Survival probabilities of plaice, sole and turbot discarded by beam trawl and flyshoot fisheries
The objectives of this study were 1. to establish baseline survival probabilities for undersized plaice, sole and turbot discarded by tickler chain beam trawl fisheries with two 12 m wide gears and for plaice discarded by flyshoot fisheries, and 2. to establish the effect of replacing the conventional mesh lengthener and cod-end of a 12 m wide tickler chain beam trawl by a modular harvest system (MHS) on discards survival probabilities of undersized plaice, sole and turbot. Discards survival was measured by captive observation of test fish sampled at sea during regular commercial beam trawl and flyshoot fisheries. During all trips, control fish of the same species were used to separate potential effects of the experimental procedures on mortality from fisheries-induced mortality. Average survival of control fish was 83.7% for plaice employed in the flyshoot trips and 86.8% for plaice, 83.3% for sole and 100% for turbot employed in the beam trawl trips. We conducted three trips with a flyshooter in April, May and July 2023 and sampled a total of 675 plaice. Discards survival probability estimates ranged from 39.7% in April to 1.2% in July. The overall survival probability estimate based on these three trips is 15.0% (95% confidence interval 12.5%-17.9%). We conclude that plaice discards survival is highly variable in flyshoot fisheries for reasons that are not yet fully understood. We conducted five trips with a tickler chain beam trawler with two 12 m wide gears in the period February to August 2023. During two out of these five trips we tested the effect of the Modular Harvesting System (MHS) on discards survival. The MHS is a novel cod-end consisting of a membrane-like fabric tube with escapement holes that replaces the mesh cod-end of a trawl. The MHS aims to reduce fish damage during trawling, haul back and unloading. Based on the strong relation between the condition in which fish are discarded and their survival probability, this was predicted to increase discards survival probabilities. To test the MHS, the conventional mesh cod-end of one of the two trawls of a double rigged beam trawler was replaced with a MHS and paired samples of test fish were collected from the catches by both gear types. In total 579 plaice, 294 sole and 128 turbot were sampled from the conventional beam trawl gear. From the beam trawl with MHS we sampled a total of 160 plaice, 80 sole and 37 turbot. The overall discards survival probability estimates for the conventional beam trawl are 8.1% (95% confidence interval 6.2%-10.7%) for plaice, 20.4% (95% confidence interval 16.3%- 25.6%) for sole and 15.6% (95% confidence interval 10.4%-23.3%) for turbot. For plaice the highest survival (19.9%) was observed in February and the lowest (2.5%) in August. For sole the highest survival (34.1%) was observed in May and the lowest (12.5%) in August. For turbot the highest survival (35.5%) was observed in June and the lowest (0%) in February. Because our observations include annual temperature extremes we conclude that overall discards survival probability estimates for conventional beam trawling lie within these ranges. However additional observations in the periods that were not covered by the current project are needed to determine overall discards survival probabilities that are representative for year-round fisheries. This also applies to flyshoot fisheries. We also recommend to consider discards survival probability data in conjunction with data on discards amounts to obtain insight in absolute fisheries mortality due to discarding. Survival probabilities of plaice and turbot were significantly higher for the fish sampled from the beam trawl equipped with a MSH. On average the MHS resulted in an elevenfold increase in survival for plaice and a sixfold increase in turbot. For sole no significant effect was detected. This study confirms and provides the first quantitative insight in the potential increase in survival probability of replacing the conventional mesh cod-end of a beam trawl with a MHS. Further increase in discards survival probability is expected with the refinement of the fishing technique. To fully utilize the potential increase in discards survival that can be achieved by the MHS, we also recommend research into the refinement of the catch sorting process to maintain the good condition of the fish while on board.
| Main Authors: | , , , , , , , |
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| Format: | External research report biblioteca |
| Language: | English |
| Published: |
Wageningen Marine Research
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| Subjects: | Life Science, |
| Online Access: | https://research.wur.nl/en/publications/survival-probabilities-of-plaice-sole-and-turbot-discarded-by-bea |
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| Summary: | The objectives of this study were 1. to establish baseline survival probabilities for undersized plaice, sole and turbot discarded by tickler chain beam trawl fisheries with two 12 m wide gears and for plaice discarded by flyshoot fisheries, and 2. to establish the effect of replacing the conventional mesh lengthener and cod-end of a 12 m wide tickler chain beam trawl by a modular harvest system (MHS) on discards survival probabilities of undersized plaice, sole and turbot. Discards survival was measured by captive observation of test fish sampled at sea during regular commercial beam trawl and flyshoot fisheries. During all trips, control fish of the same species were used to separate potential effects of the experimental procedures on mortality from fisheries-induced mortality. Average survival of control fish was 83.7% for plaice employed in the flyshoot trips and 86.8% for plaice, 83.3% for sole and 100% for turbot employed in the beam trawl trips. We conducted three trips with a flyshooter in April, May and July 2023 and sampled a total of 675 plaice. Discards survival probability estimates ranged from 39.7% in April to 1.2% in July. The overall survival probability estimate based on these three trips is 15.0% (95% confidence interval 12.5%-17.9%). We conclude that plaice discards survival is highly variable in flyshoot fisheries for reasons that are not yet fully understood. We conducted five trips with a tickler chain beam trawler with two 12 m wide gears in the period February to August 2023. During two out of these five trips we tested the effect of the Modular Harvesting System (MHS) on discards survival. The MHS is a novel cod-end consisting of a membrane-like fabric tube with escapement holes that replaces the mesh cod-end of a trawl. The MHS aims to reduce fish damage during trawling, haul back and unloading. Based on the strong relation between the condition in which fish are discarded and their survival probability, this was predicted to increase discards survival probabilities. To test the MHS, the conventional mesh cod-end of one of the two trawls of a double rigged beam trawler was replaced with a MHS and paired samples of test fish were collected from the catches by both gear types. In total 579 plaice, 294 sole and 128 turbot were sampled from the conventional beam trawl gear. From the beam trawl with MHS we sampled a total of 160 plaice, 80 sole and 37 turbot. The overall discards survival probability estimates for the conventional beam trawl are 8.1% (95% confidence interval 6.2%-10.7%) for plaice, 20.4% (95% confidence interval 16.3%- 25.6%) for sole and 15.6% (95% confidence interval 10.4%-23.3%) for turbot. For plaice the highest survival (19.9%) was observed in February and the lowest (2.5%) in August. For sole the highest survival (34.1%) was observed in May and the lowest (12.5%) in August. For turbot the highest survival (35.5%) was observed in June and the lowest (0%) in February. Because our observations include annual temperature extremes we conclude that overall discards survival probability estimates for conventional beam trawling lie within these ranges. However additional observations in the periods that were not covered by the current project are needed to determine overall discards survival probabilities that are representative for year-round fisheries. This also applies to flyshoot fisheries. We also recommend to consider discards survival probability data in conjunction with data on discards amounts to obtain insight in absolute fisheries mortality due to discarding. Survival probabilities of plaice and turbot were significantly higher for the fish sampled from the beam trawl equipped with a MSH. On average the MHS resulted in an elevenfold increase in survival for plaice and a sixfold increase in turbot. For sole no significant effect was detected. This study confirms and provides the first quantitative insight in the potential increase in survival probability of replacing the conventional mesh cod-end of a beam trawl with a MHS. Further increase in discards survival probability is expected with the refinement of the fishing technique. To fully utilize the potential increase in discards survival that can be achieved by the MHS, we also recommend research into the refinement of the catch sorting process to maintain the good condition of the fish while on board. |
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