Effects of differing ground‐truth data,transect design and statistical analysis on the repeatability of hydroacoustic assessments of pollan Coregonus autumnalis pollan

Pollan, Coregonus autumnalis pollan Thompson, requires an appropriate standardised sampling protocol for conservation assessments. The suitability of hydroacoustics with gillnetting was evaluated, and the effects of sampling design, effort and statistical analysis on the repeatability of results were tested. Summer hydroacoustic surveys appear appropriate as pollan were not abundant in acoustically unsampled areas. However, pollan density estimates were significantly affected by the gillnet sampling design used to ground‐truth the acoustic data. Density estimates from the >12‐ m layer were more robust to gillnet design than estimates from the 3‐ to 12‐ m layer. Estimates from different transect designs yielded some statistically significant results. Comparison of transects common to both transect designs suggests that observed differences reflect temporal changes rather than transect design effects. The reduction in the systematic parallel transect effort by half significantly affected target strength distributions but not pollan density estimates. Density estimates were affected by statistical analysis method where pollan density was highest, with geostatistical analysis providing higher estimates than the arithmetic mean. Hydroacoustic assessments may provide a reliable index of pollan abundance over time and between lakes but future research should investigate the effects of temporal and abiotic factors.