Assessing Spawning Runs of Anadromous Fishes Using a Bayesian Analysis of Split-Beam and DIDSON Count Data

Riverine hydroacoustics offer a non-invasive technique to assess anadromous populations during upstream migrations (spawning runs) to riverine habitats.  We monitored spring spawning runs of alosine and moronid species using side aspect split-beam echosounding and DIDSON transects to obtain point counts across the river channel and throughout the water column.  The split-beam covered ~30% of the river cross section, while DIDSON transects provided resolution in unsampled areas (opposite bank and near bottom).  Upstream migrants were bottom and shore oriented, with 75% within 1 m of the bottom and 82% within 20 m of each bank.  We estimated daily run size using modeled spatial densities from the combined sonar count datasets, and a conditionally autoregressive model to smooth spatial trends to unsampled areas.  Daily estimates in both years were strongly correlated in space and time, explained by spatial covariates included in the model (e.g., distance from bank).  We identified run timing of species using drift gillnetting and boat electrofishing, and peaks in catch were significantly correlated with daily estimates.  We partitioned estimates to species specific run sizes using a 7 d moving average of daily catch data.  Results indicate a currently low population size of American shad, a consistent result with other river-specific assessments of this population.  Run estimates for other targeted anadromous fishes appear relatively stable.  Our survey methods and analysis pathway enhance run size monitoring techniques, especially in rivers where fixed side-aspect hydroacoustics monitoring covers only a fraction of the entire river cross section.

¹Current address for M. W. Waine: Atlantic States Marine Fisheries Commission, 1050 N. Highland Street, Suite 200 A-N, Arlington, VA 22201