North Pacific Spiny Dogfish (Squalus suckleyi) Presence in Eelgrass Habitat in the Salish Sea, Washington

Eelgrass (Zostera marina) is the dominant seagrass along the Pacific North American Coast, providing complex physical structure, high trophic productivity, and protection from predators (Phillips 1984; Simenstad 1994). Because of these multiple functions, many fish species and life stages use Eelgrass beds more than other nearshore habitat types (Dean and others 2000; Murphy and others 2000; Johnson and Thedinga 2005). Currently, there is growing evidence identifying nearshore habitats as places that may be regularly inhabitated by large-bodied fish, including sharks (Tobin and others 2014), but it is not clear how much sharks use Eelgrass habitat. In other parts of North America, several sharks have been reported inhabiting Eelgrass habitat, including Sandbar Sharks (Carcharhinus milberti) in Chesapeake Bay, Maryland and Virginia (Orth and Heck 1980), Leopard Sharks (Triakis semifasciata) in Humboldt Bay, California (Ebert and Ebert 2005), and Gray Smooth-hound Sharks (Mustelus californicus) in Bolsa Chica, Orange County, California (Espinoza and others 2011). Conversely, reports of sharks are rare in Eelgrass beds in the Salish Sea region, which is located between the southwestern tip of British Columbia and the northwestern tip of Washington (Fig. 1; Farrer 2009). In this region, North Pacific Spiny Dogfish (Squalus suckleyi Girard 1854; Ebert and others 2010), hereafter Spiny Dogfish, are currently considered an abundant upper trophic level predator (Harvey and others 2010). Although Spiny Dogfish have been documented as making local movements along the nearshore in Puget Sound, Washington (Andrews and Harvey 2013), they have not generally been reported in Eelgrass habitat. Spiny Dogfish, however, have been captured in sand and Eelgrass habitat in Samish Bay, Washington, using overnight commercial gill net sets (Farrer 2009). Here we document the use of Eelgrass habitat by Spiny Dogfish. We conducted our study in Padilla Bay National Estuarine Research Reserve, Mount Vernon, Washington. Padilla Bay sustains one of the most extensive Eelgrass beds in the inner Salish Sea region (Berry and others 2003; Fig. 1). The shallow estuary covers nearly 4500 ha, of which .3200 ha is Eelgrass of both the native Z. marina and the non-native Z. japonica (Shull 1999). Padilla Bay differs from most estuaries in the region because it has become isolated from major rivers as it has filled with sediment from the Skagit River to the south, and the Eelgrass has thrived in the absence of a major freshwater input. However, drainage from adjacent agricultural lands enters via sloughs with tidegates. Other nearby sources of freshwater are the Samish River to the north and the Skagit River via the Swinomish Channel to the south. Tides in the region are mixed semidiurnal. We sampled 3 sites along major tidal channels in contiguous Z. marina habitats that were located over mud bottoms. These sites were located in the north (Short Channel), middle (Middle Channel), and south (Bayview Channel) portions of Padilla Bay (Fig. 1). At each site, we sampled 2 adjacent locations to have replicate samples, thereby completing 6 overnight gill-net sets for each month of May, June, and July of 2004. We trawled during the day to characterize the fish assemblage in the shallows (Penaluna 2006), and we set gill nets overnight in Eelgrass locations parallel to the adjacent tidal channel to document nocturnal movements of larger fishes. Each 15-m by 2-m experimental gill net was composed of five 3-m panels, with square mesh size increments of 12, 25, 38, 50, and 68 mm, with leadand foam-core lines. We sampled when tidal elevations ranged from –1.5to –3.0-m mean lower low water. Catch per unit of effort (CPUE) was measured as number of fish per net-hour. The highest CPUEs of Spiny Dogfish captured during overnight gill-net sets were recorded in June and July (Table 1; Fig. 2). Spiny GENERAL NOTES