Shifting Distributions of Adult Atlantic Sturgeon Amidst Post-Industrialization and Future Impacts in the Delaware River: a Maximum Entropy Approach

<div><p>Atlantic sturgeon (<i>Acipenser oxyrinchus oxyrinchus</i>) experienced severe declines due to habitat destruction and overfishing beginning in the late 19^th century. Subsequent to the boom and bust period of exploitation, there has been minimal fishing pressure and improving habitats. However, lack of recovery led to the 2012 listing of Atlantic sturgeon under the Endangered Species Act. Although habitats may be improving, the availability of high quality spawning habitat, essential for the survival and development of eggs and larvae may still be a limiting factor in the recovery of Atlantic sturgeon. To estimate adult Atlantic sturgeon spatial distributions during riverine occupancy in the Delaware River, we utilized a maximum entropy (MaxEnt) approach along with passive biotelemetry during the likely spawning season. We found that substrate composition and distance from the salt front significantly influenced the locations of adult Atlantic sturgeon in the Delaware River. To broaden the scope of this study we projected our model onto four scenarios depicting varying locations of the salt front in the Delaware River: the contemporary location of the salt front during the likely spawning season, the location of the salt front during the historic fishery in the late 19^th century, an estimated shift in the salt front by the year 2100 due to climate change, and an extreme drought scenario, similar to that which occurred in the 1960’s. The movement of the salt front upstream as a result of dredging and climate change likely eliminated historic spawning habitats and currently threatens areas where Atlantic sturgeon spawning may be taking place. Identifying where suitable spawning substrate and water chemistry intersect with the likely occurrence of adult Atlantic sturgeon in the Delaware River highlights essential spawning habitats, enhancing recovery prospects for this imperiled species. </p> </div

Contemporary MaxEnt mean response curves of the covariates distance to the salt front (km), mixed reworking (mixed gravel, sand, and mud), nondepositional (cobble and bedrock), uniform reworking (mud), bedload (moderately well sorted sand and gravel), coarse grained reworking (poorly sorted sand and gravel), and fine deposition (mud and fluid mud).

<p>(0 = absence of covariate, 1 = presence of covariate). Mean is in red with ± one standard deviation in blue (two shades for the categorical substrate covariates). </p

Number of days adult Atlantic sturgeon were observed occupying each substrate type with expected number of days by each sediment type: FD- Fine Deposition, UR- Uniform Reworking, MR- Mixed Reworking, CR- Coarse Reworking, BL- Bedload, ND- Nondepositional.

<p>Number of days adult Atlantic sturgeon were observed occupying each substrate type with expected number of days by each sediment type: FD- Fine Deposition, UR- Uniform Reworking, MR- Mixed Reworking, CR- Coarse Reworking, BL- Bedload, ND- Nondepositional.</p

Projected areas of adult Atlantic sturgeon occurrence for 4 different salt front location scenarios: a- contemporary location (rkm 103), b-historical location (rkm 92), c- location given climate change (rkm 114), d- location given drought conditions (rkm 164).

<p>Projected areas of adult Atlantic sturgeon occurrence for 4 different salt front location scenarios: a- contemporary location (rkm 103), b-historical location (rkm 92), c- location given climate change (rkm 114), d- location given drought conditions (rkm 164).</p

Density of observations by distance from salt front for telemetered adult Atlantic sturgeon in the Delaware River during the likely period of spawning from 2009-2012.

<p>Density of observations by distance from salt front for telemetered adult Atlantic sturgeon in the Delaware River during the likely period of spawning from 2009-2012.</p

Acoustic receiver locations and 1 km detection radius overlain on the sediment composition map of the Delaware River (adapted from Sommerfield and Madson 2003), location of the salt front shown given four different scenarios, urban areas indicated through dark shading.

<p>Acoustic receiver locations and 1 km detection radius overlain on the sediment composition map of the Delaware River (adapted from Sommerfield and Madson 2003), location of the salt front shown given four different scenarios, urban areas indicated through dark shading.</p