Estimating and Evaluating Mechanisms Related to Walleye Escapement from Rathbun Lake, Iowa
This article is from North American Journal of Fisheries Management 33 (2013): 642, doi: 10.1080/02755947.2013.788588. Posted with permission.
Reservoir fisheries present managers a unique set of challenges. One of the most obvious, yet overlooked, challenges to maintaining sustainable reservoir fisheries is escapement. Yet, little is known about the escapement of reservoir fishes or factors influencing escapement. From June 2009 through March 2011, 4,137 Walleyes Sander vitreus >300 mm were collected in the tailrace of Rathbun Lake, Iowa, individually tagged with visual implant tags, and returned to the reservoir to estimate escapement rates and identify influential factors. During this time, escapement was estimated as (1) the percentage of tagged Walleyes at large that were recaptured in the tailrace and (2) the proportion of tagged to untagged Walleyes present in Rathbun Lake. Tailrace recapture rates during individual sampling events ranged between 0.0% and 2.1% and minimum annual tailrace escapement was estimated as 13.6% (SE, 3%). In spring 2010, 8% (SE, 1%) of the Rathbun Lake Walleye population had been tagged and by spring 2011, 26% (SE, 2%) of the Walleyes were tagged, indicating that they had previously escaped. To understand factors related to escapement, individual fish capture–recapture data were analyzed in Program MARK using a multistate model to estimate apparent survival, detection, and escapement probability. Probability of escapement increased with increasing mean daily discharge and decreased with increasing fish length and release distance from the dam. Variable weights indicated that discharge was the primary factor related to escapement. Escapement probability increased exponentially with daily discharge and doubled as discharge increased from 40 to 60 m3/s. Our results suggest a substantial proportion of the Rathbun Lake Walleye population has been lost recently, due in part to record water releases, making management of this reservoir fishery challenging.