Reducing bottom trawl seabed impacts and bycatch in the Eastern Canada offshore Northern shrimp (Pandalus borealis) fishery

Abstract

The offshore Northern shrimp (Pandalus borealis) fishery in Eastern Canada is currently harvested by factory freezer vessels using bottom trawls. This fishery is a major contributor to the regions’ economy. However bottom trawling typically is associated with negative benthic impacts and bycatch. To address seabed impact, this thesis evaluated the at-sea performance of a traditional roller footgear using underwater cameras. Results showed that footgear sections were rotating at extremely low rates and offered essential information to further develop an innovative footgear with reduced seabed impact. Secondly, an innovative aligned-rolling footgear was designed and evaluated for use in the fishery. I compared traditional and experimental footgears using engineering models and simulated seabeds in a flume tank. Results revealed that the aligned-rolling footgear bottom trawl produced significantly lower warp loads and reduced seabed contact up to 71.5% depending upon seabed penetration depth modelled. Next, I addressed the issue of a recent increase of juvenile redfish (Sebastes spp.) bycatch in the shrimp fishery. This study investigated the effectiveness of 17 and 15 mm bar spacing Nordmøre grids in a twin-trawl configuration against the traditional 22 mm bar spacing grid. Results showed that smaller bar spacing grids resulted in no significant reduction in shrimp catch across all length classes. Conversely, catch of juvenile redfish was significantly reduced with the smaller bar spacing grids. However, the overlap in body/carapace width between redfish and Northern shrimp limits the overall sorting efficiency of the grids, leaving some juvenile redfish still vulnerable to capture. Finally, I investigated the behaviour of juvenile redfish in response to Nordmøre grids. Behaviours were analyzed for 22 mm and 19 mm bar spacing grids. Reducing bar spacing to 19 mm slightly reduced the number of redfish retained (1.83% reduction) and behaviours exhibited by redfish were similar for both grids. The most common behaviours that led to escapement were redfish that approached upwards, had no contact with the grid, and swam upwards to finally escape through the grid opening. This thesis provides important advancement toward the development of bottom trawls with reduced environmental impacts and contributes to the sustainable development of the fishery.