Intercalibration of hydroacoustic and mark-recapture methods for assessing the spawning population size of a threatened fish species

Hydroacoustic counting and a three‐year mark–recapture study with passive integrated transponders (PIT tags) were used to estimate the size of a spawning population of nase Chondrostoma nasus, a threatened potamodromous cyprinid that undertakes annual spawning migrations into a tributary of the Danube River. In 2005, the estimates of the size of the spawning population from the hydroacoustic counts (N = 2234, 95% CL 1929–2538) and from the Jolly–Seber model (N = 1198, 95% CL 461–5842) corresponded well. Estimates from the jackknife‐estimator based on the hydroacoustic counts yielded slightly higher values (N = 2783, 95% CL 2529–3037), but were still in the same order of magnitude as those from the hydroacoustic and mark–recapture approach. At low run‐size, hydroacoustic counting was more time consuming and technically demanding than mark–recapture studies. At the same time, it was non‐invasive, provided real‐time data on a fine temporal scale, and estimates showed less variability than the Jolly–Seber model. Mark–recapture of fish in spawning streams involved substantial disturbance at a sensitive stage of the life cycle. Hence, hydroacoustics is highly suited for population estimates of threatened potamodromous fishes, where interference needs to be minimized.