What will it take to get fish out of the trawl? Creating a Low-Flow Zone within and outside the boundaries of a trawl to incentivize fish escape

A promising, yet under-utilized approach to bycatch reduction in trawl fisheries is one that exploits and manipulate the swimming orientation and flow refuging behaviour of bycatch species to encourage their escape from inside a trawl. We investigated whether a low flow zone area, expanding both within and outside the extension section of a demersal trawl, can be used to increase fish escape. A 360° radial escape opportunity, in the form of a gap in the trawl between the extension and codend, was offered to the fish in the low flow area. The catch was guided into the codend by an impermeable tarpaulin funnel, which generated a low flow area in the wake region behind it, where the gap was located. In principle, only individuals that exploit the low flow area can swim forward and escape through the gap. To prevent fish with strong flow refuging behaviour to hold stationary in the low flow zone, without exiting the gap, a deflector flange also made of an impermeable tarpaulin was mounted outside the entrance of the funnel to expand the low flow area in the open water surrounding the extension piece. This concept was designed using flume tank trials and computational fluid dynamic modelling and tested during experimental trials at sea. Substantial escape rates were found for all three roundfish species considered: cod (Gadus morhua), haddock (Melanogrammus aeglefinus) and whiting (Merlangius merlangus). Moreover, based on the fluid dynamic models, we tested two different gap sizes, with the smaller gap (1 m) moving the codend entrance into the recirculation area created by the low flow zone. We found a significant difference in the escape of undersized haddock and whiting, but not cod, which suggest complex and possibly species-dependent interactions between hydrodynamic flow and fish behaviour.