Foraging and Fisheries

Foraging Behavior

Belugas are known to feed on prey that concentrate, including shrimp and schooling or spawning fish (Seaman et al. 1982), and beluga presence has been used by fish harvesters as indicators of fish abundance. Feeding both independently and cooperatively, belugas capture and swallow their prey whole, using blunt teeth to grab prey. Quakenbush et al. (2015) noted that because belugas swallow their prey whole, the diet of smaller (young) belugas is limited by the size of the esophagus to smaller prey. The suitability of some adult salmon as prey for young belugas may thus be limited, even when salmon are available. While belugas are known to eat large amounts of fish in spring and summer, little is known about winter distribution and less about winter feeding. An extensive review of potential CI beluga prey species, including their distribution and known abundances, is presented in Appendix F – CI Beluga Prey Supplement.

Current data on the foraging ecology of CI belugas are quite limited and based primarily on visual observations of whales in areas of seasonal prey concentrations. However, dive behavior data was obtained through satellite tags deployed on 11 belugas during 1999 to 2002 (Goetz et al. 2012a). Dives were significantly shorter and shallower June to November versus December to May. Over 50% of the dive effort occurred in shallow, nearshore areas of Chickaloon Bay, Susitna Delta, Knik Arm, Turnagain Arm, and Trading Bay, a behavior suggesting feeding in these areas. These locations are also recognized as areas where anadromous prey concentrate when entering river mouths. Belugas in northern Cook Inlet likely benefit from the tendency of anadromous prey species to be concentrated by shallow water and the time required to transition from salt water to fresh as these prey enter the stream mouths, which presumably makes them easier to capture.

Belugas in Cook Inlet appear to feed extensively on concentrations of spawning eulachon in the spring; CI belugas then shift to foraging on salmon species as eulachon runs diminish and salmon return to spawning streams. While winter foraging is not well known, some components of beluga whale populations in other areas forage more on benthic species (DFO 2011). It is presumed that CI belugas in winter forage more on benthic species or opportunistically on infrequently encountered pelagic species. Analysis of CI beluga stomach contents indicated gadid and flounder species were relatively important prey items in spring and fall (and likely winter), seasons when fewer salmon are available (Quakenbush et al. 2015). The degree of prey switching, either seasonally or on longer time scales, is not well understood, although belugas must be somewhat opportunistic with respect to foraging selectivity relative to prey availability.

Diet

The diet of belugas throughout their circumpolar range is dominated by fish and invertebrate prey. While published reports on beluga diets are available from Canada (Vladykov 1946, cited by Seaman et al. 1982; Doan and Douglas 1953, cited by Seaman et al. 1982; Sergeant 1973), Russia (Kleinenberg et al. 1969, cited by Seaman et al. 1982; Tomlin 1967, cited by Seaman et al. 1982), and Europe (Lono and Oynes 1961, cited by Seaman et al. 1982), published data for Alaska are limited to one published report (Seaman et al. 1982; n = 119 belugas from three stocks) and several unpublished reports from Bristol Bay (Brooks 1954, 1955, 1956, 1957; Lensink 1961, cited by Seaman et al. 1982; Klinkhart 1966, cited by Seaman et al. 1982). Diet data for CI belugas are currently limited to a relatively small sample of stomach contents (Quakenbush et al. 2015), stable isotope analyses (Nelson and Quakenbush 2014), as well as observations from Alaska Native subsistence hunters (Fall et al. 1984; Huntington 2000).

A total of 53 stomachs from CI belugas were collected from 1992 to 2010 (Quakenbush et al. 2015). Stomachs collected from 1992 to 2001 (April to October; n = 24) were analyzed separately from stomachs collected during 2002 to 2010 (March to November; n = 27). Thirty five non-empty stomachs were sampled; 17 from the earlier and 18 from the later time periods. For 1992 to 2001, the only prey items identified were eulachon and Chinook (king) salmon, with additional items identified only as “salmon.” However, because only a portion of the contents from each stomach collected was analyzed, additional prey items were likely present. For non- empty stomachs from 2002 to 2010, fish were identified in 18 stomachs and invertebrates in nine (Table 2). Fish prey included seven families and at least 12 species. Fish frequencies of occurrence were greatest for salmon (67%), gadids (39%), smelts (11%), and flounders (11%); salmon frequencies included coho (28%), Chinook (11%), and chum (17%). Gadid frequencies included saffron cod (22%), walleye pollock (17%), and Pacific cod (6%). Eulachon was the only smelt identified, whereas two flounder species, yellowfin sole (11%) and starry flounder (6%), were identified. A longnose sucker was the only freshwater fish found. Seven types of invertebrates were found in the beluga stomachs, with the frequency of occurrence among non- empty stomachs being highest for shrimp (33%), followed by polychaetes (11%) and amphipods (11%). Other invertebrates included Tanner crab (6%) and sponges (6%). Because fish appearing in beluga stomachs have also consumed a variety of prey, including polychaetes, shrimps, amphipods, and other fishes (Clausen 1981, 1983; Seaman et al. 1982), some prey items in the beluga stomachs could have resulted from secondary ingestion.

Alaska Natives have reported CI belugas feeding on freshwater/brackish fish, including trout, whitefish, northern pike, grayling, and Pacific tomcod (Fall et al. 1984; Huntington 2000).

Stomach samples from CI belugas are lacking for the winter months of December to February. Dive data from belugas tagged with satellite transmitters suggest whales feed in deeper waters south of the Forelands during winter (Hobbs et al. 2005), possibly on prey such as flatfishes, sculpins, and gadids. Diet data for early spring are limited to one dead whale found in March 2003, which had thinner blubber than beach-cast belugas found in summer. This early spring beluga stomach contained saffron cod, walleye pollock, Pacific cod, eulachon, Tanner crab, shrimp, and polychaetes (NMFS unpub. data; Table 2).

Table 2. Prey items from CI beluga stomachs, 2002 to 2010.
Mar
Jun
Jul
Aug
Sep
Oct
Nov
Total among months
Percent frequency (a)
Total no. stomachs 1 3 4 7 3 8 1 27
Total no. stomachs with prey 1 2 4 5 1 5 0 18 67
Stomachs that contained fish 1 2 3 5 1 5 0 17 94
Salmon 0 2 3 4 1 2 0 12 67
Gadid 1 0 0 1 1 4 0 7 39
Eulachon 1 0 1 0 0 0 0 2 11
Flounder 0 0 0 1 0 1 0 2 11
Other identified fish 0 0 0 2 0 1 0 3 17
Unidentified fish 1 0 0 1 0 0 0 2 11
Stomachs that contained invertebrates 1 0 3 1 0 4 0 9 50
Shrimp 1 0 1 1 0 4 0 7 39
Amphipod 0 0 1 0 0 1 0 2 11
Polychaete 1 0 0 0 0 1 0 2 11
Other identified invertebrates 1 0 2 1 0 0 0 4 22
Unidentified invertebrates 0 0 0 0 0 1 0 1 6

Percent frequency is the number of stomachs containing a prey item relative to the total number of non-empty stomachs. Source: L. Quakenbush, ADF&G, unpub. data.


Analysis of stable carbon (δ13C) and nitrogen (δ15N) isotopes in 23 archived skull bones revealed a depletion of both δ13C and δ15N values between 1964 and 2007 (Nelson and Quakenbush 2014). Annual growth layers from teeth (1961–2007) also showed a decline in isotope values (Nelson and Quakenbush 2014). However, the decline in δ13C appeared steady over time, while the decline in δ15N was steep from 1970 to 1978 and more gradual after 1978. The authors noted that the decline in δ13C is consistent with the reduction of the CI belugas’ range into the upper more freshwater reaches of Cook Inlet, where their prey may have a greater freshwater influence and thus be more depleted in δ13C than the same prey from marine waters. The overall decline in δ15N indicates a decline in trophic level. Prey isotope signatures were not identified to species. However, possible examples of such a change to lower trophic level prey include a switch from pollock or Pacific herring to capelin or sandlance, or a switch from older piscivorous pollock to younger planktivorous pollock.

Caution is warranted regarding interpretation of diet information. For example, more-recently ingested prey items are likely to be more identifiable owing to less digestion, although hard parts of prey may accumulate in the digestive tract. However, recently eaten prey are also more likely to be regurgitated from stimuli such as stress. The cause of mortality may create additional bias, as stranded belugas may have fed differently, due to poor health, compared to harvested belugas. Thus, depending on beluga health, prey type, and time since consumption, some prey items may be over or under-represented in diet analysis from stomach samples. The relatively small sample size for CI beluga stomachs remains a concern as aspects such as feeding preferences by individual whales may be underrepresented in the current analysis. While salmon is obviously important as a prey item throughout the spring to fall season, some whales may be more proficient at foraging on salmon, while other whales supplement salmon with other prey items. Thus, a better understanding of foraging selectivity by individual whales is compromised by the low sample size.

Fisheries Management

Management of salmon fisheries in Alaska attempts to constrain harvests to be no greater than the level of surplus production, defined as returning adult salmon in excess of the spawning production that is needed to maintain productive salmon populations (Quinn and Deriso 1999). In addition to reproductive needs, harvest considerations must include commercial fisheries and upstream consumptive uses such as recreational, personal use, and subsistence fisheries (Shields 2010), as well as allowances for natural mortality, which includes predation by belugas, bears, and other species. However, it is unlikely that escapement goals will be met in all tributaries across all years. Thus, while fishery management, on average, should maintain sufficient total numbers of prey for belugas, the timing of prey concentration or densities in the river mouths can vary and may not always be adequate for efficient feeding by belugas.

An important concern is that salmon are an essential feature of CI beluga critical habitat (76 FR 20180; 50 CFR part 226.220), and some species of salmon, most notably Chinook salmon, have had reductions in run strength in Cook Inlet and throughout Alaska. In 2012, the U.S. Secretary of Commerce determined in response to a request from the Alaska governor that commercial fishery failures due to fishery resource disasters had occurred for Chinook salmon stocks in the Yukon, Kuskokwim, and Cook Inlet regions. The declaration acknowledged hardships for commercial, sport, and subsistence users as a result of the Chinook fishery failures. To identify key knowledge gaps and discuss how best to address those gaps, ADF&G sponsored a Chinook salmon symposium, “Understanding the Abundance and Productivity Trends of Chinook Salmon in Alaska,” in Anchorage during October 22–23, 2012. Subsequently, ADF&G worked collaboratively with federal agencies and academic partners to develop a stock assessment and research plan with recommended studies to address critical knowledge gaps (ADF&G Salmon Research Team 2013). Implementation of strict fishery management actions has been necessary to meet escapement objectives, and many fisheries have been curtailed to protect Chinook salmon. In 2016, runs improved for the westward stocks (i.e., Yukon, Kuskokwim, and Nushagak), as well as in Kodiak and Cook Inlet, but overall these runs were still below long-term averages. Runs in these regions are expected to continue to improve.

More information on this topic is presented in Appendix F – CI Beluga Prey Supplement.