Aerial surveys in the 1960s, 1970s, and early 1980s counted belugas in Cook Inlet but only a few of these had sufficient coverage to estimate the population size (Calkins 1984, 1989). A survey in 1979 resulted in an estimate of 1,293 whales using a correction factor of 2.7 developed to account for submerged whales under similar conditions in Bristol Bay (Calkins 1989). This is the best available estimate of historical beluga abundance in Cook Inlet, and represents the maximum observed size of this population. Therefore, based on the best information available, NMFS has adopted this maximum historical abundance estimate, rounded to 1,300 belugas, as the estimated carrying capacity to be used for management purposes (65 FR 34590, May 31, 2000). We have no data at this time to indicate whether this carrying capacity may have changed.
Between 1979 and 1994, the CI beluga population declined from 1,300 to 650 belugas, which represents an average annual decline of around 5% (i.e., 650 = 1300*0.955(1994-1979)). While the decline between 1994 and 1998 is well documented and attributed to unsustainable subsistence harvest, empirical data are lacking for the period between 1979 and 1994 to identify a mechanism of decline. Native subsistence harvest (enumerated through hunter interviews) was significant during the 1970s and 1980s and may have been at levels similar to the hunts reported in the mid-1990s, but there was no comprehensive count of subsistence harvest until the 1990s (Mahoney and Shelden 2000). Commercial and sport hunts also occurred during the 1960s and 1970s, but no information is available to assess whether the 1979 abundance estimate of 1,293 (based on the 1979 ADF&G survey; Calkins 1989) may represent a partially depleted population, and thus a conservative estimate of Cook Inlet carrying capacity for belugas.
Recent Abundance Estimates and Population Trends
NMFS began conducting comprehensive, systematic annual aerial surveys of the beluga population in Cook Inlet in 1993 (Hobbs et al. 2015b). Beginning with the 2012 annual survey, the survey schedule was switched from annually to biennially, to occur in even numbered years (see Hobbs 2013). These surveys occur in early June (except in 1995 when the survey was in late July), include the upper, middle, and lower sections of the Inlet, and are stratified to focus survey effort in the areas of the upper Inlet where belugas are typically at their highest concentrations during June. At the time of publication of this document, aerial surveys had been flown in 2016; however, the data analysis to determine the 2016 abundance estimate had not been completed.
Annual estimates of the numbers of CI belugas resulting from these surveys documented a decline in abundance of nearly 50% between 1994 and 1998, from an estimate of 653 whales to 347 whales (Table 3). This period of rapid decline was associated with a substantial, unregulated subsistence hunt; although the hunt was regulated starting in 1998, CI beluga numbers did not increase. An analysis indicated the decline in beluga abundance from 1994 to 1998 was adequately explained by the estimated take from the subsistence hunt. With the very limited hunt beginning in 1999 (a total of five whales hunted from 1999–2014, 16 years) NMFS anticipated that the population would begin to increase at a growth rate of between 2% and 6% per year. The 2014 abundance estimate was 340 belugas, with a declining trend for both the most recent 10- year time period (–0.4% per year; standard error [SE] = 1.3%) and since the hunt was managed in 1999 (–1.3% per year, SE = 0.7%) (Shelden et al. 2015a; Figure 13). Thus, the population is not growing as expected despite the regulation of the subsistence harvest.
|2005||May 31-June 9||278||0.1|
|2011||May 31-June 9||284||0.09|
|2012||May 29-June 7||312||0.13|
a Surveys in 1993 were not suitable for analysis using the abundance estimation methods of 1994 and later. No surveys were conducted in 2013 or 2015.
b CV estimates prior to 2011 used the method of Hobbs et al. (2000) in previous publications. These have been recalculated using a revised CV formula based on the standard error of the daily abundance estimates and an estimate of the variance in behavior of the whales which better reflects the sources of variability in the estimate. The method for calculating the CVs was revised in 2011; CV’s for older estimates have been recalculated using the new formula.
Source: Shelden et al. 2015a; Hobbs et al. 2015b.
Small Population Dynamics
Small populations, such as the CI beluga population, may face inherent risks that large populations do not, simply as a result of their small population size. Small population dynamics may be at play when the impact to individual survival and fecundity increases as the population abundance decreases, or when there are persistent effects that result from a population having been small at an earlier time. These small population dynamics may manifest in various ways, including inbreeding, loss of genetic or behavioral diversity, or Allee effects. The Allee effect refers to a positive relationship between individual fitness and either abundance or densities of individuals (Stephens et al. 1999). For example, a very small population may experience Allee effects such as reduced reproductive success due to difficulties finding mates or reduced foraging success due to difficulties in locating prey. Reduced population sizes could mean reduced breeding opportunities and an increased potential for breeding with relatives.
If a population remains small, genetic diversity will decrease with each generation, resulting in a greater risk of extinction. Even if the population later increases in size, there may still be lingering consequences of the low genetic diversity. Reduced genetic diversity could result in:
- Increased susceptibility to disease due to reduced variety of immune responses within inbred individuals, such that each beluga is more susceptible to a disease organism and also more likely to suffer severe symptoms.
- Increased risk of epidemic disease due to loss of variability among individuals. With more similarity among individuals, the disease organism also requires less adaptation among individuals, resulting in greater virulence and more rapid spread.
- Decreased resilience to environmental change at both individual and population levels. Individual belugas will have a more limited phenotypic (i.e., the observable properties of an organism that are produced by the interaction of the genotype and the environment) response to changes in the environment; this limited response will narrow the adaptive range for the population.
- Decreased fecundity due to failed pregnancies and birth defects. With loss of diversity in the population, the likelihood increases for a fetus to develop a phenotype with decreased survival, resulting in a lost reproductive opportunity and reducing the net number of offspring that a female produces over her lifetime.
While these are potential consequences of small population size, NMFS concluded that the Allee effect is not a relevant concern for CI belugas unless the population size is smaller than 50 animals (Hobbs et al. 2006). Similarly, inbreeding depression and loss of genetic diversity do not pose a significant risk to CI belugas unless the population is reduced to fewer than 200 whales (Hobbs et al. 2006).
Little is known about the social structure of CI belugas and how it relates to effective population size. Social structure may limit who and how many belugas breed, resulting in a lower effective population size and reproductive capacity than the population size and age-sex composition would indicate. In addition, some behaviors are transmitted from parents to offspring in other better-studied matriarchal odontocetes. In these other matriarchal odontocete groups, behavioral variation of females is passed to their offspring, much like genetic variation (e.g., Würsig and Pearson 2015). As a result, social units or groups within the same population might display significant behavioral differences. Seasonal foraging strategies and site fidelity are examples of learned behaviors. Belugas show strong site fidelity, which may be learned during the period of dependence when the mothers teach the weaning calves to forage. Loss of behavioral diversity could result in:
- Reduced spatial distribution, increased risk of stranding, reduced prey choices, and reduced predatory efficiency due to fewer learning opportunities and greater similarity of experiences among remaining females.
- Decreased juvenile survival due to a reduction of learned recognition of habitat and resources, such as alternative prey, refuge from predators or disturbance, or other use- specific areas (Wade et al. 2012).
- Reduced socialization with fewer opportunities to learn foraging techniques, mating, group cohesion, and hierarchical definition or strengthening, as well as a reduction in mutual defense against, or avoidance of, predators. A decline in the population will be paralleled by a reduction in behavioral diversity.
- Overall fitness and resilience to perturbations such as catastrophic events.
CI belugas have exhibited a marked contraction of their summer habitat range. If CI belugas are matriarchal and pass knowledge from female to offspring, it is possible that some knowledge regarding preferred summer habitats in mid- and lower-Inlet might not have been passed on to the current generation. If this is the case, it is unknown how long it would take for these habitats to be recognized again by individuals in the current or a recovered CI beluga population. Our knowledge regarding CI beluga social structure and differences in behavior among groups is quite limited, but the available information indicate that large groups of CI belugas observed in the Susitna River Delta do not appear to be segregated by color or age-class, with most groups consisting of both white and gray animals (McGuire et al. 2014b). Photo-identification studies of the upper CI also suggest that most, and perhaps all, of the CI beluga population uses Eagle Bay seasonally, with 90% of CI belugas also having been seen elsewhere in upper CI (McGuire et al. 2014c). Thus there seems to be significant intermixing of age groups/color classes and a high resight rate of individuals in multiple locations, and at this time we have no information to suggest there has been a loss of behavioral diversity in the CI beluga population.
Although reduction of range likely increases the risk of extinction, the implications of this shift are not entirely clear and are in need of investigation. Range contractions generally increase vulnerability to catastrophic loss from stochastic events and point sources of disturbance, disease, and mortality. These risks may have become exacerbated in the CI beluga population by a range contraction to the area of greatest human impact. It is not known how the range contraction may have altered behavior and habitat use within the consistently occupied areas in the upper Inlet. With fewer whales, prey may be relatively more abundant, thus reducing competition and the need for more wide ranging movements. Concentrating in large numbers in discrete areas appears to be a basic trait of belugas and a strategy by the Cook Inlet population. While likely increasing vulnerability to catastrophic events, such behavior may reduce risk from other factors such as predation. It is essential to focus research on understanding both the cause and implications of the range contraction in CI belugas.
CIB Population Viability Analysis (PVA)
A population viability analysis (PVA) for CI belugas that was completed at the time of listing under the ESA indicated a risk of extinction in 100 years between 1% and 27% (Hobbs and Shelden 2008). The model that NMFS considered to best represent the risk to the CI beluga population indicated a 26% probability of extinction in 100 years. The detailed PVA population model used the abundance estimates for 1994 to 2008 and accounted for immature and mature stages of both sexes (Hobbs and Shelden 2008). The PVA was based on a Bayesian analysis using a population model that accounts for the removals from the population by the subsistence hunt, births and deaths in the population, and time lags in the response of the population to changes. More recently, Hobbs et al. (2015c) developed another PVA analysis that incorporated five additional abundance estimates (for the years 2010, 2011, 2012, and 2014). This recent PVA included some model scenarios that were not included in the 2008 analysis, and the values for some model parameters differed from those used in corresponding models in Hobbs and Shelden (2008). There was considerable variation in risk of extinction at 100 years among the models, with the probability of extinction (among the five models that best fit the existing CI beluga data, along with the model accounting for risk of catastrophic events) ranging between 0% and 14%. Based on the modelling results, the authors concluded it is likely the CI beluga population will continue to decline or go extinct unless factors determining its growth and survival are ameliorated.