Predation by harbour seals and sea lions on adult salmon in Comox Harbour and Cowichan Bay, British Columbia / by M.A. Bigg, G.M. Ellis, P. Cottrell and L. Milette. : Fs97-6/1769E-PDF

"The movements, feeding behaviour and predation rate on prespawning salmon by harbour seals (Phoca vitulina) at Comox Harbour and Cowichan Bay and by sea lions (Zalophus californianus, Eumetopias jubatus) at Cowichan Bay were examined during 1989-90. Tidal variation largely governs the daily movement of harbour seals over the estuaries. Sea lions tended to remain in slightly deeper water than harbour seals and their movements were affected less by tides. Peak harbour seal abundances of 750 and 175 were recorded in Comox Harbour and Cowichan Bay respectively. A maximum of 30 sea lions were counted at Cowichan Bay. The seasonal movement of these pinnipeds into the estuaries coincided with the migration of prespawning salmon into nearby rivers. Harbour seals and sea lions fed on salmon mainly along the outer edge of the estuaries, although harbour seals also commonly fed in the lower reaches of the Puntledge River which drains into Comox Harbour. Two to four harbour seals usually preyed together upon a single salmon. A combination of cooperation and opportunism seems involved in group predation. Sea lions tended to feed individually. Based on the number of salmon seen eaten hr⁻¹ in 1989, an estimated 46% of the fall run of chinook, 4% of pinks, 8% coho and 7% chum salmon were taken by harbour seals at Comox Harbour. In Cowichan Bay an estimated 1% of coho and 1% of chum were eaten by harbour seals and 3% of coho and 3% of chum were eaten by sea lions during 1989. The fall run of chinook salmon at Comox Harbour were particularly vulnerable to predation by harbour seals due the low number of fish and their availability for predation in both the Comox Harbour estuary and the slow moving Puntledge River"--Abstract, page iii.

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