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 depressed 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.

BRUCELLA PINNIPEDIALISINFECTIONS IN PACIFIC HARBOR SEALS (PHOCA VITULINA RICHARDSI) FROM WASHINGTON STATE, USA

“…Adult harbor seals are primarily piscivorous, feeding on fish that are seasonally abundant (Olesiuk et al 1990;Lance et al, 2012), suggesting fish as a possible route of B. pinnipedialis exposure. Weaned pups also consume shrimp (Pandalus spp.…”

Food Habits of Harbor Seals (Phoca vitulina) in Two Estuaries in the Central Salish Sea

“…Scats were collected in Padilla Bay and Drayton Harbor during two seasons in 2006: spring (late March through June) and summer/fall (July through September) after Lance et al (2012). In Padilla Bay, samples were collected every 10 to 14 d during daytime low tides from 30 March to 7 September.…”

“…In addition, harbor seals may impose significant predation on out-migrating juvenile and returning adult salmonids (National Marine Fisheries Service [NMFS], 1997;Yurk & Trites, 2000). Given their potential to impact prey populations, several recent studies have examined the diet and foraging behavior of harbor seals in the central Salish Sea, focusing on rocky haulout sites in non-estuarine habitats (Thomas et al, 2011;Lance et al, 2012;Peterson et al, 2012). However, knowledge of the diet of harbor seals in soft-bottomed, estuarine habitats in this region is still quite limited.…”

Fine-scale habitat use and behavior of a nearshore fish community: nursery functions, predation avoidance, and spatiotemporal habitat partitioning

“…Pacific herring Clupea pallasii, surf smelt Hypomesus pretiosus), which we refer to in functional groupings (see Table 2) based on similarities in morphology, life history, and habitat use. These fish primarily consume small invertebrates and are potential prey for larger fish, marine mammals, and birds (Buchanan 2006, Duffy & Beauchamp 2008, Lance et al 2012, Munsch et al 2015a. They are mobile and in some cases migratory; thus, it is unlikely to observe the same individuals in replicate surveys.…”