Grey whale (Eschrichtius robustus) COSEWIC assessment and status report: chapter 8
Biology
General
The migration of the eastern North Pacific population of grey whales between its subtropical breeding grounds and temperate or arctic feeding grounds is the longest known migratory route of any species of mammal. Grey whales are generalist feeders and are the only species of baleen whale known to regularly feed on benthic prey. For large baleen whales, grey whales have a relatively high reproductive capacity.
Reproduction
Calving is largely confined to the subtropical breeding lagoons (Rice et al. 1981), although occasionally females give birth off the California coast (Sund 1975). Sexual activity has been observed year-round (Wilson and Behrens 1982; Clarke et al. 1989), but most calves are conceived in late November and early December during the southbound migration (Rice and Wolman 1971). The animals arrive at the breeding lagoons between December and January, and the median date of birth lies in late January (Rice and Wolman 1971). The gestation period is 13-14 months and females give birth to a single calf (Rice and Wolman 1971; Rice 1983). Only a single incident of twin foetuses has been reported (Blokhin 1987). Photographically identified females are usually seen with a calf every other year (Jones 1990), although the interbirth interval is probably larger in years of poor food abundance. During the 6-month lactation period, calves grow from 4.6 m at birth to 7.0 m. At one year of age, they typically measure 8.0 m (Sumich 1986). Grey whales continue to grow until they are approximately 40 years old (Rice and Wolman 1971). Male and female grey whales attain sexual maturity at an average age of eight years (Rice and Wolman 1971) and Heppell et al. (2000) estimate the average generation time to be 22 years. Data from subsistence whaling (Blokhin 1984) suggest that sexually mature individuals comprise approximately 60% of the population. An annual shore-based census of the population during its northbound migration at various locations along the migratory route determined that calves represented between 4.6 and 5.2% of the population (Herzing and Mate 1984; Poole 1984a, b). However, more recently, percentages as low as 1.1% have been reported (Le Boeuf et al. 2000; Perryman et al. 2002).
Survival
The age of dead grey whales can be determined from growth rings in the ear plugs thought to be deposited annually (Rice and Wolman 1971). Since growth rings may fuse in mature animals, age estimates from ear plugs could underestimate the true age of individuals (Rice and Wolman 1971; Reeves and Mitchell 1988). Rice and Wolman (1971) report a male with 70 growth rings in his ear plugs. No estimates of maximum life expectancy are currently available from studies of photographically identified individuals. However, one summer-resident grey whale first seen as an adult in 1974 was still alive 25 years later (Darling 1984; Deecke 2003).
Mortality of grey whales is highest during the first year of life. Swartz and Jones (1983) estimated calf mortality on the breeding grounds at 5.4%. Sumich and Harvey (1986) suggest that 75% of first-year mortality occurs within a few weeks of birth. Until recently, calves represented by far the largest proportion (91.4%) of dead grey whales on the breeding grounds, followed by yearlings (0-19.5%) and adults (0-5%; Jones and Swartz 1984). Analyses of population estimates suggest overall adult mortality rates of between 0.1 and 5.0% (Punt and Butterworth 2002; Wade 2002). High re-sighting rates of individual summer-resident grey whales between years similarly suggest relatively low levels of adult mortality (Darling 1984; Deecke 1996; Calambokidis et al. 2002). Recently, however, higher levels of adult mortality have been reported (see section ‘Population Sizes and Trends’).
Physiology
Since grey whales feed almost exclusively in the summer, female grey whales depend primarily on stored fats for reproduction (Perryman et al. 2002). A pregnant female has to nourish her foetus on the southward migration, give birth, and nurse her calf on the northward migration without any major intake of food. During this time, her offspring nearly doubles in length (Sumich 1986). Since females with calves are the last to depart from the breeding lagoons, her subsequent feeding season will be severely shortened (3.5 months as opposed to 6.9 months for newly pregnant females and males; Swartz 1986). The energy stores of a female grey whale are therefore probably severely depleted after she weans her calf. This means that interbirth intervals would lengthen under suboptimal feeding conditions, either through suppressed ovulation (Rice and Wolman 1971) or premature termination of pregnancies (Perryman et al. 2002).
Movements/dispersal
Calves stay with their mothers during their first northward migration and possibly learn the location of feeding grounds. In studies using photographic identification of individuals, a few calves were resighted on the same feeding ground as their mothers in following years suggesting some degree of maternally directed site fidelity (Weller et al. 1999; Calambokidis et al. 2002). However, genetic studies so far have failed to find evidence for such maternal transmission (Steeves et al. 2001).
Nutrition
Grey whales were believed to be highly specialized for feeding on benthic invertebrates. However, recent research has shown that grey whales feed on a much broader range of species, suggesting that grey whales are opportunistic feeders (Reeves and Mitchell 1988; Darling et al. 1998).
On the feeding grounds in the Bering, Chukchi and Beaufort Seas, grey whales predominantly feed on epibenthic and infaunal amphipods of the genera Ampelisca, Atylus, and Anonyx (Bogoslovskaya et al. 1981; Nerini 1984). Quantitative studies have shown that amphipods represent 95% of the diet on arctic feeding grounds (Nerini 1984). Grey whales obtain these amphipods by diving to the bottom, rolling over on their side and sucking sediment into their mouth and straining the associated invertebrates through their baleen plates (Ray and Schevill 1974; Bogoslovskaya et al. 1981; Johnson and Nelson 1984; Nerini 1984; Oliver and Kvitek 1984). This benthic feeding can be easily identified by the plumes of mud trailing behind the animals as they surface, and the feeding activity leaves characteristic feeding pits in the sea floor (Johnson and Nelson 1984; Oliver and Kvitek 1984; Nerini 1984; Kvitek and Oliver 1986; Nelson et al. 1987; Weitkamp et al. 1992). Grey whales on arctic feeding grounds also occasionally forage on sand shrimp (Crangon sp.; Gill and Hall 1983).
It appears that the diet of summer-resident grey whales is more varied than that of grey whales feeding in the Arctic. Amphipods (Ampelisca spp., Atylus borealis, Corophium spinicorne) are also an important prey on the temperate feeding grounds off the west coast of North America (Oliver et al. 1984; Avery and Hawkinson 1992; Darling et al. 1998; Dunham and Duffus 2001; 2002), and grey whales feed preferentially in areas where large individuals (> 6 mm) are common (Dunham and Duffus 2001; 2002). Summer-resident grey whales have also been observed to feed extensively on ghost shrimps (Callianassa californiensis) and associated small clams (Cryptomya californica) in shallow muddy bays along the west coast of North America (Weitkamp et al. 1992; Darling et al. 1998; Dunham and Duffus 2001).
In addition to this benthic prey, summer-resident grey whales feed extensively on planktonic invertebrates. In the exposed waters off the west coast off Vancouver Island and elsewhere, mysid shrimps (primarily Holmesimysis sculpta, Neomysis rayii, Acanthomysis spp.) are an important prey (Wellington and Anderson 1978; Murison et al. 1984; Deecke 1996; Darling et al. 1998; Dunham and Duffus 2002). In addition, the animals commonly feed on planktonic crab larvae (Pachycheles rudis, Petrolisthes spp., Cancer magister; Darling et al. 1998; Dunham and Duffus 2001; 2002).
In many areas along the British Columbia coast, the arrival of northbound grey whales coincides with the spawning of herring (Clupea harengus) on coastal eelgrass beds. Grey whales have been observed foraging on herring spawn and larvae in Barkley Sound (Gisborne, pers. comm.), Clayoquot Sound (Darling et al. 1998), as well as the Queen Charlotte Islands (Nichol and Heise 1992; Ford et al. 1994). Feeding on herring spawn may represent an important ‘refuelling stop’ for grey whales migrating to arctic feeding grounds.
Very little feeding is thought to occur on the winter breeding grounds, but where feeding has been observed, amphipods (Ampelisca spp., Aoroides columbiae) and crab larvae again were the dominant food source (Oliver et al. 1983; Nerini 1984; Tershy and Breese 1991; Sánchez-Pacheco et al. 2001). In addition, unidentified bait fish have also been documented as grey whale prey on the breeding ground (Nerini 1984).
Interspecific interactions
In addition to exerting substantial predation pressure on many benthic and planktonic invertebrate communities in temperate and arctic waters, grey whales are of ecological importance to a number of species throughout their range. Grey whales are the hosts of many endo- and ectoparasites (e.g., Blokhin 1984; Dailey et al. 2000), and since grey whales lack close taxonomic relatives, they are the exclusive hosts for many of these species (e.g., grey whale barnacles, Cryptolepas rachianecti and the cyamid crustacean Cyamus scammoni).
Grey whales are part of a variety of symbiotic and commensal interactions. For example, Swartz (1981) describes a cleaning symbiosis between topsmelt (Atherinops affinis) and grey whales on the Mexican breeding grounds. On the arctic feeding grounds, many species of seabirds (e.g. northern fulmar, Fulmarus glacialis; red phalarope, Phalaropus fulicaria; black-legged kittiwake, Rissa tridactyla; and thick-billed murre, Uria lomvia) feed on invertebrates from grey whale mud plumes. Grey whales represent the only means of accessing benthic prey for these species of seabirds (Obst and Hunt 1990; Grebmeier and Harrison 1992).
Killer whales (Orcinus orca) frequently attack grey whales during the migration and on the feeding grounds (Ljungblad and Moore 1983; Lowry et al. 1987; Goley and Straley 1994; Craighead and Suydam 1998). They appear to target predominantly calves and may pose an important source of mortality for immature grey whales. Eighteen percent of grey whales landed at California whaling stations bore scars attributed to killer whale attacks (Rice and Wolman 1971).
Behaviour/adaptability
As generalist feeders, grey whales are probably able to adapt to changes in the abundance of certain food sources. However, it appears that the health of the eastern North Pacific population largely depends on the productivity of benthic habitats in the Arctic.
Increased human activity is probably the main factor affecting grey whale habitat along the migratory corridor. This includes increased industrial noise and increased vessel traffic for shipping, resource extraction and recreation. Grey whales have been shown to avoid loud sources of industrial noise (Richardson et al. 1995; Moore and Clarke 2002). Behavioural responses to boats range from actively seeking contact with boats to active avoidance (Jones and Swartz 1984; Heckel et al. 2001).
Although the importance of acoustic signals for communication and orientation in grey whales is currently poorly understood, it is known that grey whales produce a variety of communicative sounds both on the breeding and feeding grounds (Dahlheim et al. 1984; Moore and Ljungblad 1984; Crane and Lashkari 1996). An increase in anthropogenic noise associated with increased human activity in grey whale habitat may negatively affect this acoustic communication.
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