Review of mitigation techniques to reduce benthic impacts of trawling

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Summary

Remedial efforts through gear modification are described, and their potential application by the New Zealand bottom trawl fleet is discussed, including relative impact on seabed contact (footprint), fisher profitability, and handling and operation of the trawl. Multiple recommendations are also provided to guide next-steps towards reducing seabed contact by this trawl fleet, including both inshore and deepwater vessels.

Around the world there have been numerous attempts by fishing technologists, fishers, and others to mitigate the benthic impacts of bottom trawling through gear modification. Most of these efforts have focused on eliminating seabed contact, and thus avoiding habitat impact by lifting trawl components into the water column, including the use of semi-pelagic trawl doors, elevated sweeps and bridles, and groundrope removal. Other efforts that have attempted to minimise or reduce seabed contact include increasing upper bridle length to lighten groundrope contact and increasing the diameter (surface area) of sweeps and lower bridles to reduce impact per unit area. These efforts have been tried in many fisheries, although efficacy is questionable because fishers cannot precisely control and regulate trawl contact with the seabed. Subsequently, quantifying the efficacy of these modifications is extremely difficult, and no reports were found in the literature describing the success of these modifications. Therefore, efforts to reduce seabed contact using these latter methods is not included in this report.

Not all methods described to reduce seabed contact are expected to be equally applicable across all New Zealand bottom trawl fisheries. Some may be better suited to inshore fisheries than deepwater fisheries. Several may also not be applicable at all, but their inclusion serves to stimulate ideas that may ultimately result in the development of new methods to reduce seabed contact by bottom trawl gear in New Zealand fisheries.

Publication details

Eayrs, S., Craig, T. and Short, K. 2020. Review of mitigation techniques to reduce benthic impacts of trawling. MIT2019-02 final report prepared by Terra Moana Limited for the Conservation Services Programme, Department of Conservation. 135 p.