Harbour porpoise (Northwest Atlantic population) COSEWIC assessment and status report: chapter 10

Population size and trend

There are no range-wide estimates of the abundance of harbour porpoises in eastern Canada and, in fact, much of the range of the species has never been surveyed. Surveys have been conducted in the Gulf of St. Lawrence and the Bay of Fundy-Gulf of Maine, but there are no estimates of abundance from Newfoundland or Labrador (Department of Fisheries and Oceans 2001). 

Aerial line transect surveys were conducted for cetaceans in the Gulf of St. Lawrence during the summers of 1995 and 1996 by Kingsley and Reeves (1996). The 1995 survey was conducted in late August and early September and sampled most (69%) of the Gulf. The 1996 survey was conducted in late July and early August and focused on the shelf adjacent to the north shore of the Gulf, so the two surveys are not directly comparable in extent or timing. The estimates of abundance for the 1995 and 1996 surveys were 12 100 (CV = 0.26) and 21 720 (CV = 0.38), respectively (Kingsley and Reeves 1996). The highest densities were observed in the northern Gulf and particularly along the north shore shelf. Neither survey design allowed for correction of g(0), the probability of detecting an animal on the survey trackline. Some porpoises were submerged when the survey plane passed and were unaccounted for; thus, estimates of density derived from both surveys are negatively biased. 

Four shipboard and aerial line transect surveys were conducted by the U.S. National Marine Fisheries Service to estimate abundance of harbour porpoises in the Bay of Fundy and Gulf of Maine (summarized in Waring et al. 2001). These surveys were conducted in July-September of 1991, 1992, 1995 and 1999 (Table 2). The surveys conducted in 1991, 1992 and 1995 sampled the northern Gulf of Maine and lower Bay of Fundy; in 1999 survey coverage was expanded to include the entire Gulf of Maine, including northern Georges Bank, and the upper Bay of Fundy. In 1999, porpoises were seen in areas not surveyed during previous years. All estimates were corrected for g(0), the probability of detecting a group of porpoises on the survey trackline, using the direct-duplicate mark-recapture method (Palka 1995a). The shipboard components of all four surveys used two independent teams, searching with naked eyes in non-closing mode. This approach was used to correct for both perception and availability bias. The estimates of abundance resulting from these surveys are provided in Table 2.

 

Table 2. Estimates of harbour porpoise abundance in the Bay of Fundy and Gulf of Maine (data from Waring et al. 2001).
Year Estimate of Abundance CV Abundance in Common Survey Area
1991 37 500 0.29 29 000
1992 67 500 0.23 57 600
1995 74 000 0.20 71 900
1999 89 700 0.22 67 600


The 1991 survey produced a much lower estimate of abundance than the other three surveys (Waring et al. 2001). This difference may have been due, in part at least, to inter-annual changes in porpoise distribution, caused by variation in water temperature and the distribution of prey (Palka 1995b).

All four surveys in the Bay of Fundy and Gulf of Maine covered a common area; the estimate of abundance for this area is presented in the last column of Table 2 (this estimate forms part of the total estimate of abundance). It is not possible to use these latter data to estimate a trend in abundance because an unknown proportion of the population likely would have been outside the common survey area in any given year (Waring et al. 2001). If, for example, more of the population was outside this common area (and perhaps in an unsurveyed area altogether) in 1991, it would not be appropriate to compare the results of this survey with those from more recent years. Thus, even for the best-studied portion of the eastern Canadian population, we have no data on trends in abundance.

There are no estimates of the number of mature individuals in any subpopulation or the effective size of any subpopulation of harbour porpoises in eastern Canada, because of a lack of information on the true sex ratio or age structure (Caswell et al. 1998). Existing information on sex ratios and age structure have been obtained from samples of fisheries bycatches and strandings, which are unlikely to be representative of the populations from which they were derived.

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