Gizzard Shad Target Strength-To-Body Size Equations At Multiple Hydroacoustic Frequencies

Species-specific target strength (TS)-to-length (L) and TS-to-weight (W) equations can reduce bias in biomass estimates from hydroacoustic surveys, yet these equations do not exist for most fishes. Equations specific to the Gizzard Shad Dorosoma cepedianum, a wide-ranging and often highly abundant prey fish in North American reservoirs and rivers, do not exist. Herein, we sought to develop TS-L and TS-W equations for Gizzard Shad by insonifying free-swimming individuals of known sizes (36-209 mm TL) in a net cage at three transducer frequencies (70, 120, and 200 kHz). We derived TS-size relationships using major-axis regression (MAR) and least-squares regression (LSR), comparing our resultant TS-L equations to a commonly used multispecies equation (Love 1971). To determine how our Gizzard Shad-specific equations affected estimates of prey fish biomass, we conducted mobile hydroacoustic surveys in four small, shallow Midwestern reservoirs and then estimated biomass using each equation. In general, for TS-L equations, MAR produced the highest estimates of biomass, followed by LSR and then the multispecies equation. Similarly, the TS-W equation derived using MAR produced greater biomass estimates than the equation using LSR. Our findings highlight the value of using species-specific TS-size equations over multispecies equations in ecosystems dominated by a single fish species (e.g., Gizzard Shad in Midwestern reservoirs). They also demonstrate the value of using MAR to develop TS-size equations, owing to its ability to account for fish size measurement error. The use of species-specific TS-size equations and MAR should lead to robust estimates of biomass in single-species-dominated ecosystems, thus benefiting fisheries assessment and decision making.