Distribution and Abundance of Porpoises In the San Juan Channel: Effects of Bathymetry and Tides

The San Juan Archipelago is a dynamic cluster of islands supporting seven species of cetaceans including the harbor and dall’s porpoises. Under recognized in the San Juan Archipelago conservation and better management of these species are required. Harbor porpoises were seen in higher densities than dall’s porpoises concurrent with past data. Supported by the habitat preference of each species. Densities of harbor porpoises were highest in the central part of the San Juan Channel and densities of dall’s porpoises were not a regular pattern observed. The bathymetry in this section (3 and 4) described by a sudden change from deep (~ 165 m) to shallow (~70 m) water depth effects turbulent mixing and prey aggregation dependent on current speed and direction. Harbor porpoise densities changed considerably due to this effect. Harbor porpoises densities changed when considering tidal factors. Highest densities of harbor porpoises were counted during fast flood (3.02/km2) and fast ebb (1.75/km2). Increased density with increased current speed was observed over 67% of cruise days