Under the “high survival” exemption of the European landing obligationor discard ban, monitoring vitality and survival of flatfish such as Europeanplaice (Pleuronectes platessa)becomes relevant to a discard-intensive beam trawl fishery.
For Belgianfisheries, previous research addressed the impact of gear deployment duration,and air exposure on vitality and mortality of plaice and also indicated therelevance of temperature changes to fish when being trawled through the watercolumn, exposed to air on deck, and being re-submerged under water (Uhlmann et al., 2016). Similarly to van Beek etal.
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(1990), Uhlmann et al. (2016)repeatedly observed higher survival at cooler days in winter compared withtrips in summer onboard the Eurocutter vessels. It is unclear, however, whetherhigher temperature per se orthermoclines during the hauling process cumulatively stressed captured fish. Theobjective of this proposed study is to address temperature tolerance oftrawl-caught plaice and determine the effect of temperature changes on survivalprobability. The experiments tookplace on board RV Simon Stevin (three trips) and the monitoring in land basedfacilities at a research laboratory in Ostend. The sampling was performed bythe research vessel with a beam trawler. After every trawl twenty individualsof different sizes (mainly undersized between 10 and 25cm in total length),were collected randomly. Then, they were divided into four batches of five andevery batch underwent a different treatment.
The treatments consisted of air exposureand then recovery in water. The temperature of both mediums was controlled. Forthe air treatment, it was kept about 5 degrees colder or 5 degrees warmer thanthe water ambient temperature and the water was either ambient water or watercooled down by 10 degrees. Sixty individuals were collected per trip and theirvitality was quantified and scored based on a reflex impairment index as it wasproposed in Uhlmann’s et al. (2016).Once treated the fish were kept onboard in tanks. Afterwards, the fish werethen transported to the laboratory.
The fish were retained in tanks for a week for monitoring, whereseawater was regulated through a circulatory system. An important (total)survival rate of about 25% was observed in the results, which is higher thanprevious studies Van de Reijden et al. (2017), where survival rate of plaicewas estimated at 15%. Out of all the treatments, the transition from exposureto warm air (about 25oC) and to recovery in cold water (about 10oC)caused the biggest loss of vitality, while the exposure to warm air andrecovery in ambient water (about 20oC) had the least. Concluding,our results support that there is a considerable survival rate of plaice evenafter the treatments, supporting the exception of plaice from the no disposal policy for by catch by fisheries. But,consideration should also be given in the temperature conditions of the sortingroom, since our results suggest that when there is a wide gap between the seawater temperature and the air temperature that fish are exposed to there is adrop to the survival rate of plaice.