Assessing Horizontally-oriented Acoustic Methods for Gizzard Shad Abundance Assessments: Accuracy, Precision and Target Orientation

Abstract

Gizzard Shad (Dorosoma cepedianum) are an important prey species that are commonly sampled with gill nets. However, horizontally-oriented methodologies have the potential to produce better Gizzard Shad data with less effort. Before horizontal beaming can be used as a sampling gear for Gizzard Shad, accuracy and precision need to be examined to determine if this approach provides reliable and consistent data. Further, Gizzard Shad-specific relationships between acoustic target strength (TS) and total length (TL) should be derived to ensure density estimates are accurate. I tested the accuracy and precision of horizontal beaming by sampling known populations of Gizzard Shad in a net pen (15-m long x 15-m wide x 4.5-m deep with 6.35-mm square mesh). I found horizontal beaming accurately detected changes in density (R2=0.63) with increased precision (mean CV of 6% among all trials) than other gears used to sample Gizzard Shad. Given that TS changes with fish orientation, I developed an orientation-based TS-TL equation to increase accuracy of hydroacoustic estimates. A catenary (U-Shape) function was best at representing the change in TS at different fish orientations (conditional R2 = 0.71 and marginal R2 = 0.67). I also compared echo integration results using six different TS-TL equations (2 from this paper and 4 from previous literature) using 23 fish aggregations imaged in the field. Equation choice had a significant effect on density estimates (P<0.01) indicating care should be taken when selecting TS-TL equations for use in hydroacoustic surveys.