Elasmobranch Bycatch in the Canadian Northwest Atlantic and Arctic Adjacent Seas: Composition, Biogeography, and Mitigation

Abstract

BACKGROUND: Industrial fishing has profoundly changed the biological state of oceans and seas. While the direct impacts of overfishing on target stocks are being increasingly addressed, unwanted bycatch and discarding of non-target species remains a key challenge of contemporary fisheries management. Excess bycatch mortality is particularly threatening for intrinsically vulnerable species, such as sharks, skates, and rays (Class Chondrichthyes). QUESTION: In order to address these pressing concerns we ought to know where bycatch occurs in time and space, and how it can be solved through spatial-temporal management and modification of fishing technique. METHOD: These questions were addressed by developing novel geostatistical methods to track and quantify bycatch "hotspots" in the Northwest Atlantic and adjacent Arctic seas. These tools were applied to analyse (i) bycatch patterns of Greenland shark (Somniosus microcephalus), an emerging species of concern in expanding Arctic fisheries; (ii) bycatch of large sharks in pelagic longline fisheries; and, (iii) to estimate total discards of all elasmobranch species in Eastern Canadian fisheries. Additional research also clarified the role of circle hooks in reducing post-release mortality for sharks and field-tested electropositive metals as bycatch deterrents in longline fisheries. RESULTS: Areas of high bycatch were concentrated in southern Canadian waters bordering the United States: upper Bay of Fundy, Georges Bank, Browns Bank, and Emerald Basin, however northern latitudes were associated with bycatch of deep-water species - which are in need of greater attention. These patterns were driven by both species' abundance and fishing intensity. In 2012, total discard amounts were estimated at 3250 mt (2722-3849, 95% credible intervals) for sharks and 1772 mt (1642-1911) for skates and rays. Total bycatch of large sharks was higher than previously assumed in pelagic longline fisheries. For bycatch mitigation circle hooks reduced post-release mortality, but electropositive shark repellents were not effective in reducing bycatch in pelagic longline fisheries. CONCLUSION: This dissertation clearly demonstrates the high spatiotemporal variability and inherent complexity of bycatch and supports the use of geostatistical models and fine-scale spatial management for elasmobranch conservation.