Trawling is a fish harvesting process that catches many non-target species which derive little income for fishers. Legislation is being adopted by countries including Iceland, Norway, the Faroe Islands, the USA, the United Kingdom and New Zealand to limit the catching of these non-target species. The adoption of gear that reduces bycatch for the groundfish trawl fisheries has mostly involved changing trawl net mesh orientation and sizes to date. These changes have shown to be effective at reducing bycatch. However, this thesis identifies possibilities to innovate new trawling gear, which may have an advantage over mesh changes in improving selectivity.

A novel passive sorter was developed for the ground fish trawl fishery. The system was manufactured, and a preliminary test carried out on a commercial fishing boat which indicated a reduction in bycatch of juvenile gurnard (Chelidonichthys cuculus) by 68%. It is a low-cost system, satisfies fisheries regulations, and is simple to handle and stow on a fisheries vessel. This novel system is an entirely new method to improve the selectivity of catch in trawling and meets many of the requirements necessary to see adoption by fishers. It is currently being utilised in the day-to-day operations of a trawling company located in Napier.

The second part of this research consisted of developing a novel active sorter concept, using a camera to identify species and a gate to retain or release each fish, developed in previous work. This system images species inside the trawl and with a mechanical actuator can make real-time decisions to release any non-target fish species. To determine whether this concept design was optimal, several alternative concepts were generated and then evaluated. However, none of these alternatives were thought to improve selectivity over the existing active gate design. However, some features were identified that may increase the probability of a successful active fish sorter. These being a system that images and sorts fish in the same location. Also, a screening room that traps fish in a location by closing the entrance they arrived through, images them and then opens an exit to release or capture. This research can be used as a framework to generate novel active bycatch reduction systems in the future.

Overall, the results from this research further our understanding of the gear solutions that can be created to improve selectivity in the trawling industry.