Sei whale (Balaenoptera borealis) COSEWIC assessment and status report: chapter 8
Limiting Factors and Threats
General factors with the potential to affect baleen whale populations are discussed by Clapham et al. (1999) and include fisheries interactions, ship strikes, whaling, pollution, disease and habitat degradation. However, there are no species-specific factors limiting the recovery of sei whales, and the threats to which they are exposed are indirect.
On the Atlantic coast, where vessel traffic is a serious threat to other whale species, there is a single reported ship strike of a sei whale (Waring et al. 2001). However, due to the pelagic nature of the species, a higher incidence of undetected vessel strikes is possible on both coasts. There are no reports of fisheries-related mortality, entanglement or injury on either coast, (Perry et al. 1999, Waring et al. 2001) although the offshore drift net fishery poses a potential threat (Barlow et al. 1997).
Acoustic disturbance and habitat loss are emerging concerns for all cetacean species, but the effects of acoustic pollution on rorquals is poorly understood. Sources of acoustic pollution include shipping, recreational speed boats, sonar, industrial and military activities, and acoustic thermography (Gordon and Moscrop 1996). The effect of vessel noise appears to depend on the behaviour of the animals and the approaching vessel (Perry et al. 1999). Sei whales are reported to exhibit more avoidance behaviour than fin whales when approached (Gunther 1949 cited in Perry et al. 1999).
Acute, intermittent noise from mineral exploration or military exercises is likely to elicit strong avoidance and at sufficiently high levels may result in mortality for some species (Gordon and Moscrop 1996). Chronic noise, such as that originating from oil platforms, has been shown to cause avoidance behaviour and change in habitat use.(Gordon and Moscrop 1996, Schick and Urban 2000). Recent oil exploration activities and the potential exploitation of reserves throughout Atlantic Canada (on the Nova Scotian shelf, the Grand Banks and the southern Labrador Shelf) could therefore result in habitat degradation for the Nova Scotian stock.
Factors that could potentially limit the persistence and recovery of this species are primarily indirect, and are a reflection of the overall state of the oceans. These include bioaccumulation of toxins, and inter-specific competition for prey items. While marine mammals generally appear to be at risk from immunotoxic chemicals (Ross 2002), O’Shea and Brownell (1994) concluded that there is no evidence of toxic effects from metal or organochlorine contamination on baleen species, largely because they feed at low trophic levels.
Perhaps the greatest threat is the potential replacement of large baleen whales in the ecosystem by ecologically equivalent finfish stocks (Payne et al. 1990). An initial attempt to answer a similar question for the Bering Sea suggests that some species of fish are significant competitors of whales (Trites et al. 1999).
The degree to which sei whales continue to suffer from baleen shedding and parasitic infections is unknown. Some information on mortality may be available from the examination of strandings; however, very little mortality data currently exist. The contribution of these infections to the natural mortality of the species may increase if the animals’ immune systems are compromised by other factors, such as toxin loading.
The sei whale’s use of relatively remote habitats may reduce the impact of some of the threats to which the species may be exposed. However, habitat loss through competition with commercial fisheries, vessel noise and traffic, or seismic exploration cannot be ruled out.
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