Harbour porpoise (Pacific Ocean population) COSEWIC assessment and status report: chapter 4

Population Size and Trends

Little information is available on current or historic abundance or trends of harbour porpoises in British Columbia, and the information that is available is restricted to the southern inshore parts of the province (Juan de Fuca Strait, southern Strait of Georgia).

There have been some indications and anecdotal reports of declines of harbor porpoise in some areas of southern British Columbia. Cowan (1987) stated that harbour porpoise abundance in British Columbia seems to be declining, though no information was presented to assess the validity of this statement. One individual who spent considerable time on the Victoria waterfront from the 1940s through the 1980s recalls seeing harbour porpoises from shore on virtually a daily basis in the 1940s and early 1950s, and relatively few after that point (M. Goodwill, personal communication). Harbour porpoises are still found in that area (Baird and Guenther 1994), although sightings are not common, and sightings from shore in the same area as reported from the 1940s are extremely unusual (Baird unpublished). In the nearby waters of Puget Sound, Washington, harbour porpoise were considered the most common cetacean in the area in the 1940s (Scheffer and Slipp 1948), yet are largely absent from the region today (Osmek et al. 1996). Flaherty and Stark (1982) note anecdotal evidence that the population around San Juan Island in 1980 was considerably lower than it had been 20 years earlier. A comparison of sighting records from the area about 10 years after Flaherty and Stark’s (1982) surveys (see also Everitt et al. 1980) suggest a further reduction in numbers or a contraction of range size (see maps in Baird and Guenther 1994). Considerable effort was expended in eastern Haro Strait in the Baird and Guenther (1994) study, shown by large numbers of Dall’s porpoise sightings in the area, so the relative differences in sightings between the studies is likely not due to differences in effort. These sources, albeit anecdotal, suggest that abundance today is likely much smaller than was historically present in this area.

Flaherty and Stark (1982) estimated population size in the adjoining waters around the San Juan Islands in the early 1980s using mark-recapture from photo-identification and sighting rates from a small vessel. Sixteen individuals were photo-identified in each of two periods, and three of the individuals in the second period had been documented in the first. Two population estimates were given, the higher of which was 408 individuals (SE = 358). Flaherty and Stark (1982) also estimated a population of 176 individuals around the San Juan Islands from line-transects surveys.

Estimates of the harbor porpoise abundance in the inside waters of Washington and southern British Columbia were made in 1991 and 1996 (Calambokidis et al. 1992, 1997, Laake et al. 1997). A survey in 2002 has also been conducted but not yet analyzed. Estimates from the 1996 surveys are therefore the most recent available for southern BC waters and are summarized below.

Harbor porpoise abundance estimates for south British Columbia inside waters based on 1996 aerial survey data from Calambokidis et al. (1997).
Region south British Columbia (BC) inside waters Area
(km²)
Effort
(km²)
Number
sighted
Group
size
Density
(animals)
Abundance
Uncorrected

CV
Abund
Corrected

CV
British Columbia Juan de Fuca Strait
1,531
728
60
1.43
0.236
362
0.18
1,239
0.41
BC Gulf Islands
1,350
546
31
1.42
0.161
217
0.38
745
0.53
BC Strait of Georgia
6,370
1,102
20
1.15
0.042
266
0.45
911
0.58
south BC Total
9,251
2,376
111
1.38
0.091
845
0.18
2,895
0.41
United States (US) inside waters
Straits & San Juan
5,108
2,117
148
1.42
0.201
1,025
0.15
3,509
0.40
US & South BC inside waters Total
14,359
4,493
259
1.40
0.130
1,870
0.12
6,404
0.38

Quantitative estimates of harbor porpoise abundance in the Strait of Juan de Fuca and San Juan Islands have not provided any indication of a recent decline. Estimates of abundance for the San Juan Islands from aerial surveys in the 1990s (1,121 and 1,616) were considerably higher than either the estimates from vessel transects or photo-ID made in the early 1980s by Flaherty and Stark (1982). This comparison is complicated by differences in methods and areas surveyed. Estimates from aerial surveys revealed higher density rates in 1996 than 1991 for the two subareas of British Columbia and two in Washington inside waters surveyed in both years, although these differences were not statistically significant (Calambokidis et al. 1992, 1997).

Assessments of harbor porpoise in US coastal waters to the north and south of British Columbia have revealed fairly large populations. In addition to the harbor porpoise in inland waters of Washington State, the stock for coastal waters of Oregon and Washington from Cape Blanco to Cape Flattery was estimated at about 40,000 in 1997 (Laake et al. 1998). To the north of British Columbia, populations of harbor porpoise in southeastern Alaska including both inside and outside waters have totalled about 10,000 animals (Angliss et al. 2001).

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