Pacific Sand Lance and the Pelagic Ecosystem in the
San Juan Islands

by Matthew Baker, Olivia Graham, Emily Burke, Kailee Bynum, Jan Newton, W. Breck Tyler

The University of Washington Friday Harbor Laboratories (FHL) are situated at the interface of the Salish Sea estuary and the influence of the greater Pacific Ocean. What is the influence of tides, season, and inter-annual variation on the open water environment in this region? Are these patterns reflected in forage fishes, which occupy a critical link in the ecosystem, serve as indicators of ocean productivity, and channel the flow of energy from plankton to a broad suite of predators, such as salmon, groundfish, seabirds, and marine mammals?

Tidal currents pulse through the San Juan Channel a few hundred meters east of FHL. In this complex marine environment, inputs from the Pacific Ocean and several major river systems mix in powerful tidal regimes to create habitat for a diverse community of prey and predator species. Since 2005 the Pelagic Ecosystem Function (PEF) Research Apprenticeship has provided an opportunity for students to explore the habitats and organisms of the pelagic ecosystem (Figure 1) each fall at FHL.

Our challenge has been to investigate the patterns, interactions, and linkages among various components of this complex system and to understand how oceanographic processes shape spatial and temporal patterns in open water biotic communities. Individual research projects are the cornerstone of the apprenticeship (Figure 2). Over the past five years several students have examined the role of Pacific sand lance (PSL) in this ecosystem, pursuing analyses on diet, foraging patterns, and habitat preference. Through these efforts, we are developing a time series (successive measurements made over time) for this critical species, increasingly integrating results with other research conducted at FHL on biomechanics of sand lance burrowing, stable isotope and fatty acid content analyses on diet and energetics, acoustic surveys of distribution in the water column, and mapping of benthic habitats.

Pacific sand lances (Figure 3) occupy a unique position in the ecosystem and comprise a large proportion of the diet of most upper level predators. In the Fall FHL apprenticeship, PSL are sampled in two separate methods – in the water column in nearshore environments (Figure 4) and in sediments in deep water habitat (Figure 5). Examining this species over multiple years has allowed us to investigate seasonal and annual trends, including research on fish densities and abundance across space and time, fish length and condition, diet and trophic role in the ecosystem, population dynamics, sediment association, and the influence of tidal, seasonal, and plankton production on foraging patterns. Research also aims to characterize how fish distribute in the natural habitat available to them (Figure 6). Our analyses are designed to complement other research efforts at FHL, including laboratory studies by Adam Summers, Joe Bizzarro, and Nick Gidmark on biomechanics of sand lance burrowing behavior, mapping of benthic habitat by H. Gary Greene, and studies implemented by Ken Sebens, Tina Wyllie-Echeverria and others.

The PEF apprenticeship is a collaborative endeavor with an accumulative approach, designed to build on past years data and integrate across elements of the system. This has resulted in an emerging time series, which we are increasingly able to mine for patterns and employ in retrospective and trend analyses to relate physical variables to biological trends. Individual student projects are integrated to link across disciplines and ecosystem components to better understand broad-scale ecosystem patterns relating oceanography, plankton, fish populations and fish habitat, sea birds, and marine mammals.

Research on PSL has distinguished juvenile (ages 0-1, Jackson Beach) and adult populations (ages 1-3, San Juan Channel) and identified critical nearshore habitat for spawning and preferred sediments for adult fish in sand fields; PSL demonstrate strong aversion to sediments with high levels of silt and gravel and a preference for coarse sand (φ=0-3; Figure 7). Diet analyses suggest a reliance on copepods in juveniles and adults, but significant differences in the relative contribution of pelagic versus benthic inputs between nearshore rearing and offshore sites in the sand wave field. Stable isotope results, which integrate diet over longer time periods, also suggest differences in feeding patterns and sources between these populations (distinct δ¹³C signatures). Foraging patterns suggest crepuscular (twilight) feeding in the upper water column, with a pronounced decrease in foraging and a subsequent decrease in condition (weight at length) throughout the fall season, eventually leading to the cessation of feeding, increased residency in sediments, and the onset of a winter dormant phase coincident with the fall transition in the system overall.

Sampling within the system has also provided coarse-scale population estimates for PSL in the San Juan Channel (N≈100 million). As we develop a time series, we are recognizing patterns in population structure, recruitment, and pulses in year-class strength (potential differences in age structure in odd and even years; Figure 8). We are also able to relate environmental conditions to fish size, condition and abundance. This past season was an anomalous year in many respects, with reduced productivity and relatively lower expected abundance of PSL, seabirds and harbor porpoises and increased incidence of herring and other large cetaceans. Notably, PSL condition was also markedly reduced in 2014 in contrast to other years, which may suggest effects that translate through the system and suggests the lower abundance may reflect higher mortality (Figure 9). PEF research this year on fish also examined Pacific herring (Clupea pallasii) and Surf perch (Hyperprosopon ellipticum) diets to provide greater contrast across forage fish species in the region. Generalist diets were found in surf perch and high reliance on copepod spp. for both Pacific herring and PSL. This may suggest resource partitioning between surf perch and juvenile PSL in nearshore habitat in contrast to diet overlap and resource competition between herring and adult PSL (though spatial partitioning may occur).

Some persistent questions remain. These include the population structure and inter-annual dynamics in the PSL population in this area, movement and behavior in the water column, the influence of tidal cycles and tidal currents on foraging, linkages between fish and higher order predators, and surveys of other sand wave fields in the area. We are also interested in determining whether PSL remain in this system or move to oceanic offshore habitat as older adults. Understanding more about this critical species will provide greater understanding the system and better inform conservation and management of the San Juan Archipelago and Salish Sea.