Assessing Spawning Runs of Anadromous Fishes Using a Combination of Split-Beam and DIDSON Count Data

Fixed-location riverine hydroacoustic techniques are an effective, non-intrusive way of monitoring upstream migration of anadromous fishes.  We used a combination of side-looking split-beam and side- and down-looking DIDSON sonars in a Bayesian framework to assess spawning run size of striped bass Morone saxatilis, American shad Alosa sapidissima, hickory shad A. mediocris, alewife A. pseudoharengus, blueback herring A. aestivalis, and semi-anadromous white perch M. americana, in the Roanoke River, NC during 2010 and 2011.   The 430 kHz split-beam transducer was aimed cross-channel, perpendicular to flow, to gather count data on upstream moving fishes in the mid-channel and near-bottom zones of the river.  Although split-beam monitoring is an effective and proven method, unevenness in the bottom profile caused ‘blind-spots’ in beam coverage near bottom, leading to fish moving past the sonar site undetected.   Another factor leading to underestimation is the narrow beam width close to shore, where migration rates are highest. We used a down-looking DIDSON deployment to address blind-spots in the split-beam coverage and monitor cross-channel and vertical distributions.  The side-looking DIDSON deployment was more effective than the split-beam system in covering the first 10 m.   DIDSON deployments also provided size and shape information that is useful in partitioning run size estimates by species. This combination of techniques and gears and our focus on areas with highest concentrations of upstream migrants resulted in improved accuracy and precision of our estimates.  We recommend this as a replicable and reliable protocol for sampling southeastern US rivers.