Using ferry monitoring data to explore the importance of isotherms on the winter survival of Northern anchovy in Puget Sound

The Salish Sea displays strong seasonality in water temperature which can impose physiological limits on temperature sensitive species. Puget Sound, in winter, relies on ocean water as a heat source whereas in summer, the gradient is reversed. The dynamic exchange of Puget Sound with coastal water dictates the spatial and temporal patterns of isotherms that are relevant to temperature sensitive species. Recent winters with increased water temperature may expand the range of certain species to be able to survive in Puget Sound over the winter. Northern anchovy (Engraulis mordax) are pelagic spawners and survive in between 8 and 25 °C water. We examine near surface isotherms to describe the dynamic of Northern anchovy access to spring plankton blooms using continuous, geo-referenced data from physical and bio-optical sensors from an 80-mile long en route ferry system between Seattle, WA and Victoria, BC. Spatial and temporal patterns reveal which part of the year near-surface temperature conditions may be favorable for anchovy exploitation of the spring plankton blooms. If filter-feeding prey species such as Northern anchovy can reside in Puget Sound during winter, then they can exploit the spring plankton blooms and potentially change the structure of the food web