Harbour seal (Phoca vitulina) COSEWIC assessment and status report: chapter 8
Limiting Factors and Threats
Phoca vitulina concolor
Harbour seals were the target of bounty programs in eastern Canada as recently as the 1970s, This drastically reduced the size of the population and eliminated them from some areas of their range (Boulva and McLaren 1979). Some direct hunting of harbour seals still goes on (Sjare et al. 2005; Yetman 2005) although the species is officially protected from hunting in Atlantic Canada. Stewart et al. (1986) note a low-level hunting effort for harbour seals in communities throughout the Arctic.
Harbour seals are also shot by fishermen because of interactions with fish and aquaculture operations (Jacobs and Terhune 2000; Conway 2005). Though difficult to quantify, this mortality may be significant in some areas (Conway 2005). In St. Pierre and Miquelon a small number of seals are shot by hunters and tourists each year (Lawson 2006). Harbour seals are also known to be killed incidentally in fishing gear in Newfoundland, Labrador, Gulf of St. Lawrence and Bay of Fundy groundfish gillnets, Atlantic Canada and Greenland salmon gillnets (which are still pursued for subsistence), Atlantic Canada cod traps, and Bay of Fundy herring weirs (Read 1994; Réseau d’observation de mammifères marins 2004). In addition, harbour seals are killed in PEI fixed gear fisheries and at least one mobile fishery for bluefin tuna (Thunnus thynnus) (Cairns et al. 2000). The scale of this incidental mortality is unknown (Baird 2001).
Harbour seals occur in coastal areas and frequently come into contact with humans. Robillard et al. (2005) summarized a variety of studies that have pointed to motorized boats, kayaks and canoes, barking dogs and people walking on beaches as important sources of disturbance of seals at haul-out sites. Experimental evidence indicates that harbour seals are easily disturbed by small boats (Henry and Hammill 2001). Seismic activity from proposed oil and gas exploration (e.g. southeastern Gulf of St. Lawrence) could cause physical damage to hearing and result in distribution changes due to noise or changes in food distribution (Hammill et al. 2001).
Contaminants in eastern Canadian harbour seals such as heavy metals, trace elements, persistent organic pollutants (POPs) and polybrominated diphenyl ethers (PBDEs) (Lebeuf et al. 2003; Sjare et al. 2005) are potentially toxic for marine wildlife (Ross et al. 1997; O’Hara and O’Shea 2001). Sjare et al (2005) reported that the levels in harbour seals were generally low. However, there is uncertainty about the levels of exposure that are potentially harmful to marine mammal populations. Disease may be a limiting factor for harbour seals, particularly morbillivirus (Duignan et al. 1995) and influenza A (Nielsen et al. 2001), though other diseases such as giardiasis may be relevant (Measures and Olson 1999).
Competition with gray seals has been mentioned as a potentially major contributing factor in the exclusion of harbour seals from some areas of habitat (Robillard et al. 2005; Bowen et al. 2003), though it is unclear whether this competition results in their moving to other areas that are equally suitable. The grouping of harbour seals on haul-out sites likely has an anti-predator function (Terhune and Brillant 1996), and given the significance of killer whale (Orcinus orca) predation on harbour seals in the Pacific (Deecke et al. 2002) this may be a factor in the Atlantic as well. Shark predation has been a major contributor to the decline of harbour seal numbers on Sable Island in recent years (Lucas and Stobo 2000).
In the Arctic, seals are killed opportunistically in many communities (Stewart et al. 1986; Bernhardt 2005). Given the distribution of harbour seals in small groups, the impact of this low-level hunting could be significant. In the Churchill area, hunting is conducted by a few residents who kill less than 5 animals a year (Bernhardt 2005). Historically, Manitoba Conservation personnel killed harbour seals to be used for bait in their polar bear control program. Up to 10 animals were killed annually, but this has not occurred since 1999, when a voluntary hunting moratorium was implemented. Harbour seals are killed during the open water hunt at Arviat (Bernhardt 2005). Shipping traffic into Churchill is a potential disturbance to harbour seals, but the current rate of traffic is low and is not thought to have any effect on harbour seals in the Churchill estuary.
Hydroelectric development along the Churchill and Nelson rivers has included the diversion of flow from the Churchill River into the Nelson during the mid-1970s. It is not known what effect this may have had on harbour seals, but anecdotal accounts indicate that harbour seals may be less numerous in the Churchill River since the diversion. More recently, a rock-fill weir was constructed across the Churchill River at a location just upstream of tidal influence. This project resulted in the displacement of harbour seal haul-out sites downstream by approximately 1 km. Numerous hydroelectric generating stations have been developed along the Nelson River but the effect these stations may have on harbour seals is unknown (Bernhardt 2005).
Phoca vitulina mellonae
At present, the only known cause of human-induced mortality is occasional hunting of this small seal population by Aboriginal people (Clouston 1820; Low 1898; Flaherty 1918; Doutt 1942; Doutt 1954; Consortium Gilles Shooner & Associés et al. 1991; Petagumskum 2005).
None of the habitat of this population is protected. It is entirely on Crown land that could be adversely affected – through changes to lake and river levels – by Hydro-Québec's construction of the proposed Grande Baleine hydroelectric project (Rosenthal and Beyea 1989; Rougerie 1990; Woodley et al. 1992; Smith 1999), which, though indefinitely postponed by the Québec government, has not been cancelled altogether. The Rupert Diversion, a new Hydro-Québec project further south of Grande Baleine, has already begun and overlaps with areas where some freshwater seals have been sighted.
Current fossil fuel energy prices, and concerns regarding air pollution, have prompted a renewed interest in aggressive new hydroelectric development right across Canada (Canadian Hydropower Association 2005). The 2004 Hydro-Québec Annual Report, entitled “Growing Strong”, indicates that “The past year confirmed the resumption of major hydroelectric development projects” (Hydro-Québec 2004, p. 4). One of the results of the Grande Baleine environmental assessment process has been that Hydro-Québec is now required to evaluate the likely impacts of the project on the population, prior to construction (Review Bodies 1994). Some of these potential impacts include the disappearance of ice-free areas and under-ice shoreline shelters, upon which the seals may rely in the winter, in watercourses with altered flows arising from hydroelectric development. The Grande Baleine project may also affect the distribution and abundance of the seals' prey, and contaminate the animals with methyl mercury released from the flooded, decomposing vegetation (Woodley et al. 1992). The negative effects of this habitat destruction could lead to a decline in the seal population and a reduction in its genetic diversity (Alfonso and McAllister 1994).
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