Several factors may result in the reduction of the abundance, quality, availability, or seasonality of CI beluga prey. The impact of reduction of available prey on CI belugas is poorly understood, but may be the result of competition with humans or other animals. It may also result from habitat disturbances or modifications as a result of anthropogenic or natural factors. Factors, whether anthropogenic or natural, that affect the available prey species may have a greater impact on one prey species or species subcomponent (e.g., age or size-related). Resultant changes in relative abundance of prey will affect the prey composition available (Pyke et al. 1977).
Competition for Prey Resources
CI belugas compete with humans and other animals for prey resources, particularly salmon and eulachon. Quantitative data on the spatial and temporal distribution of beluga prey in upper Cook Inlet are limited (see Appendix IX.F – CI Beluga Prey Supplement). Although management of fisheries targeting anadromous species in Alaska attempts to constrain harvests to be no greater than the level of surplus production, it is unlikely that escapement goals will be met in all tributaries across all years.
Effects of fishing by humans on beluga foraging success are not well known, yet may include spatial and temporal components for any specific prey resource. Effects on belugas will depend on the extent to which a reduction occurs to the abundance, quality, or availability of prey (localized or Inlet-wide), and if the belugas can compensate for losses of preferred prey by shifting to other feeding sites or less-preferred prey. If a non-preferred prey species is reduced, the relative or absolute abundance of preferred prey may increase over time, depending on the ecological linkages and response times. The temporal distribution of these prey resources may be as important as their magnitude, particularly for growing juveniles and pregnant and/or lactating female belugas. Changes in seasonality of prey may occur due to seasonality and species preference of fisheries, changes in seasonal fish habitat, or seasonal environmental changes affecting Cook Inlet. The extent to which shifts in the seasonality of prey species or temporal gaps in prey availability impact reproductive success and survival of belugas, particularly during critical life stages, is unknown. However, these impacts are likely to be most important if affecting temporal availability of energy-rich high-lipid prey. Alternatively, events that result in decreases of specific runs or changes in the availability of prey (e.g., by changing schooling patterns or altering nearshore terrain) may leave temporal gaps in the availability of prey at sufficient densities resulting in the reduction in total days when beluga blubber fat storage can occur. For more information see Appendix IX.F – CI Beluga Prey Supplement.
CI belugas may also compete against other predators (harbor porpoise, harbor seals, killer whales, sea lions, large whales, sea otters, sea birds, etc.) for available prey resources, particularly in upper Cook Inlet where the available prey resources may be more limited in abundance or diversity. Although there may be some foraging specialization upon available prey species, there is also likely to be a high degree of dietary overlap due to the limited prey diversity available. In upper Cook Inlet, belugas are most likely to compete for prey resources with harbor seals and harbor porpoises, which have been documented also to be present in Cook Inlet year round and co-occur in the same general locations as CI belugas (Small et al. 2011; AEA 2013; T. McGuire, LGL, unpub. data).
Disturbance or Modification of Prey Habitat
The amount or types of prey available to CI belugas may also be reduced as a result of disturbances or modifications to prey habitat. Anthropogenic activities that may detrimentally affect prey habitat and possibly reduce the availability of prey to belugas are present both seasonally and continuously in Cook Inlet. Anthropogenic activities in Cook Inlet that may disturb or modify the habitat of beluga prey include dredging; oil or gas activities; hard rock quarrying; laying of electrical, communication, or fluid lines; construction of docks, bridges, breakwaters or other structures; and other activities. These activities may cause avoidance or destruction of an area used by beluga prey as a result of anthropogenic disturbance. Permanent structures, such as docks, platforms, or bridges, alter the Cook Inlet habitat by altering local tidal flow, among other potential effects. However, the net effect of anthropogenic structures on beluga prey remains unknown.
In addition to loss of habitat available to beluga prey species by displacement or avoidance, anthropogenic activities may reduce the quality of the prey as a result of contamination of the habitat. For example, mechanical disturbance of the seafloor (e.g., dredging) re-suspends silt, and potentially buried chemicals, into the water column. A sewer outfall plume alters both the abiotic and biotic environment, releasing various hormones, pharmaceuticals, and other chemicals into Cook Inlet. Catastrophic events such as oil or chemical spills are infrequent, but may have significant effects on beluga prey, whether through changes to spawning or migration patterns, direct mortality, or potential long-term sub-lethal impacts (Moles et al. 1994; Marty et al. 1997; Murphy et al. 1999). While some of these contaminants are known to bioaccumulate and be passed up the food chain, they also may impact the survival, quality, and reproduction of the prey species itself. For example, elevated copper concentrations can harm salmon and other CI beluga prey.
The habitat upon which beluga prey depend may also be affected by natural events, including: Pacific decadal oscillation, an El Niño-like pattern of Pacific climate variability (potentially affecting rainfall, freshwater runoff, water temperature, and water column stability); climate change (potentially affecting glacial output and siltation and salinity in downstream estuarine environments); volcanic ash outfall (affecting siltation and water chemistry); and earthquakes and associated landslides, elevation changes, and tsunami waves. Some of these natural threats are infrequent, but may have instantaneous and substantial impacts upon abundance, quality, or seasonality of CI beluga prey. However, other threats, such as Pacific decadal oscillations, may occur more regularly, may or may not be readily detectable, may develop over an extended time period, and may have long-lasting ecological effects.
Ecological regime shifts, in which species composition is restructured in association with abrupt changes in climate, have been identified in the North Pacific (Hollowed and Wooster 1992; Anderson and Piatt 1999; Hare and Mantua 2000; Spies 2007) and are believed to have affected prey species availability in Cook Inlet. For example, in the 1970s, dominance in the Gulf of Alaska ecosystem transitioned from crustaceans to groundfish, particularly gadid (e.g., cods) species. In another analysis, Hare and Mantua (2000) reaffirmed the 1976 to 1977 ecosystem change in the Gulf of Alaska and identified a less dramatic shift in 1989. Analyses of multi-decadal data from small-mesh trawl surveys conducted by NMFS and ADF&G showed ecosystem reorganization in the 1970s at Kachemak Bay in southern Cook Inlet and around Kodiak Island and in Shelikof Strait located in the northern Gulf of Alaska south and west of Cook Inlet Gulf waters (Bechtol 1997; Anderson and Piatt 1999). Of particular note was a decline in forage species, particularly pandalid shrimp and capelin, and increases in cod, pollock, and flatfish.
Changes to the marine, coastal, and freshwater ecosystems are known to be occurring as a result of global climate change and the associated occurrence of shifts in temperature, oxygen content, ocean acidification, and other physical and chemical changes (Doney et al. 2012), and are expected to continue and even increase with continued changes in the earth’s climate system (IPCC 2013). Climate-driven change in the environment could strongly influence CI beluga prey distribution and population size through changes in growth, survival, reproduction, and spawning distribution, but the possibilities are complex (e.g., Tillman and Siemann 2011; Hollowed et al. 2013; Link et al. 2015; Sydeman et al. 2015).
Anthropogenic Noise Effects on CI Beluga Prey
Recent literature reviews on the effects of sound on fish (Popper and Hastings 2009) conclude that little is known about these effects and that it is not yet possible to extrapolate from one experiment to other signal parameters of the same noise, to other types of noise, to other effects, or to other species. Limited available scientific literature indicates that noise can evoke a variety of responses from fish. Pile driving can induce a startle response, an avoidance response, and can cause injury or death to fish close to the noise source (Caltrans 2001, Abbott and Bing- Sawyer 2002, NMFS 2011, Halvorsen et al. 2011).
Some noises may evoke flight and avoidance response in juvenile salmon. Other studies have shown that the avoidance response is temporary. Salmon have been found to respond to low frequency sounds, but only at very short ranges (Chamberlin 1991). Carlson (1994), in a review of 40 years of studies concerning the use of underwater sound to deter salmonids from hazardous areas at hydroelectric dams and other facilities, concluded that salmonids were able to respond to low-frequency sound and to react to sound sources within a few feet of the source. He speculated that the reason that underwater sound had no effect on salmonids at distances greater than a few feet is because they react to water particle motion/acceleration, not sound pressures. Detectable particle motion is produced within very short distances of a sound source, although sound pressure waves travel farther (USDOT 2005). It is likely that fish will avoid sound sources within ranges that may be harmful (McCauley et al. 2003).
Of all known CI beluga prey species, only coho salmon have been studied for effects of exposure to pile driving noise (Casper et al. 2012, Halvorsen et al. 2012). These studies defined very high noise level exposures (210 dB re 1µPa2.s) as threshold for onset of injury, and supported the hypothesis that one or two mild injuries resulting from pile driving exposure at these or higher levels are unlikely to affect the survival of the exposed animals, at least in a laboratory environment. Hart Crowser Inc. et al. (2009) studied the effects on juvenile coho salmon from pile driving of sheet piles at the Port of Anchorage in Knik Arm of Cook Inlet. The fish were exposed in-situ (in that location) to noise from vibratory or impact pile driving at distances ranging from less than 1 meter to over 30 meters. The results of this studied showed no mortality of any of the test fish within 48 hours of exposure to the pile driving activities, and for the necropsied fish, no effects or injuries were observed as a result of the noise exposure.
The effects of noise on other CI beluga prey species, such as eulachon, gadids, and flounder species is unknown.
While the potential exists for human fishing pressure to change the abundance, seasonality, or composition of beluga prey, for targeted species, fisheries are managed with in-season reductions or closures if those fish stocks appear to be weak. However, not all fish stocks are assessed, and it is unknown whether management of fisheries for optimal returns provides sufficient densities in beluga feeding areas for efficient foraging by belugas. In addition, a fishery would not be reduced or closed if escapement goals are met. But if the escapement goal arrived in a shorter time period (e.g., 30 days instead of 90 days), the benefit of optimal returns to CI beluga energetics may be very different. It is likely there is interspecific competition for limited prey resources between CI belugas and other predators in Cook Inlet (e.g., harbor seal, harbor porpoise). However, the impact of this competition on the availability of prey to CI belugas has not been determined.
Habitat modification may result in changes in prey species availability and/or species composition throughout the range of CI belugas. While potentially having substantial effects on local ecosystems, natural threats are difficult to predict and mitigate. Many changes are tied to infrequent, short-term, uncontrollable events such as earthquakes or volcanic eruptions. Habitat disturbances may cause beluga prey to avoid an area, reduce viability of prey species, or interfere with belugas’ predation success. Anthropogenic noise may also have negative effects upon CI beluga prey. Noise impacts on fish may range from temporary displacement to barotrauma induced death (Popper and Hastings 2009). Moreover, as noted in Section III.A.3 (Threat Type: Noise), anthropogenic noise may affect beluga foraging performance.
Depending on the source, prey reduction can be a local or rangewide event, with a variable frequency of occurrence. While reduction of prey may result in reduced carrying capacity of the environment or reduce the fitness of CI belugas, the magnitude of the impact of a reduction of prey on CI belugas is unknown, as is the trend and future probability. As such, the threat to CI beluga recovery due to the reduction of prey is of medium relative concern.