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Recovery Strategy for Blue, Fin, and Sei Whales in Pacific Canadian Waters [Proposed]

5. Threats (cont'd)

5.2.3. Pollution

Large whales may be exposed to pollution in a number of ways, including the ingestion of marine debris or contaminated prey items, or through contact with oil spills. O'Shea and Brownell (1994) concluded that there was no evidence of toxic effects from metal or organochlorine contamination in baleen species (see also Sanpera et al. 1996), largely because they feed at relatively low trophic levels. No effects of oil contamination were detected for humback whales after the Exxon Valdez oil spill in Prince William Sound (von Ziegesar et al. 1994). However other, primarily piscivorous, marine mammals are thought to be at risk from immunotoxic chemicals (Ross 2002). Pollution effects that have been observed in marine mammals include depression of the immune system, reproductive impairment, lesions and cancers (Aguilar et al. 2002).

Concentrations of organochlorines sufficient to warrant concern were found in fin whale samples taken in the Gulf of St. Lawrence in 1991-92 (Gauthier et al. 1997). However, a retrospective analysis comparing these samples to earlier ones collected in 1971-72 off Newfoundland and Nova Scotia found that the St. Lawrence concentrations were significantly lower (Hobbs et al. 2001). Fin whales feed at a similar trophic level to sei whales, thus, the risk from chemical bioaccumulation in sei whales is likely to be similar, and possibly even lower for blue whales. Decreasing trends have been found for other marine mammals (principally pinnipeds) in eastern Canada (Hobbs et al. 2001). However, Muir et al. (1999) found that organochlorine contaminants in cetaceans show both increasing and decreasing trends, depending on species and geographic location.

Marine debris is a recognized threat to smaller marine mammals but poses less risk to larger species. It is possible for balaenopterids to ingest marine debris during feeding. However, the frequency and consequences have not been quantified and no associated mortality has been reported.

5.2.4.  Habitat Displacement

Balaenopterid habitat can be displaced by changes in ocean climate, or by changes in trophic structure. The Pacific Ocean climate responds to interannual (e.g., El Niño) and decadal scale (e.g., Pacific Decadal Oscillation) variability. These natural cycles can combine to cause regime shifts – significant changes in the physical and biological structure of the ocean. Ocean climate may also be affected over the long-term due to anthropogenic causes (e.g., global warming), however the effect of human-induced changes will be difficult to distinguish from natural variability.

Regime shifts cause major changes in ecological relationships in marine systems over large-scale oceanographic areas (Francis and Hare 1994), and are manifested earlier at lower trophic levels (Benson and Trites 2002). Significant declines in zooplankton abundance have taken place off California since the 1970s and have been linked to increases in sea surface temperature (Roemmich and McGowan 1995).

The displacement of balaenopterid foraging habitat by regime shifts may occur because the timing and spatial distribution of zooplankton abundance can be directly related to physical conditions. How large whales locate suitable foraging habitats is unknown. However, matrilineal fidelity to feeding grounds has been observed in other baleen species (humpback, right, and grey whales). Such fidelity implies a limited ability to locate new feeding grounds when changing oceanographic conditions lead to a significant shift in prey distribution.

Trophic structure can also be affected by overfishing. For example, the massive reduction in whale biomass due to large-scale commercial whaling in the Antarctic is thought to have released as much as 150 million tonnes of krill annually, resulting in an increase in smaller predators such as seals, small cetaceans, and seabirds. The loss of krill consumers in the Bering Sea due to commercial whaling may also have influenced the dominant fish species observed in the Bering Sea during the 1970s and 1980s (Trites et al. 1999), although this change has also been related to regime shifts.

The krill fishery in Pacific Canada is restricted to a few mainland inlets and the Strait of Georgia, and because there is an existing moratorium on the expansion of this fishery, blue whales are currently not at risk from direct competition with this fishery. Commercial harvesting of herring, sardines, or other forage fish is more wide-spread, and thus has the potential to alter the coastal distribution of fin whales, and, to a lesser degree reduce the frequency with which sei whales venture into coastal waters to feed. However, given the complexity of trophic interactions, it is possible that feeding habitat of balaenopterid whales could be enhanced through the removal of competing fish stocks.

Blue whales feed exclusively on zooplankton, primarily euphausiids. In Pacific Canada, fin whales eat euphausiids and schooling fish, whereas sei whales have a more diverse diet that includes copepods and forage fish (which prey on zooplankton). All three species require high density prey aggregations for successful feeding. Such concentrations depend on physical oceanographic factors such as current flows, temperature, and phytoplankton growth. Given the narrower range of prey types, the blue whale may be relatively more sensitive to declines in zooplankton production than either the fin or sei whale. However, the true impacts of changes to ocean climate on the abundance and distribution of zooplankton are unknown. A local increase in zooplankton off the British Columbia coast as a result of changing ocean conditions resulting from climate change cannot be ruled out.

5.2.5.  Other threats

While gear entanglements do result in some mortality of large whales on the east coast, there has been little evidence of gear-related injury or mortality for balaenopterid whales in the eastern North Pacific. The NMFS Pacific Take Reduction Plan, implemented in 1997 to reduce by-catch from fisheries, has not documented any blue or sei whale kills from 1997-2001 (Barlow and Cameron 2003). A review of stranding reports from 1990 to 1996 for Canada’s Pacific coast reported several incidents of entangled, unidentified large whales, and a fin whale was observed entangled in what appeared to be a crab-pot line during a 2004 survey (COSEWIC 2004). As the species begin to recover, the potential for gear interactions may increase, particularly for fin whales using more nearshore waters where such interactions are more likely to take place.

Whale watching tours targeting blue whales in the North Pacific are based primarily off California and in the Sea of Cortez, Mexico. Whale watching industries in British Columbia primarily target killer whales, grey, and humpback whales. However given their size, blue and fin whale watching would very likely increase should such trips become economically feasible for commercial operators. Potential impacts on blue and fin whales from whale watching would include injury from propellers and vessel strikes, and increased acoustic disturbance (see above). Sei whales are unlikely targets for any whale watching operation in Pacific Canadian waters because of their primarily offshore distribution.