Escape of blue whiting (Micromesistius poutassou) and herring (Clupea harengus) from a pelagic survey trawl

The escape of blue whiting and herring from the aft parts of a pelagic trawl was studied by using small-meshed collection bags. The bags were attached to 80, 100, 200 and 400 mm belly sections in front of the codend of a 640 m pelagic survey trawl. The study was conducted in the springs of 2007, 2008 and 2009 based on two acoustic biomass estimation cruises each year. The results show that, at times, there can be substantial escape from the aft belly of the trawl. For blue whiting, the mean estimated total escape was in the range of 0.6–12%, and for herring, the range was 2–21%. Blue whiting escaped mainly from the 200 mm mesh section and was not recorded behind the 100 mm mesh section. The escapement of herring was recorded in all of the sampled sections of the trawl, mainly from the 80, 200 and 400 mm mesh sections. A length dependent tendency was found for herring for all three years but only for one year for blue whiting (paired t-test). Generally the mean length of escaped fish was lower than of fish in the codend. This can affect the acoustic abundance estimations done with the aid of this trawl. A significant difference between numbers of blue whiting escaping from 100 to 200 mm mesh sections was found, whereas no significant difference was detected between numbers of herring escaping from different mesh sizes (Mann–Whitney U and Kruskal–Wallis tests).

► This study demonstrates that there can be a high number of escaping blue whiting and herring from a pelagic trawl.
► It seems to be a difference in escape behaviour between blue whiting and herring.
► One possible explanation of the high number of escapees is that there is lower water flow in the aft parts of the trawl.