Status of Shortnose Sturgeon in the Potomac River. Part 1: Field Studies

Abstract

Field studies during more than 3 years (March 2004–July 2007) collected data on life history of Potomac River shortnose sturgeon Acipenser brevirostrum to understand their biological status in the river. We sampled intensively for adults using gill nets, but captured only one adult in 2005. Another adult was captured in 2006 by a commercial fisher. Both fish were females with excellent body and fin condition, both had mature eggs, and both were telemetrytagged to track their movements. The lack of capturing adults, even when intensive netting was guided by movements of tracked fish, indicated abundance of the species was less than in any river known with a sustaining population of the species. Telemetry tracking of the two females (one during September 2005–July 2007, one during March 2006–February 2007) found they remained in the river for all the year, not for just a few months like sturgeons on a coastal migration. Further, one fish used the same freshwater reach during three summers. The two sturgeons used different reaches during some seasons, with one fish using saline water more than the other. The adults homed to small reaches in the same month each year, like shortnose sturgeon in their natal river. The total reach used by tracked sturgeons was 124 km (rkm 63–187), of which the lowermost 78 km, which was used for summering and wintering, contained the freshwater: saltwater interface. The most upstream reach used (rkm 185–187) contained potential spawning habitat. This reach was visited by one female on a pre-spawning migration in April 2006, but spawning was likely unsuccessful. Water quality (dissolved oxygen and temperature) in the summering–wintering reach was adequate all the year, although during the summer it was minimally acceptable. We periodically recaptured the same tagged female and found she healed well after tagging, appeared healthy in body and fins, grew well, and rapidly matured a new clutch of eggs. All surveys indicated adults had sufficient habitat and water quality needed to complete their life history. While we studied only two adults, all data strongly suggests shortnose sturgeons are a permanent resident of the Potomac River diadromous fish community. Life-history movements of the Potomac River adults were similar to adults in northcentral rivers, like the Delaware River, not to adults in southern rivers. We did not identify a unique life history behavior that separated Potomac River adults from other populations. Life history data indicates Potomac River shortnose sturgeons are most likely remnants of the natal population or colonizers from a north-central river, like the Delaware River.