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 odd-numbered 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.
We assess the potential of using otolith chemistry to differentiate quillback rockfish (Sebastes maliger) within Puget Sound, Washington, where two distinct population segments (DPS) have been identified. Using opportunistic collections (1993–2003) of quillback rockfish (n=77; age range of 2–65 yrs.) we first sought to determine whether fish from different sites and regions could be differentiated based on the trace elemental concentrations at the edge of their otoliths (i.e., the chemical record of the fish’s recent history). Results of our quadratic discriminant function analysis (QDFA) indicated significant spatial variability for fish collected at relatively large (regions) and small (sites) spatial scales. Specifically, fish collected from regions in 2002 (San Juan Islands and southern Puget Sound) and 2003 (eastern and western Strait of Juan de Fuca) were correctly classified with 100% and 65% accuracy (based on jack-knife classification), respectively, while fish collected from sites in 1998 (Mukilteo and Foulweather) were classified with 100% accuracy. We also investigated whether we could differentiate fish that were collected from different DPS and regions by using elemental concentrations from their whole otolith (which represents environmental information over the lifetime of a fish). Results from the QDFAs indicated relatively high classification success (80%) when comparing fish collected from either different DPS (i.e., Northern Puget Sound and Puget Sound Proper DPS) or regions (i.e., western and eastern Strait of Juan de Fuca). Findings from this study highlight the value of otolith chemistry in the study of population structure of quillback rockfish in Puget Sound.
The extent of larval retention and natal homing in demersal fish is a topic central to the design and the efficacy of Marine Protected Areas (MPAs). Unfortunately, little is known about effective larval dispersal in many marine species. The duration of the pelagic phase in many species suggests extensive dispersal, and population genetic studies suggest large-scale exchange of migrants. On the other hand, studies on the larval distribution around oceanic islands and within estuaries, as well as mark-recapture studies indicate effective mechanisms of larval retention, in part due to local oceanographic conditions (gyres, eddies), in part because of active larval behavior such as vertical migration. However, even with limited exchange between populations, levels of gene flow are usually sufficient to homogenize allele frequencies among populations, limiting the power of traditional population genetics in estimating rates of larval retention. Here, we use genetic markers to identify the offspring of resident adult brown rockfish (Sebastes auriculatus) among incoming settling juveniles on an isolated artificial reef at Pt Heyer in Puget Sound, thus directly estimating rates of self-recruitment on the reef. The project is the first application of genetic parental identification in a marine species, and will provide valuable data for the design and management of MPAs.
Available at http://depts.washington.edu/uwconf/2005psgb/2005proceedings/index.html