In this project we will use time-depth recorders to identify the current sink profile of the commercially used floated longlines on UK-flagged vessels operating in the north of Scotland. Alongside discussion with crew and skippers on the critical aspects of design, this will then inform the design - in collaboration with fisheries scientists and through numerical simulation - of an improved weighting regime on floated longlines, that sinks rapidly enough to minimise seabird bycatch, but still fishes in a way that maintains or increases target-fish catches.
This project looked at the key driver of accidental seabird captures in a floated demersal longline fishery – the availability of baited hooks to seabirds as the line is being set. Using this information, we identified potential gear adaptations to reduce the risk posed to seabirds.
We attached Time Depth Recorders to the fishing gear, which demonstrated that the current gear design results in a very slow sink speed when hooks are set, such that most fall below international recommendations to reduce seabird bycatch risks. Slow sink rates also have an economic impact, as baits and hooks lost to seabirds are no longer able to catch fish. Increasing the sink rate of the fishing gear, at least in the upper 10m depths of the ocean, would therefore be beneficial for both seabirds and the industry.
Through computer simulation, we were able to examine the sink profiles of three gear design alterations. We identified that a series of measures such as improved line weighting, increased float line length, reduced hook numbers near floats and switching to hydrodynamic floats have the best potential to reduce bycatch risk through improved sink rate. We acknowledge that even small alterations to the gear can have a significant effect on a fishery’s operations and cost. The proposed alterations take this into account and seek to maximise conservation and economic benefits.
A follow on project is planned, where in-fishery trials of this gear would provide key data on actual impacts on target catch and bycatch rates, and the potential for widespread adoption by the fishery.