Past and Future Habitat Suitability for the Hudson River Population of Shortnose Sturgeon: A Bioenergetic Approach to Modeling Habitat Suitability for an Endangered Species
Ryan J. Woodland, David H. Secor, and Edwin J. Niklitschek
Abstract.—Diadromous species encounter highly variable water quality as they traverse freshwater, estuarine, and marine environments. The U.S. federally endangered shortnose sturgeon Acipenser brevirostrum is a diadromous estuarine resident species that relies heavily on tidal freshwater regions of estuaries as spawning, nursery, and foraging habitat. A recent recovery in abundance in the Hudson River shortnose sturgeon population coincided with an ecosystem shift in the tidal freshwater estuary from hypoxia to normoxia (dissolved oxygen > 4 mg/L) during the summer juvenile rearing period. Decades of persistent summertime hypoxia encompassing as much as 40% of shortnose sturgeon nursery habitat was followed by a sudden shift to normoxia (1970 to 1978) due to the U.S. Clean Water Act legislation. Here, we evaluate how past and present water quality in the tidal freshwater Hudson River affects nursery habitat suitability. Habitat suitability, as indexed by potential instantaneous growth rate, was estimated with an empirically derived bioenergetic growth model before (pre-1978: 20% and 40% dissolved oxygen [DO] saturation) and after (1988: 85% DO saturation) the shift in seasonal ecosystem oxygenation. Habitat suitability was then forecast in the context of regional climate change and potential zebra mussel Dreissena polymorpha oxygen demand. Results from this simulation study indicated that even moderate reductions in water quality can significantly lower habitat suitability, supporting the circumstantial association between improved water quality and shortnose sturgeon recovery. Although presently occurring at high abundance levels, Hudson River shortnose sturgeon in the future may encounter diminished nursery habitat due to warming temperatures and increased benthic oxygen demand by zebra mussels.