Movements of green sturgeon, Acipenser medirostris, in the San Francisco Bay estuary, California

摘要

The green sturgeon, Acipenser medirostris, is a long-lived, iteroparous, anadromous acipenserid that is native to the San Francisco Bay Estuary, California. Sub-adult and adult fish are oceanic, but enter the estuary during the spring and remain through autumn. Little is known about green sturgeon distribution within the estuary or what, if any, physical parameters influence their movements. We report the results of a telemetry study conducted between September 2001 and November 2002. Five sub-adult and one adult green sturgeon were captured by trammel net in the San Pablo Bay region of the estuary. We implanted depth-sensing, ultrasonic transmitters within the peritoneum of individuals and tracked them from a boat for 1 - 15 h per day over periods ranging from 1 to 12 days. Salinity, temperature, and dissolved oxygen profiles of the water column were measured hourly. Observed movements were categorized as either non-directional or directional. Non-directional movements, accounting for 63.4% of observations, were closely associated with the bottom, with individuals moving slowly while making frequent changes in direction and swim speed, or not moving at all. Directional movements consisted of continuous swimming in the top 20% of the water column while holding a steady course for extended periods. Four of the five sub-adult fish remained within the confines of San Pablo Bay for the duration of their tracking period. The remaining sub-adult moved over 45 km up-river into Suisun Bay before contact was lost. The adult fish exited the bay and entered the ocean 6 h after release near Tiburon, CA, a movement of approximately 10 km. The sub-adult fish typically remained at the shallower depths (<10 m) of the estuary, but there were no apparent preferences for temperature, salinity, or dissolved oxygen, with the fish moving widely and rapidly across the range of these physical parameters. Activity is believed to be independent of light level with no discernable crepuscular, nocturnal, or diurnal peaks in activity.