Migratory Behavior of American Shad in the York River, Virginia, with Implications for Estimating In-River Exploitation from Tag Recovery Data

Abstract

Tagging of American shad Alosa sapidissima may alter their migratory behavior, causing some tagged individuals to cease or delay the spawning run. In a tag recovery study designed to assess fishery impacts, this altered behavior would reduce the number of tagged fish available to the target fishery and would bias estimates of exploitation and fishing mortality rates. To investigate this possibility, we fitted 29 prespawning adults with acoustic tags and released the fish into the middle reaches of the York River, Virginia. Movements of individuals were remotely monitored at three hydrophone stations: (1) 7 river kilometers (rkm) downriver of the release site; (2) on the Mattaponi River, 48 rkm upriver of the release location; and (3) on the Pamunkey River, 56 rkm upriver of the release location. Almost half of the fish were apparently affected by capture, handling, and tagging, as they either abandoned their migration or delayed their upstream movements. The movements of some fish appeared to be unaffected by capture; these fish were not detected at the downriver station and were detected on the spawning grounds 2–5 d after release. Eighteen fish remained on the spawning grounds for 17–51 d (average = 34.4 d) and were last detected at the downriver location, presumably during their seaward migration. Of the 26 tagged fish that migrated to either tributary after release, 15 originally selected spawning grounds on the Mattaponi River and 11 selected the Pamunkey River. One fish occupied both tributaries for several weeks each, suggesting possible spawning at both locations. We conclude that tagging protocols designed to measure the impacts of fishing on American shad should include telemetry to assess altered migratory behavior.