When target strength models meet messy biology: Uncertainty in active acoustic population estimates of fish and zooplankton

Target strength (TS) is commonly used to convert acoustic backscatter into population estimates and other biologically meaningful parameters for fish and zooplankton. Researchers often derive TS using theoretical models that depend on frequency, morphology, material properties, and orientation. But several sources of uncertainty can adversely affect the use of TS as a reliable biological estimator, including natural variability in parameters, intrinsic model biases, and poor model transferability across species and regions. Furthermore, limited time and funding for surveys often make it difficult to measure important parameters like orientation and material properties. Therefore, choosing an appropriate TS means striking a balance between biological realism, survey goals, and acceptable levels of uncertainty. In this talk, I will present case studies where biological “messiness” strongly influenced both modeled and experimentally measured TS. These examples show how small differences in model parameters yield substantial changes in population estimates. They also point toward practical strategies for improving TS model parameterization. The lessons learned from these examples are instrumental to developing decision support tools that help researchers focus limited resources, such as ship time, without compromising key survey objectives.