Insights into factors affecting long term fish surveys in the San Francisco Estuary, for a state listed species
The San Francisco Estuary is a highly invaded estuary with highly dynamic changes in species composition and abundance. Long term fish surveys have been routinely conducted throughout the year to monitor changes in the estuary. The San Francisco Bay Study is a long term fish survey that uses two types of trawls, the Otter trawl and the Midwater Trawl to evaluate the benthic and pelagic communities of fish and invertebrates. Longfin Smelt (Spirinchus thaleichthys), a California listed species has undergone a sustained decline in abundance since 2001. Longfin Smelt is a pelagic fish that can vertically stratify during the day and has been routinely collected in both trawls. The catch data from the two trawls were highly correlated but recently that relationship has diverged. A bimodal regression and occupancy analysis was initiated to determine what factor(s) were involved in the disparity.
The analyses suggest that the vertical distribution and water clarity may have affected detection and occupancy of Longfin Smelt. Field studies are being conducted to examine the occupancy in the water column and detection probabilities for the two trawls.