Use of Split-Beam Sonar to Estimate Anadromous Fish Runs in the Roanoke River, North Carolina.

Abstract

ABSTRACT MAGOWAN, KEVIN JAMES. Use of Split-beam Sonar to Estimate Anadromous Fish Runs in the Roanoke River, North Carolina. (Under the direction of Joseph E. Hightower.)

Annual in-river population estimates of anadromous fishes in the Roanoke River would be useful for managing this resource. The goal for some species (American shad Alosa sapidissima, alewife Alosa pseudoharengus, and blueback herring Alosa aestivalis) is to monitor current restoration efforts. For other species (striped bass Morone saxatilis, hickory shad Alosa mediocris and white perch Morone americana), the goal is to manage sustainable commercial and recreational fisheries. Hydroacoustic methods are useful for estimating the abundance of anadromous fishes, because these methods are non-intrusive, have been proven to work in other river systems, and are capable of generating real-time population estimates. The goal of this project was to estimate the numbers of anadromous fishes returning to the Roanoke River in 2006 and 2007. We used fixed-location split-beam sonar in conjunction with drift gill netting and boat electrofishing to estimate run size of alewife, American shad, blueback herring, hickory shad, striped bass and white perch migrating upstream past a study site at river kilometer 64 near Williamston, North Carolina. The default estimates (and SEs) for 2006, assuming a uniform cross-channel distribution of upstream migrants and a fixed river depth cut-off point to separate fish migrating near the river bank (i.e., in water < 3 m deep) from fish migrating in the river channel (i.e., in water > 3 m deep), were: total run size 819,063 (± 31,234), alewife 60,622 (± 7,871), American shad 35,483 (± 4,936), blueback herring 88,281 (± 8,689), hickory shad 40,465 (± 6,155), striped bass 217,439 (± 14,236), white perch 83,035 (± 8,054), and other fish species 293,737 (± 13,356). In 2006, the estimates were generated for fish that migrated past the monitoring site from March 18 to May 29. In 2007, under the same assumptions, an estimated total of 1,137,921 (± 32,310) fish migrated upstream past the study site from March 5 to May 26. Species-specific run size estimates were: alewife 270,478 (± 16,138), American shad 5,175 (± 1,401), blueback herring 173,796 (± 16,660), hickory shad 102,645 (± 12,720), striped bass 139,517 (± 14,890), white perch 45,522 (± 2,057), and other fish species 400,787 (± 22,224). The profile of the river channel at the monitoring site allowed for the acoustic beam to sample about 59% of the cross-sectional area of the river channel. This was an increase in the percentage of area covered by the acoustic beam, relative to an upriver site used in 2004-2005. Increasing the area sampled should lead to a more accurate estimate of the total number of upstream migrants. However, the reliability of species-specific run size estimates is hard to assess because high species diversity and low catches made estimating species composition difficult. There were strong diurnal patterns in migration. Mean fish passage was significantly higher during daylight hours (06:00 to 20:59). Based on hydroacoustic results, upstream migrants at night were larger, swam slower and were located closer to the river bank. Upstream migrating fish also responded to changes in flow, with fish being evenly distributed across the river channel during flows < 4,000 ft3s-1 (113 m3s-1), but closer to shore during periods of higher flows. Most projects using sonar in rivers to count fish are done in rivers with low species diversity, so obtaining accurate species-specific run-size estimates in a species-rich environment like the Roanoke River, North Carolina is challenging. The greatest improvements in the reliability of hydroacoustic estimates will likely come from increasing the area of the sampled river cross-section, eliminating bottom interference, and refining the methods used to estimate species composition.