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Fishery Ecology Diet & Zooplankton

Exploring Conspecific Predation of Pacific Salmon at Sea in Alaska

Conspecific predation on juvenile Pacific salmon has been considered a potential factor in regulating marine recruitment of salmon. In his 1962 treatise on the regulation of abundance of pink salmon populations, the Canadian fisheries biologist W.E. Ricker developed a series of hypotheses around potential factors that could affect populations of this species. One hypothesis involved marine depensatory mortality, in which inbound migrant adult pink salmon of one brood line could become predators on outbound seaward migrant juveniles of the opposite brood line. However, surprisingly little has been published on the topic of marine predation on juvenile salmon by salmon or other species in the decades since Ricker formed his hypotheses.

Researchers from ABL's Southeast Coastal Monitoring (SECM) project have monitored potential predators of juvenile salmon during monthly surveys in Southeast Alaska since 1997. During these May-September surveys, predator complexes have been sampled in addition to other biophysical features associated with juvenile salmon habitat use and stock distribution. In the 14 years of sampling epipelagic habitat with a Nordic surface trawl, more than 1,000 guts of immature and adult salmon have been examined, including approximately 400 pink, 90 chum, 20 sockeye, 118 coho, and 386 Chinook salmon.

Until 2010, predation on juvenile salmon was only observed in adult coho (13%) and immature Chinook salmon (0.2%). Two incidents of predation by adult pink salmon on pink salmon juveniles were observed in June 2010. In other years, coho and Chinook ate juvenile pink and chum salmon ranging from approximately 80 to 200 mm fork length from June to September, with up to four juvenile salmon prey per gut. These predation events occurred in Icy and Chatham Straits, Cross Sound, and in coastal waters off Icy Point. In addition to conspecific salmon predation, other juvenile salmon predators were identified from another 14 non-salmonid species representing 1,100 potential predators. The principal salmon predators identified were spiny dogfish, Pacific sandfish, pomfret, walleye pollock, salmon shark and, in one year (1999), age 1+ sablefish. Overall, predation rates were low in Southeast Alaska, but occasional episodes of high predation by non-salmonids on juvenile salmon could have impacted harvest of returning adult salmon.

To explore conspecific predation of salmon in another Alaska region, SECM scientists collaborated with biologists from the Cordova office of the Alaska Department of Fish and Game (ADF&G) to examine Ricker's hypothesis of predation by the odd-year broodline of pink salmon in Prince William Sound (PWS) as a potential cause of depressed returns of even-year pink salmon in 2009. During seine test fisheries, returning adult pink (214) and chum (42) salmon were examined in southwest PWS in summer. Of all the adult salmon sampled, four predation events were recorded in June and July, by both species. However, the five salmon prey encountered included only juvenile chum salmon and two unidentified salmon. Since the latter were observed in adult pink salmon, we could not rule out that cannibalism by alternate broodlines occurred, but evidence from Southeast Alaska suggests that such predation is uncommon. Results from 2010 studies in PWS are not yet available.

By M. Sturdevant, R. Brenner, J. Orsi, E. Fergusson, J. Moss, and B. Heard

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