European plaice (Pleuronectes platessa) is a commercially essential fish species in the North Sea, Skagerrak, Kattegat, and Baltic Sea. It is mostly caught with the Danish seine, an active fishing gear targeting demersal species like the plaice. Researchers from Aalborg University in Denmark, and Norwegian University of Science and Technology recently published a study on catch-induced injuries, post-capture recovery and short-term survival of European plaice. 

Salinity, temperature and depth measured during towing
During the summer period when the air temperature and sea salinity were high, experiments were conducted from a commercial fishing vessel in the Western Baltic Sea. Three deployments were sampled from a Danish seine and Star-Oddi’s DST CTD loggers measuring salinity, temperature and depth were attached to the codend, the closed end of the trawl net. Deck temperature was also measured using Star-Oddi loggers. Other parameters as tow speed, tow time, gear configuration, and video footage were collected.  

Injuries and survival observed 
A total of 98 plaice were collected, individually tagged and housed in live wells for ten days to observe their survival and injures. Reflex impairments and external injuries were assessed after capture and at the end of the observation periods using reflex action mortality predictor (RAMP) and catch-damage index (CDI) methodologies. External injuries were primarily minor bruises, fin fraying, and net marks and they changed little from after capturing to the end of the observation period.

Higher survival rate than expected
Survival was expected to be negatively affected by high temperatures, high salinity gradients and time on deck. Average seabed and surface temperatures were 7.2°C and 14.3°C, average seabed and surface salinities 27.0 PSU and 9.2 PSU, and average deck temperature was 16.5°C. The air temperature reached about 30°C during mid-day. The experiment showed that 87 % of the studied fish survived after 10 days of observation. The survival rate was significantly higher for the fish that had remained on deck for 30 min or less, compared to those that remained on deck for more than 30 minutes. 

The researchers suggest that more studies involving other vessels fishing in other periods of the year are essential to obtain average estimates for this fishery. 

The study was recently published in the journal Fisheries Research and can be found here

Photo: LeDrezen