Harbour porpoise (Pacific Ocean population) COSEWIC assessment and status report: chapter 5
Limiting Factors and Threats
A number of potentially limiting factors have been identified from both anthropogenic and natural sources, though the actual impacts of these on the British Columbia population(s) are unclear. Anthropogenic factors fall into two categories: 1) acute impacts (those which directly cause the death of individuals); and 2) long-term impacts, for example habitat loss, changes in prey availability, disturbance from vessels or other sound sources, or factors which might result in reduced reproductive rates or a compromised immune system.
Up until about 1900, harbour porpoises were regularly killed by First Nations people in British Columbia (Boas 1909; Drucker 1951; Suttles 1951; Barnett 1955; Waterman 1973). Today, the primary source of direct mortality is animals being killed incidentally in fisheries. Harbour porpoises seem to be extremely prone to entanglement in fishing gear (Jefferson and Curry 1994). Within British Columbia animals have been documented as being killed in salmon and dogfish drift gillnet fisheries, salmon troll and hake trawl fisheries (Pike and MacAskie 1969; Stacey et al. 1989; Langelier et al. 1990; Baird et al. 1991; Guenther et al. 1993, 1995; Baird and Guenther 1995). The complete loss of animals from the highly developed area of Puget Sound, and the apparent reduction in numbers in areas around Victoria and Haro Strait, are indicative of its sensitivity to human activities. There is little evidence of re-colonization of southern Puget Sound in the last 20 years (Flaherty and Stark 1982; Osmek et al. 1996), suggesting that when a local population is extirpated, recolonization by individuals from surrounding areas is unlikely (see also Chivers et al. 2002).
It is unclear whether incidental mortality may be limiting population growth. Stacey et al. (1990) used a questionnaire survey to examine small cetacean mortality in British Columbia fisheries, and estimated that a minimum of 43-59 deaths per year occurred of three different species (harbour porpoise, Dall’s porpoise and Pacific white-sided dolphins). More recently, Hall et al. (2002) estimated harbour porpoise mortality using a questionnaire and fishery observer reports, based on 5% observer coverage of fishing vessels in southern British Columbia. No entanglements were reported in 2001 in seine net or troll salmon fisheries. However, salmon gillnet fisheries in southern British Columbia killed an estimated 80 harbour porpoise in 2001. Surveying licence holders (1997-2001) suggested a province-wide by-catch of <100 harbour porpoises (Hall et al. 2002), though questionnaire surveys are known to be negatively biased in estimating incidental mortality of cetaceans (Lien et al. 1994).
Anthropogenic influences that could result in reduced reproductive rates include effects from accumulation of persistent toxins, disturbance by vessel traffic, and displacement from prime habitat by sources of high-level underwater sounds (e.g., high amplitude seal "scarers" used at aquaculture operations). In terms of effects of toxins, both long-term and acute effects might be important. Harbour porpoises appear to have among the highest levels of dioxins and furans of any cetacean in the Strait of Georgia, as well as high levels of organochlorines and heavy metals (Muir and Norstrom 1990; Baird et al. 1994; Jarman et al. 1996). High levels could affect reproduction, immune function and endocrine function (Ross et al. 1996a, 1996b, 2000). Immune function suppression can result in acute (immediate) impacts on individuals or on the population. For example, the 1988 morbillivirus-associated mass mortality of harbour seals (Phoca vitulina) in northern Europe, which resulted in a population reduction of over 50%, may have been exacerbated by immunotoxic effects of contaminants (Ross et al. 1996a, 1996b; de Swart et al. 1996). However, whether toxins are currently impacting harbour porpoise in British Columbia is not known. Another source of mortality (most likely of young animals) could be ingestion of marine debris (Kastelein and Lavaleije 1992; Baird and Hooker 2000).
Harbour porpoise appear to be easily disturbed by vessels as well as other sources of high-amplitude underwater sounds, such as acoustic deterrent devices associated with finfish aquaculture operations (Nichol and Sowden 1995). Ferries between southern Vancouver Island and the mainland often travel through areas where harbour porpoise have been frequently observed (Keple 2002). No information is available to assess the impacts of high-speed (and loud) vessel traffic, nor quantify the impacts of sound sources associated with finfish aquaculture operations. However, given the wide distribution of such aquaculture operations within the province and their frequent use of acoustic deterrent devices, it is possible that this source of disturbance may be impacting populations.
Indirect effects, including reduction of their prey base due to habitat degradation or overfishing, may also be important. Assessing the magnitude of this threat is difficult, however. It should be noted that herring, one of the important prey items for this species in British Columbia and elsewhere, are currently being considered for listing as “threatened” in Washington State under the United States Endangered Species Act.
Natural limiting factors may include diseases, predation by killer whales (Orcinus orca) or sharks, competitive or agonistic interactions with other cetaceans (e.g., Dall’s porpoise), and die-offs due to biotoxins. In British Columbia, shark predation is probably quite rare (only one case has been documented – Baird and Guenther 1995, though see Arnold 1972; Anselmo and van Bree 1995), but this species is regularly killed by killer whales (e.g., Morton 1990; Baird and Dill 1995). Rates of mortality from killer whales have not been calculated, but they likely vary among different parts of the province, depending in part on the relative abundance of other preferred prey for mammal-eating killer whales (Baird and Dill 1995). Read (1999) notes that, in some areas, it is possible that agonistic interactions with bottlenose dolphins (Tursiops truncatus) may play a role in determining the limits of harbour porpoise range (see Ross and Wilson 1996). In British Columbia two cases of apparent agonistic interactions with Pacific white-sided dolphins have been documented (Baird 1998; Morton 1999). Osmek et al. (1995) discuss the possible role of competitive interactions with Dall’s porpoise in the decline of harbour porpoise in southern Puget Sound. The diet of the two species around southern Vancouver Island does overlap substantially (Walker et al. 1998). Dall’s porpoise were rarely seen in the Strait of Georgia in the 1950s and 1960s (Pike and MacAskie 1969), yet are relatively common there today (R.W. Baird, unpublished data). Similarly, in southern Puget Sound, when harbour porpoise were common in that area in the 1940s, Dall’s porpoise were never seen (Osmek et al. 1995), yet Dall’s porpoise are relatively common in the area today (Osborne et al. 1988; Miller 1990). Biotoxins or disease outbreaks could, in theory, result in large mortality events. Harbour porpoises are the most frequently reported cetacean to strand on the British Columbia coast (Baird and Guenther 1995), though all strandings are of single individuals, and most (over 90%) animals are dead when found. A number of disease processes contributing to morbidity of harbour porpoises in British Columbia have been identified, including parasitic cholangitis, zygomyotic gastritis, parasitic pneumonia, suppurative pneumonia, and parasitic brochnopneumonia (Guenther and Baird 1993). Unlike the eastern coast of the United States, where there is a spring peak in strandings (Polacheck et al. 1995), strandings are most frequently recorded in late spring and summer (May through September) in British Columbia (Baird and Guenther 1995). Baird and Guenther (1995) suggest that this is due primarily to effort, although the summer occurrence of the salmon fisheries in B.C. (where animals are incidentally caught) may be partly responsible. Two small-scale mortality events (where 10s of animals probably died in each) have been recorded in southern British Columbia. The cause of either event was not determined (Baird et al. 1994; T. Guenther, personal communication). A small-scale mortality event off the Washington coast in 1992 appeared to be caused by bio-toxin poisoning (Osmek et al. 1996).
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