Research Document - 2004/133

Distribution and abundance indices of marine mammals in the Gully and two adjacent canyons of the Scotian Shelf before and during nearby hydrocarbon seismic exploration programmes in April and July 2003

By Gosselin, J-F., J. Lawson

Abstract

The Sable Island Gully is a submarine canyon on the eastern Scotian Shelf that provides habitat to a wide diversity of species including the endangered northern bottlenose whale (Hyperoodon ampullatus). Seismic surveys for hydrocarbons were conducted in waters adjacent to the Gully in the spring and summer of 2003. An effort to evaluate marine mammal species composition, distribution, and abundance within the Gully prior to, and during these seismic surveys was coordinated by the Centre for Offshore Oil and Gas Environmental Research (COOGER). Vessel-based line transect surveys were conducted in the Gully over areas of 1565 km² and 2218 km² before and during seismic operations and over an area of 1851 km² covering two adjacent marine canyons (Shortland and Haldimand Canyons) only before seismic activities. Visual detections were accomplished by a team of two observers and a recorder from a location 7 m above the sea aboard a research vessel, 37 m in length, following a saw-tooth transect design at 18.5 km/h. In the Gully, 148 km were surveyed on 30 April prior to seismic data acquisition, and a total of 395 km were surveyed on 8, 10 and 11 July while seismic operations were underway. In the Shortland and Haldimand Canyons, 175 km of lines were surveyed on 1 May. Seven species of marine mammals in 45 groups (84 individuals) were identified in both areas in spring, with northern bottlenose whale being the most abundant of detected species with three groups (13 individuals) in the Gully and one group (5 individuals) in Shortland Canyon. In July, 11 species in 207 groups (563 individuals) were identified in the Gully, where northern bottlenose whales (8 groups, 35 individuals) were outnumbered by common dolphins (Delphinus delphis), pilot whales (Globicephala sp.) and grey seals (Halichoerus grypus). Four species of large whales were identified during the surveys, with fin (Balaenoptera physalus) and sperm whales (Physeter macrocephalus) detected in both spring and summer, and blue (Balaenoptera musculus) and humpback whales (Megaptera novaeangliae) detected only in summer. Estimated abundance, not corrected for animals missed on the track-line (i.e. g(0)=1), of northern bottlenose whales in the Gully were 44 (95%CI: 19–105) in April and 63 (95% CI: 20–230) in July. Fin, humpback, sperm and blue whales, combined into a large whale category for the Gully, were estimated to number 89 (95% CI: 31-254) in April and 114 (95% CI: 61-214) in July. Abundance in the Gully of common, Atlantic white-sided (Lagenorhynchus acutus), bottlenose dolphins (Tursiops truncatus), and harbour porpoises (Phocoena phocoena), combined as one group, was estimated at 121 (95% CI: 21–686) in April and 1763 (95% CI: 849-3659) in July. Changes in composition, distribution and abundance of marine mammal species between the spring and the summer surveys most likely represent seasonal variation rather than an effect of seismic activity. Since we had to use a uniform model for density estimation of northern bottlenose whales, and did not correct for sighting availability and detection on the track-line for any species, the densities and abundances presented here are likely underestimated.

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