A Contemporary Understanding of the Delaware River Atlantic Sturgeon: Survival in a Highly Impacted Aquatic Ecosystem
Philip C. Simpson and Dewayne A. Fox
The Delaware River and Bay, while once supporting the largest known population of Atlantic sturgeon Acipenser oxyrinchus, has long suffered under multiple anthropogenic stress sources. These include both point and nonpoint pollution sources, which resulted in considerable hypoxic zones and large-scale channel deepening projects. When coupled with these human-induced habitat alterations, historic harvest levels were not sustainable and quickly lead to the demise of the Delaware River stock. At present, there is still much uncertainty surrounding the status of Atlantic sturgeon in the Delaware River. We attempted to ascertain the contemporary population status of Atlantic sturgeon in the Delaware River, with a specific focus on the identification of spawning and juvenile riverine habitats.
Atlantic sturgeon were collected during 2005– 2007 through independent gill-net sampling and a reward program established with local striped bass Morone saxatilis gill-net harvesters. Captured Atlantic sturgeon were measured, processed for both genetic and gonad (>1.3 m full length [FL]) samples, and, if large enough (>800 mm FL), implanted with an ultrasonic transmitter (Vemco V-16, 6-H). We monitored telemetered Atlantic sturgeon through both passive (Vemco Ltd. [VR2]) and active tracking. The majority of passive hydrophones were affixed to U.S. Coast Guard navigation aids with the remainder being moored independently. Habitat use comparisons were made using a combination of real-time water quality measurements, U.S. Geological Survey water quality data, and available sediment records (Sommerfield and Madsen 2003). Habitat selection was assessed using an individualbased chi-square model with a significance level of P < 0.05. We attempted to document the location of spawning sites with egg collection mats in areas where telemetry results and substrate data indicated that spawning may be occurring. Egg mats were inspected at least every third day for the presence of eggs.
To date, 32 Atlantic sturgeons have been implanted with transmitters resulting in a total of 184 active and 182,740 passive relocations. Telemetered juvenile Atlantic sturgeon began entering the Delaware estuary in early spring and then gradually moved into the river (Figure 1A, 1B). Juvenile Atlantic sturgeons are typically concentrated into one of three geographically constrained areas in the Delaware River during the summer months. Their summer movements were generally reduced compared to spring and fall where movements have exceeded 100 km/d. Telemetered juvenile Atlantic sturgeon selected deep water (>8 m) habitat but showed no statistical preference to substrate type (Figure 2). We have also documented that six telemetered juvenile Atlantic sturgeon have entered the Chesapeake and Delaware Canal, with at least three individuals passing completely through its length, transiting between the two bays. Our results indicate primarily unidirectional movements from east to west, although there is some evidence of peripheral movements into the canal throughout the spring and summer.