Beluga whale (Delphinapterus leucas) COSEWIC assessment and status report: chapter 7
The ages of belugas are determined by counting annual growth layer groups (GLGs) in the dentinal or cemental tissues of their teeth. Uncertainty still exists as to whether one or two GLGs represent a year of growth (Sergeant 1973, Perrin and Myrick 1980, Brodie 1989). It is still generally accepted, and there is some pertinent evidence (Goren et al. 1987, Heide-Jørgensen et al. 1994), that two GLGs represent a one-year interval. The following description of life history is based on this assumption.
Female belugas mature sexually between 4-7 years, males somewhat older at 6-7 years (Brodie 1971, Sergeant 1973, Burns and Seaman 1985, Doidge 1990a, Heide-Jørgensen and Tielmann 1994). Mating occurs during late winter to early spring; there is likely a spread in the timing of implantation from March until late June (Kleinenberg et al. 1964, Brodie 1971), with the peak of mating occurring before mid-April (Burns and Seaman 1985). It is generally held that there is no delay in implantation of the single blastocyst but Burns and Seaman (1985) note that this cannot yet be stated with certainty. Gestation is estimated to be in the order 12.8 (Doidge 1990b) to approximately 14.5 months (Kleinenberg et al. 1964, Brodie 1971, Sergeant 1973).
The peak calving time is not well established. It appears to occur during the late spring migration in offshore areas (Béland et al. 1990) and thus is not easily observed. Calving in estuaries, in early summer, has been postulated (Sergeant 1973), but detaÎled studies, in at least two Canadian estuaries, have never recorded a calving event (Caron and Smith 1990, Smith et al. 1994). Instead it was seen that females arriving in these areas were already with neonate calves. Their number would gradually increase from mid-June to early July. It is assumed that calving is probably spread out from June to August (Brodie 1971, Sergeant 1973), with the peak probably occurring in mid-June to early July. Lactation is estimated to last from 20 months (Brodie 1971, Sergeant 1973, Burns and Seaman 1985, Heide-Jørgensen and Teilmann 1994) to 32 months (Doidge 1990).
Doidge (1990a) estimated that there was a period overlap of lactation and pregnancy of 175 days, resulting in a complete cycle between successive pregnancies of 3.25 years. This is similar to the estimates of other authors for full reproductive cycles of 36 months (Sergeant 1973, Burns and Seaman 1985), but differs in that the actual lactation period is estimated to be much longer and overlapping the new pregnancy.
Almost nothing is known of the breeding behaviour of belugas since mating occurs in the offshore ice-filled waters. Adult males segregate from females with calves and juvenÎles during their period of summer aggregation in estuaries (Michaud 1993, Smith and Martin 1994, Smith et al. 1994). Some evidence from commercial net fishing in Russian and Norwegian (Ognetov 1981, Lønø 1961) waters indicates that there is also sexual segregation during the seasonal migrations. No direct information exists on mating systems in belugas which, in the future, might be elucidated by nuclear DNA studies.
Aboriginal traditional knowledge on the reproductive biology of belugas varies considerably between Inuit hunters and also differs depending on the location of their hunting areas. Mating is reported to occur along ice floe edges in the spring or far offshore. The timing of calving is reported to occur from spring to late autumn (Thomsen 1993, McDonald et al. 2002). Inuit from areas with major estuarine concentrations of belugas, such as Cumberland Sound or the Mackenzie Delta, report calving from July to September (Byers and Roberts 1995, Stewart et al. 1995, Stewart 2001).
Inuit hunters believe that adult females still accompanied by calves do give birth (Thomsen 1993). They cite this as evidence that the three year reproductive cycle postulated by scientists is wrong (Anon. 2001b, McDonald et al. 2002). In contrast, the Inuvialuit of the Beaufort Sea rarely see females with more than one calf. They feel that females with dependent offspring will not give birth (Byers and Roberts 1995).
Belugas are long-lived mammals with mean life spans in the range of 15-30 years. Results from aging of beluga whale jaws in the Eastern Beaufort Sea have identified whales reaching the age of 63 years, with many harvested whales of this population estimated to be in their 40s and 50s (Harwood et al. 2002). Most population parameters available for belugas have been derived from samples of animals harvested by Inuit.
The age specific frequencies obtained from hunted belugas, which form a survivorship series (lx) type of life table, are subject to various collecting biases that make the accurate calculation of the survivorship of the neonate and juvenÎle age classes virtually impossible. Estimates of adult survivorship are in the 0.90 or greater range. In the older age classes, inaccuracy occurs because of tooth wear, which leads to underestimation of age. An independent estimate of the instantaneous rate of increase of the population is rarely available for whale populations so that accurate estimates of survival rates are difficult to obtain (Caughley and Birch 1971).
A different approach, but not free of sampling bias, is the attempt to obtain mortality rates from the age structure of recovered carcasses of animals that were not hunted. The problems of estimating such mortality factors as they occur in the live population (Martineau et al. 2002a) stem from the lack of representativeness in the age structures of the recovered carcasses (Theriault et al. 2002). Additionally when these rates are compared to other beluga populations, the comparisons are invalid largely because the age structures are derived using different sampling methods (Hammill et al. 2003).
From other lines of evidence in similar odontocete populations (Smith 1999) and a limited amount of data from repeated aerial surveying (Kingsley 1998), it is felt that unexploited beluga populations, which are below carrying capacity, could increase at a rate in the range of 2.5 to 3.5 percent per annum.
Belugas have the lowest body fat content when they first occupy their estuarine summer habitat (Doidge 1990b). In late summer they appear to begin to feed intensively, often in deep areas somewhat distant from their centres of summer distribution (Smith and Martin 1994, Richard et al. 2001a,b). WhÎle we have as yet no direct evidence of the prey species involved, the Arctic cod, Boreogadus saida, is known also to have a deep water distribution in these same areas (Bradstreet et al. 1986, Welch et al. 1993). Large concentrations of Arctic cod in surface waters are also occasionally seen in the early autumn, which provide important feeding opportunities for belugas and other Arctic marine mammals and birds (Finley et al. 1990, Welch et al. 1993). The Inuit of Arctic Bay and Cumberland Sound also report that belugas feed extensively on Greenland halibut, Reinhardtius hyppoglossoides, at the floe edge (Stewart et al. 1995, Kilabuk 1998). In the Beaufort Sea they feed on a variety of fish (Byers and Roberts 1995). Elsewhere in the Arctic they consume capelin (Mallotus villosus) and saffron cod (Eleginus novaga). In most areas invertebrates are also frequently found in the stomachs of belugas (Vladykov 1946, Seaman et al. 1982, Sergeant 1962, 1973, Watts and Draper 1986).
To date there has been no indication of inter-annual changes in the body condition or nutritional state of wild-harvested belugas in any scientific studies. In Cumberland Sound, Inuit harvesters have noted that belugas appear to be thinner than in the past. They relate this to increased energy expenditure caused by avoidance of increased boat traffic (Kilabuk 1998). No detailed studies of energy budgets of free ranging belugas have yet been conducted.
Belugas are vulnerable to predation and to over-hunting, and to other anthropogenic threats because of their strong philopatry to certain sites of summer aggregation. Belugas occupy these estuarine sites to moult, to avoid predation or to feed. It is thought that exposure to warm fresh water helps the sloughing of the dead stratum externum, and accelerates the epidermal cell growth (St.-Aubin et al. 1990). Past theories postulated that estuaries provided a thermal benefit to neonates and that they were calving areas (Sergeant 1973, Fraker et al. 1979), but this has been contradicted by behavioural studies in estuaries (Finley 1982, Smith et al. 1994), and by measurement of the insulative values of the epidermis of newborn belugas (Doidge 1990b). In some areas, river estuaries might be feeding sites and also provide shelter from predators such as killer whales (Brodie 1971).
Belugas are extremely tenacious in their occupation of their traditional centres of aggregation, even in the face of continued disturbance and threat of being killed (Caron and Smith 1990). In the past, commercial hunts, which harvested large numbers of belugas from such areas as the Mucalic Estuary in Ungava Bay (Finley et al. 1982, Reeves and Mitchell 1989) and the Great Whale River in eastern Hudson Bay (Francis 1977, Reeves and Mitchell 1987), are thought to have exterminated whole populations. The Inuit of Ungava Bay, Hudson Strait and eastern Hudson Bay all mention that many small river estuaries, formerly frequented by belugas are now rarely used by the whales. It is generally held by Inuit that increased noise from outboard motors is the primary cause and that this has shifted distribution of belugas offshore (Doidge et al. 2002, Lee et al. 2002). In the St. Lawrence, the southernmost beluga population lives in a heavily traveled maritime route. There, its range has been reduced significantly and the remaining belugas continue to face anthropogenic challenges, such as increasing noise (Lesage 1993, Lesage et al. 1999), loss of habitat, and pollution (Béland et al. 1993).
In the autumn belugas leave their summer areas and migrate long distances to their winter habitats. Often these areas are shared by more than one population, but details of behaviour and distribution are still lacking from this time of year. It is in those areas that mating takes place.
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