Research Document - 2003/069
Incidental mortality of harbour porpoises in the gillnet fishery of the Estuary and Gulf of St Lawrence, 2000-2002
By Lesage, V., Keays, J., Turgeon, S. and Hurtubise, S.
Abstract
The incidental mortality of harbour porpoises as a by-catch of the gillnet fishery of the Estuary and Gulf of St. Lawrence was evaluated using questionnaires to fishermen in 2000 and 2001, and At-sea Observers and Sentinel Fisheries programs in 2001 and 2002. Of the 2277 fishermen receiving the by-catch questionnaire, 215 (9%) responded, and 165 reported being actively fishing in 2000 or 2001. Of these, 34 (23%) and 45 (27%) fishermen recorded having taken a total of 181 and 291 harbour porpoises in 2000 and 2001, respectively. The largest takes were in July and August from zones 4R, Miscou and the North Shore. These takes resulted in mean by-catch rates of 1.25 (SD = 5.0) and 1.76 (SD = 4.7) porpoises per reporting fisherman in 2000 and 2001, respectively. Extrapolation of these by-catch rates to the entire gillnet fishing fleet resulted in an estimated total by-catch of 2180 (95% CI 1012 - 3802) and 2478 (95% CI 1591 - 3464) porpoises for the Estuary and Gulf of St Lawrence in 2000 and 2001, respectively.
For 2001 and 2002, a total of 786 and 882 bottom-set gillnet hauls that were monitored by At-sea observers recorded harbour porpoise by-catches of 4 and 6 individuals, respectively. At-sea observer activities were conducted in close conjunction with the Atlantic cod and Greenland halibut commercial fishery. However, the low number of hauls that were monitored by At-sea observers prevented the calculation of by-catch estimates for several zones and the study area as a whole, and provided only imprecise estimates for all other zones. Sentinel fisheries resulted in 86 and 77 by-catches in 2001 and 2002, respectively. Depending on the year, incidental takes of harbour porpoises by this fishery peaked in late August or early September, even though their activity peaked earlier, in late July to late August in 2001 and 2002, respectively. The number of takes per haul for the Sentinel fishery was higher than that reported through the At-sea Observer program, even though the former was spread over a longer period, when target species of the fishery might have been less abundant. Significant differences in fishing behaviour were observed between commercial fisheries, commercial fisheries with At-sea observers on board, and Sentinel fisheries. Specifically, Sentinel fisheries soaked nets of similar length but of smaller mesh, at deeper depths, for longer periods, and for a lesser quantity of landed fish than was the case for commercial fisheries with an observer on board. In addition, plotting the fishing locations in the Miscou area (NAFO 4Tn) indicated that at least in August and early September 2001, not only was there no overlap in fishing location between Sentinel fisheries and commercial fisheries under the At-Sea Observer program, but there was also no overlap between commercial fisheries with observers on board and commercial fisheries not subject to an at-sea observation. Commercial and Sentinel fisheries generally followed the 60-m isobath, whereas fishing activities with At-sea observers on board occurred in shallower waters, inside Miscou bank. In 2002, periods of activity by At-sea Observer and Sentinel fisheries in Area 4Tn did not overlap in time, but did overlap spatially.
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