Incidental capture of harbour porpoise, Phocoena phocoena, in three gillnet fisheries of the northwest Atlantic - an investigation of possible factors

Hood, Catherine Catania (2001) Incidental capture of harbour porpoise, Phocoena phocoena, in three gillnet fisheries of the northwest Atlantic - an investigation of possible factors. Doctoral (PhD) thesis, Memorial University of Newfoundland.

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Abstract

There is increased concern for harbour porpoise (Phocoena phocoena) susceptibility to incidental mortality in commercial fisheries throughout their range. In order to obtain information on the incidental capture of harbour porpoise in the western North Atlantic, research was conducted in three fishing regions (St. Bride's, Newfoundland during the summer of 1993, Jeffreys Ledge in the Gulf of Maine during the fall of 1993 and Grand Manan Island in the Bay of Fundy during the summers of 1994 and 1995) where incidental capture of harbour porpoise in groundfish gillnets was occurring. Data were collected on the procedures used in fishing, the environmental conditions at the time of fishing, characteristics of the porpoise caught and the views of the fishermen regarding the issue of harbour porpoise incidental capture in their nets. -- A total of 124 harbour porpoises were captured during 465 observer days when 17,363 nets were hauled. Over three seasons, significant relationships were found between harbour porpoise capture, duration of net soak time and distance of net placement from shore. The depth at which the net was set and the number of nets in a string were related to harbour porpoise bycatch over two seasons. Target species capture varied between seasons, altering the relationship of target species fish and bycatch. For one of the two seasons where mesh size varied, it showed a relationship to harbour porpoise bycatch. Of 85 animals retrieved, 50 were male and 35 female. Lengths and weights of females were greater than males. Estimated age of animals ranged from 0 to 7+ years. Of the total number, 64% of the porpoises were sexually mature, 23% were immature, and 13% were calves. -- Newfoundland porpoise primarily foraged for capelin, sand lance and herring, while Gulf of Maine/Jeffreys Ledge animals ate pearlsides, silver hake and herring; in the Grand Manan Island/Bay of Fundy region the diet was primarily Atlantic herring and silver hake. Atlantic herring occurred in 80% of the stomachs analyzed and was the longest prey fish (44-332 mm). -- Environmental data were collected over the 159 days of the study. Bycatch of the harbour porpoise was correlated with wind speed during both seasons in Grand Manan Island/Bay of Fundy, with cloud cover during the 1993 summer season in Newfoundland, and with water temperature during the 1994 Grand Manan Island/Bay of Fundy season. -- Assessment of elapsed time since death was undertaken to examine the diagnostic usefulness of the vitreous humour and core body temperature in determining postmortem interval. Twenty-four animals from Bay of Fundy bycatches were examined for core temperature and concentrations of various constituents of vitreous humour (glucose, urea, sodium, potassium, chloride, magnesium, calcium, and phosphorus) and the data were compared with published data of rectal temperature and serum concentrations of similar elements in live harbour porpoise. Vitreous humour glucose decreased from antemortem serum values, and the level was positively correlated with core temperature. Potassium and magnesium increased from antemortem serum values. Data suggest nearly all the animals had been dead for several hours. -- Seventy-one fishermen from the Gulf of Maine/Bay of Fundy region were surveyed; most believed soak time of the net, depth of net set and target species harvest are factors related with harbour porpoise capture in gillnets.

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