Acoustic telemetry informs conditional capture probability of an anadromous fish

Information on movement ecology and susceptibility to fishing gears is becoming increasingly employed in the management of commercial fisheries. This study combined acoustic telemetry (n = 51 and 52) and a simple passive integrated transponder (PIT) tag (n = 1499 and 1113) mark-recapture study, across two successive years on a commercial river lamprey (Lampetra fluviatilis) fishery, to inform exploitation rates and the influence of conditional capture probability on expected catch-per-unit-effort (CPUE). The movements of acoustic-tagged lamprey were used to refine estimates of the number of PIT-marked individuals vulnerable to the fishery. In 2018, this increased the recapture rate estimation by 0.99% and reduced the estimated run size by 152,101 (21.6%; 95% CL, 148,683, 155,688) individuals, with corresponding values of 0.12% and 114,639 (25.0%; 95% CL, 112,900, 116,448) in 2019. Lamprey movements were similar between years, with the number of trap line encounters by individuals between trap lifts used to inform expected CPUE for each lift. Conditional capture probability was mainly dependent on environmental conditions (e.g., river flow) with most trap lifts in the expected CPUE range, although the impacts of behaviour on vulnerability to capture were difficult to disentangle. This study highlights how the incorporation of acoustic telemetry increased the accuracy of, validated, and complemented mark-recapture data, without which management decisions (e.g., quota size) would have been based upon over 100,000 more individuals (27.5% higher than the adjusted run size in 2018 and 33.3% higher in 2019), with potentially severe consequences for the population. These findings demonstrate the importance of understanding fish movement to improve and inform fishery management. The study also presents a framework to quantify conditional capture probability and its influence on CPUE; knowledge that is widely applicable across aquatic systems for management and sustainability of fisheries.