Didson Assessment of American Shad Approaching a Hydroelectric Dam

Although American shad were historically abundant in the Penobscot River, Maine, the current population is poorly characterized and presumed to be small. Implementation of the Penobscot River Restoration Project, including the upcoming removal of Veazie Dam, is expected to improve marine-freshwater connectivity and facilitate American shad recovery throughout this system. A major question surrounding shad restoration is whether the current run approaches Veazie Dam and is therefore available to recolonize upstream habitats post dam-removal.

In the spring of 2011, we surveyed migratory fishes approaching Veazie Dam using Dual Frequency Identification Sonar (DIDSON). This work was conducted in turbulent water below the dam, making automated data processing unfeasible and requiring a team of readers to manually review the footage and measure fish. Data from a 5% subsample of DIDSON files described length distributions for imaged fish, and a Bayesian mixture model was used to apportion fish observations by species. This approach enabled us to address effects between readers, and to incorporate known size distributions for several Penobscot River fishes. Preliminary results indicate DIDSON was an effective tool for making species determinations, and that a large percentage (56.8%) of fish observations at the base of Veazie Dam were American shad.