Physical influences on spatial and temporal distribution of Pacific Sand Lance (Ammodytes hexapterus) in the sand wave fields of San Juan Channel
Pacific sand lance (Ammodytes hexapterus) inhabit subtidal sediment wave fields in the San Juan Islands of Washington state. Although they are an important forage fish, serving as a trophic link between zooplankton and larger organisms, very little is known of the life history of subtidal sand lance in the sediment wave fields. Here, I hypothesize that spatial and temporal patterns of sand lance abundance are influenced by a variety of physical factors, including sediment grain size and tidal currents. Using a Van Veen grab sampler to collect sand lance in the sediment fields, and an ADP to measure current velocities over two, 3 hour sampling periods aboard the R/V Centennial, I found anomalously high numbers of sand lance in the sediment during slack before flood tides when northward velocities measured between +/- 15 cm s-1. In contrast, negative anomalies that occurred during weak ebb tides of -20 to -40 cm s-1. This suggests fine scale temporal variation in sand lance presence in the sediment fields may be tied to tidal cycles, and that sand lance may enter and leave the fields during the fall and early winter. The sediment fields near Sucia Island composed of large (1mm) shell hash, contained only mature fish, while the finer grained (1/2 mm) sediments of the San Jean Channel wave field had larger proportions of juveniles less than 90 mm in length, suggesting a relationship between sediment grain size and life stage. Other factors including depth and tidal mixing may also play a role in the spatial distribution of sand lance.