Acoustic-tagged American Shad utilize historic habitat following dam removal in northern Delaware

Abstract

Objective

The anadromous American Shad Alosa sapidissima, a once commercially important species in the mid-Atlantic region, experienced population declines due to overfishing, poor water quality, and impediments to accessing freshwater spawning habitat. Efforts at the regional and national scale substantially addressed the former two obstacles to facilitate population growth, but dam modifications or removal are still ongoing. In 2019, the state of Delaware removed the first barrier to anadromous fish passage on the Brandywine River (a tributary of the Delaware River), Dam 1.

Methods

A total of 24 American Shad were surgically tagged with acoustic transmitters from 2021 to 2022 above recently removed Dam 1 and tracked in the Delaware River estuary and coastal waters of the Atlantic Ocean.

Result

The length of available habitat increased by 1.3 km, and 75.0% of surgically tagged American Shad used historic spawning grounds in the Brandywine River following dam removal. Nine individuals exhibited long-term residency in the Brandywine River from May to June, demonstrating the importance of this newly available habitat to spawning adults. Additionally, one fish returned to the Brandywine River between years, displaying interannual spawning site fidelity and iteroparity. We also observed exploratory behavior of three additional American Shad that exited and returned to the Brandywine River within the same year.

Conclusion

Overall, we document a successful collaborative network via receiver array maintenance to answer questions regarding anadromous fish migration and habitat use following dam removal.