Developing bottom trawls to improve size and species selectivity in North Atlantic fisheries

Nguyen, Vang Y. (2023) Developing bottom trawls to improve size and species selectivity in North Atlantic fisheries. Doctoral (PhD) thesis, Memorial University of Newfoundland.

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Abstract

Bottom trawling is an important and principal capture technique in groundfish fisheries, providing a high proportion of fisheries production to the Northern Atlantic. However, bottom trawling is associated with the bycatch of undersized fish and unwanted species, a large concern from a marine ecosystem perspective. Thus, developing gear designs to improve trawl selectivity (reduce the capture of non-target species and juveniles of target species) is necessary. This thesis has focused on developing trawl designs for an emerging redfish (Sebastes spp.) fishery in Canada and understanding the groundgear selectivity of an Icelandic commercial bottom trawl. Firstly, I developed a shaking codend by attaching an elliptical-shaped canvas at the posterior of a T90 codend (codend mesh rotated 90˚ in the transversal direction) to reduce the capture of undersized redfish in the catch. The results showed that the shaking codend had a higher amplitude ratio, period, and total acceleration and captured less redfish < 22 cm than the T90 codend without canvas. Secondly, I developed a semi-pelagic trawl to capture redfish using the French rigging technique. Semi-pelagic trawls are effective at capturing redfish off the seabed and potentially reduce bycatch of unwanted species. Next, I quantified the length-dependent escape of fish under a commercial bottom trawl in Iceland. The results showed length-dependent escape for roundfish, where more small fish escaped under the groundgear than did large fish, compared with flatfish, whose escape varied among species. Finally, I quantified fish behavior at the mouth of a bottom trawl. Small roundfish (Atlantic cod (Gadus morhua) < 20 cm and haddock (Melanogrammus aeglefinus) < 11 cm) tend to escape under the trawl at the center area of the groundgear, while larger individuals with greater swimming capacity seek escape openings under the fishing line at the wing areas. For flatfish and monkfish, the results varied. These length-dependent behaviors are related to fish response behavior, escape behavior, size, and likely swimming capacity. The findings of this thesis can have potential implications for the development of the emerging redfish fishery in Canada and for developing groundgear to improve bottom trawl selectivity in North Atlantic fisheries.

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