Assessing fish distribution in Richardson Bay, California: A preliminary Dual-frequency Identification Sonar and environmental data approach

Estuaries provide critical habitat for many economically and ecologically valuable species that are adapted to a wide range of conditions and environmental variability, but the often turbid water presents challenges to monitoring efforts. This study explored fish habitat use in Richardson Bay, CA (a sub-estuary of San Francisco Bay) at two points in time, one following a dry winter (2016) and the other following a historically wet winter (2017). Dual-Frequency Identification Sonar (DIDSON) was used to record finfish and ray (>10 cm) abundance (MaxN) and size distribution, putative ray foraging pit size and abundance (MaxN), and eelgrass (Zostera marina) presence. Temperature, salinity, and dissolved oxygen were measured at each site, and water samples were collected at a subset of sites for nutrient analysis (urea, ammonium, nitrate, silicate, phosphate). Relationships between these data were explored using an information theoretic modeling approach. Finfish abundance was best predicted by nutrient concentration in 2016 (-) and eelgrass presence in 2017 (-), whereas fish length was best predicted by salinity in 2016 (-) and eelgrass presence in 2017 (+). Foraging pit abundance was strongly related to nutrient concentrations (+) in both years. This work presents a first attempt to establish relationships between fish distributions and environmental variables in Richardson Bay and highlights the value of imaging sonar for studying fish communities in turbid estuaries.