Harbor seal diet in northern Puget Sound: Implications for the recovery of depressed fish stocks

Abstract

Recovery of severely declining resource stocks often leads to enforced quotas or reduced human access to those resources. Predators, however, do not recognize such restrictions and may be attracted to areas of increased prey abundances where human extraction is being limited. Such targeting by predators may reduce or retard the potential recovery of depressed stocks. In the San Juan Islands, northern Puget Sound, USA, marine reserves were implemented to recover depressed fish populations. We examine the role of harbor seals Phoca vitulina in the San Juan Islands food web. We describe the temporal and spatial variability in their diet, emphasizing species for which reserves were established (rockfish Sebastes spp.) and other important de - pressed stocks, including salmon Oncorhynchus spp. and Pacific herring Clupea pallasii. During winter and spring, seals primarily consumed Pacific herring, Pacific sand lance Ammodytes hexapterus, northern anchovy Engraulis mordax, and walleye pollock Theragra chalcogramma. During summer/fall, adult salmonids composed >50% of the diet and were particularly important in oddnumbered calendar years, when pink salmon O. gorbuscha spawn. Rockfish were not a primary prey species at any time of the year, suggesting that the abundance of alternative prey species may reduce predation pressure and provide a critical buffer to rockfish predation. The importance of considering increased visitation by marine predators to areas where potential prey are enhanced through restrictions on human extractions should be considered when modeling the efficacy of quotas and reduced access areas, such as marine reserves. © Inter-Research 2012.