Adult American shad Alosa sapidissima habitat occupancy in the Delaware River Basin, USA

Abstract

The anadromous American Shad (Alosa sapidissima), a once commercially important species in the mid-Atlantic, experienced population declines due to overfishing, poor water quality, and impediments to accessing freshwater spawning habitat. Regional and national scale efforts 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. Forty-four shad were tagged with acoustic transmitters from 2021–2024 above recently removed Dam 1 and tracked in the Delaware River Estuary and coastal waters of the Atlantic Ocean. The length of available habitat increased by 1.3 km and 68.0% (30/44) of tagged shad used historic spawning grounds in the Brandywine River following dam removal. Thirteen individuals exhibited long-term residency in the Brandywine River from May–June, demonstrating the importance of this newly available habitat to spawning adults. Additionally, one fish returned to the Brandywine River between years, displaying inter-annual spawning site fidelity and iteroparity. I also observed exploratory behavior of eight additional shad that exited and returned to the Brandywine River within the same year. I used generalized additive models with environmental covariates including water temperature, flow rate, lunar phase, tidal cycle, etc., matched to occupancy events of shad that exited and reentered the Brandywine River to explain occupancy events within the Brandywine River. Under the most simplified model, adult American Shad occupancy event presence from the eight individuals was significantly correlated to water temperature, day length, flow rate, and lunar phase, which explained 50-60% of deviance (P < 0.05). I also observed more presence during occupancy events at temperatures of 20°C, a day length of 14.6 hours, and flows between 5.5–12.5 m3 s-1. These conditions represent the preferred spawning conditions within the Brandywine River from 2021–2023 for shad that exited and reentered the Brandywine River, despite displaying low predictive power for hypothetical environmental conditions. As such, these potential environmental preferences should be noted when management entities design dam removal or habitat restoration within northern Delaware, or specifically within the Brandywine River.