haul in which they were caught. Whilst, the probability of delayed survival was only found to be significantly affected by the injury score. These findings provide evidence on how in-situ monitored variables (i.e., injury score) could be used as a proxy for the delayed survival of thornback rays caught by flyshooters
In addition, from these 460 rays, 80 were sampled (representing each vitality class) and selected to be monitored onboard until the end of the fishing trip, after which the rays were brought to captive holding facilities (Nausicaá) for delayed survival monitoring (n=67) for 21 days. A total of 24 rays died during the monitoring period, of which 13 died onboard of the vessel before reaching the aquaria, whilst 11 died throughout the 21 days of monitoring. Due to the high onboard mortality, estimations of delayed and total survival were performed following two methodologies: (1) including all 24 mortality events or (2) censoring the mortality events occurring at sea. Considering this, delayed survival was found to vary significantly between trips, ranging from 47.33 to 87.49% when considering all mortality events, and between 77.78 and 100% when censoring onboard mortalities. The third and fourth trips were found to register comparatively lower survival. This variation between trips was likely to be influenced to some degree by the temperature differences to which rays were exposed when transported from their natural habitat to the aquaria. The vitality class was found to significantly affect the rays’ probability to survive, with rays in worse conditions (i.e. vitality class “C”) being significantly more likely to face delayed mortality. Considering these factors, the delayed survival was estimated using a generalized linear mixed-effect model. Including all 24 mortality events, the delayed survival was estimated at 73.06% (CI: 56.5-85%), which situates flyshoot discard survival for thornbacks below the estimates reported for trammel netters (93.5%), above beam trawl (56.9%) and very close to otter trawls (76.5%) (van Bogaert et al. 2020). However, when censoring the 13 onboard mortalities, delayed survival for flyshoot finds itself closer to trammel netters than other active gears, with an estimated 91.57% (CI: 69.1-98.14) delayed survival. Considering the calculated immediate and delayed survival presented above, the total survival is estimated at 69.73% (CI: 53.92-81.12%). or 87.40% (CI: 65.95- 93.66) when including all mortalities or censoring onboard mortalities respectively.
Finally technical (i.e., exposure time, landed weight), environmental (i.e., water depth, water temperature, sea state, and substrate type), and individual variables (i.e., length, sex, and fish condition) were analysed for significance as predictors of the immediate and delayed survival status. The probability of immediate survival was found to be significantly affected by the length and injury score of each fish, as well as by the total landed weight and sea state of the haul in which they were caught. Whilst, the probability of delayed survival was only found to be significantly affected by the injury score. These findings provide evidence on how in-situ monitored variables (i.e., injury score) could be used as a proxy for the delayed survival of thornback rays caught by flyshooters
| Original language | English |
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| Publication status | Published - 31-Dec-2022 |
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