Incorporating fish orientation into target strength-total length equations: Horizontal-Aspect target-Strength equations for gizzard shad Dorosoma cepedianum

Abstract

Horizontally-oriented echosounders have become more common for sampling pelagic prey species in shallow waterbodies (e.g. < 20 m) or nearshore portions of deeper water bodies, where vertical beaming can be ineffective. To properly sample fishes with horizontally-oriented echosounders, a target-strength (TS; dB re 1 m2)-to-total-length (TL; mm) relationship must be developed to acquire reliable density data. However, when sampling with horizontal beaming, measured TS can vary greatly according to fish orientation (lateral versus head-on). Currently, a TS-TL equation that is based on TS data from individual fish measured at all orientations is used to convert between TL and measured TS. However, an orientation-based equation (equation that incorporates target orientation information when converting TS to TL) could increase the accuracy of size estimates from direct TS measurements. Target strength measurements were collected from euthanized Gizzard Shad Dorosoma cepedianum, in a tank, at orientations from 0 to 180° (0° and 180° being perpendicular to acoustic beam and 90° parallel with head facing the transducer) in 5° increments. We derived orientation-based and non-orientation-based TS-TL equations for Gizzard Shad. Eight orientation-based equations were compared with a catenary (U-Shape) function best representing the change in TS for different fish orientations (conditional R2 = 0.71 and marginal R2 = 0.67). Our orientation-based equation can be used to acquire more accurate Gizzard Shad biomass estimates when orientation information is available. We found significant errors occur when using an average-orientation TS-TL equation when fish do not have random orientation, so orientation-based equations should be used when possible. We also compared density estimates from previously published, non-orientation-based TSMean equations (TS-TL equation derived from mean TS of all target orientations) to determine if equation choice significantly affected density estimates from fish aggregations. Equation choice had a significant effect on the resulting density estimates from individual schools (P < 0.01), indicating species-specific equations provide greater accuracy.