Cook Inlet is a semi-enclosed tidal estuary located in southcentral Alaska and is approximately 370 km (230 mi) in length and extends in a northeast/southwest orientation from Knik and Turnagain Arms in the north to the southernmost reaches of Kamishak Bay in the south. Considerable amounts of sediment are naturally deposited into Cook Inlet, creating a highly turbid, low visibility environment, particularly in the northern portion of the Inlet. Cook Inlet experiences some of the greatest tidal fluctuations in the world, with the difference between high and low tide levels reaching 12 m (39 ft). These large tidal ranges, combined with broad tidal flats, can result in currents reaching 6.2 m/sec (20.3 ft/sec). In winter, ice covers much of upper Cook Inlet as rivers begin to freeze in October and November.
In Alaska, there are five recognized beluga stocks delineated based on summer range: the Beaufort Sea, the eastern Chukchi Sea, the eastern Bering Sea, Bristol Bay, and Cook Inlet. The degree of genetic differentiation among the Cook Inlet stock and the other four Alaska beluga stocks indicates CI belugas are the most isolated reproductively and demographically. This isolation is long established, resulting in localized adaptation and indicating that the possibility of rescue from neighboring populations is remote.
CI belugas are unique in Alaska given that their habitat is in close proximity to the greatest concentration of Alaska’s human population. Belugas are not uniformly distributed throughout Cook Inlet, but are predominantly found in nearshore waters of the upper Inlet. Humans use the waters and shores of Cook Inlet for fishing, hunting, timber harvest, mining, shipping, dredging, renewable energy production, wastewater discharge, military activities, oil and gas development, transportation, and residential and industrial development.
The distribution of CI belugas has changed significantly since the 1970s; as their population declined, their summer range has contracted to the upper Inlet. Belugas spend the summer and early fall months in the upper Inlet, concentrating at river mouths. In late fall, belugas disperse south into the middle Inlet and into deeper offshore waters. This pattern continues through winter, when whales exhibit the most wide-ranging movements, spanning both nearshore and offshore waters from the upper reaches of Knik Arm to the middle Inlet. Large aggregations of belugas in specific areas of upper Cook Inlet during May to October likely indicate a critical time period for foraging; it is during the ice-free months that calves are born and nursed and that the whales acquire the thick blubber layer they will need to survive through the winter months. In addition to comprising important feeding habitats, the shallow waters of the upper Inlet may also play important roles in reproduction. Other critical uses of habitat by CI belugas may include avoidance/escape from predators, transiting among feeding and/or nursery habitats, and refuge from human activities (e.g., in-water noise, ship traffic and hunting).
Belugas have low reproductive potential; that is, females have a single calf only every two or more years, and devote considerable time to caring for their young. Age at sexual maturity, length of gestation, and calving interval are unknown for CI belugas. Data are not available for CI belugas to precisely determine the generation time; however, when we consider available information regarding the age at first reproduction and age at senescence for belugas, we estimate a generation time of approximately 25 years.
Belugas make a wide variety of sounds and have highly developed echolocation capabilities. Their high auditory sensitivity, wide frequency bandwidth, and dependence upon sound to navigate, communicate, and find prey and breathing holes in the ice make belugas vulnerable to noise pollution, which may mask beluga signals or lead to temporary or permanent hearing impairment.
Belugas are extremely social animals that typically travel and hunt together. High group cohesion and large group sizes may provide benefits to group members in terms of information gathering and transfer with regard to resource availability (e.g., prey, calving sites, oceanographic conditions, etc.) and cooperation in predator avoidance and reduced predation risk. The evidence available for CI belugas suggests that individual belugas intermix and interact with various beluga groups across the Inlet.
The diet of CI belugas is dominated by fish and invertebrates. Recent analysis suggests CI beluga diets changed in the last few decades and whales have been feeding at lower trophic levels. Pacific salmon, including Chinook (king) salmon, are an essential feature of CI beluga critical habitat. There is therefore concern that recent reductions in run strength of Chinook salmon stocks across Alaska, particularly in Cook Inlet, may be affecting CI belugas.
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. A 1979 survey resulted in an estimate of 1,293 belugas in Cook Inlet; NMFS has adopted 1,300 belugas as the value for the carrying capacity to be used for management purposes. Between 1979 and 1994 the CI beluga population declined roughly 5% annually from about 1,300 whales to 650 whales. Between 1994 and 1998 the population declined nearly 50% from 650 whales to 347 whales, likely a result of unsustainable levels of subsistence harvest. Since 1999, when subsistence hunting was restricted, the population has continued to decline by 1.3% per year. The 2014 abundance estimate was 340 CI belugas.
In the past, there have been both natural and anthropogenic sources of mortality or injury of CI belugas. Natural sources include predation by “transient” killer whales, live strandings, and potentially disease; anthropogenic sources include subsistence harvest, poaching or intentional harassment, and mortalities or injuries incidental to other human activities. Although the cause of death for most CI belugas remains unknown, 38 CI belugas were necropsied between 1998 and 2013; identified causes of death included association with previous mass or single live strandings, trauma, perinatal mortality, malnutrition, and disease.