Characterizing Pacific sand lance habitat in the San Juan channel sand wave field

Abstract

In the pelagic zone Ammodytes hexapterus or the Pacific Sand lance (PSL) act as an important link in the food web feeding on planktons and being preyed on by a wide variety of higher trophic level predators (Martin et al., 1999). The San Juan Archipelago (SJA) offers a diverse, productive and tidally dynamic ecosystem for PSL to inhabit. Sand wave fields are thought to be a major habitat for PSL in the SJA. Sand waves are bed forms present under flow velocities of 0.5 to 1.0 m/s and grain size of 0.25 -2.0mm in diameter (Rebesco and Camerlenghi, 2008). Understanding how benthic substrate will shape sand lance distribution on the wave fields is critical to further understanding the life history, daily migrations and consequently the role PSL play in the pelagic environment. Understanding how fish densities change with differences with substrate, how fish size change with substrate and how does substrate change over the sand wave are important steps towards further answering questions about the life history and populations of the PSL. These questions were addressed by obtaining sediment samples from the San Juan channel sand wave field and fractioning out sediment size, measuring PSL densities and sizes in the samples. PSL preferred low gravel high sand concentrations. Sand lance densities were highest in high concentrations of sand which corresponded to the middle of the wave field.