Under were kept onboard in tanks. Afterwards, the fish

Under the “high survival” exemption of the European landing obligation
or discard ban, monitoring vitality and survival of flatfish such as European
plaice (Pleuronectes platessa)
becomes relevant to a discard-intensive beam trawl fishery. For Belgian
fisheries, previous research addressed the impact of gear deployment duration,
and air exposure on vitality and mortality of plaice and also indicated the
relevance of temperature changes to fish when being trawled through the water
column, exposed to air on deck, and being re-submerged under water (Uhlmann et al., 2016). Similarly to van Beek et
al. (1990), Uhlmann et al. (2016)
repeatedly observed higher survival at cooler days in winter compared with
trips in summer onboard the Eurocutter vessels. It is unclear, however, whether
higher temperature per se or
thermoclines during the hauling process cumulatively stressed captured fish. The
objective of this proposed study is to address temperature tolerance of
trawl-caught plaice and determine the effect of temperature changes on survival
probability.

            The experiments took
place on board RV Simon Stevin (three trips) and the monitoring in land based
facilities at a research laboratory in Ostend. The sampling was performed by
the research vessel with a beam trawler. After every trawl twenty individuals
of different sizes (mainly undersized between 10 and 25cm in total length),
were collected randomly. Then, they were divided into four batches of five and
every batch underwent a different treatment. The treatments consisted of air exposure
and then recovery in water. The temperature of both mediums was controlled. For
the air treatment, it was kept about 5 degrees colder or 5 degrees warmer than
the water ambient temperature and the water was either ambient water or water
cooled down by 10 degrees. Sixty individuals were collected per trip and their
vitality was quantified and scored based on a reflex impairment index as it was
proposed in Uhlmann’s et al. (2016).
Once treated the fish were kept onboard in tanks. Afterwards, the fish were
then transported to the laboratory. The fish were retained in  tanks for a week for monitoring, where
seawater was regulated through a circulatory system.

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            An important (total)
survival rate of about 25% was observed in the results, which is higher than
previous studies Van de Reijden et al. (2017), where survival rate of plaice
was estimated at 15%. Out of all the treatments, the transition from exposure
to warm air (about 25oC) and to recovery in cold water (about 10oC)
caused the biggest loss of vitality, while the exposure to warm air and
recovery in ambient water (about 20oC) had the least. Concluding,
our results support that there is a considerable survival rate of plaice even
after 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 sorting
room, since our results suggest that when there is a wide gap between the sea
water temperature and the air temperature that fish are exposed to there is a
drop to the survival rate of plaice.

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