These are not the trends you are looking for: poorly calibrated single-pass electrofishing data can bias estimates of trends in fish abundance

There is increasing interest in the potential of single-pass and timed electrofishing to assess status and trends in fish populations. However, where capture probability varies over time, there is a risk that uncalibrated electrofishing data could fail to detect, or provide biased estimates of trends. This study analysed a long-term electrofishing dataset collected over 50 years in an intensively studied catchment where egg deposition and emigrant production declined by c. 82% and 35% over the same time. The electrofishing data were used to illustrate the effects of changing capture probability on estimated trends in juvenile Atlantic salmon Salmo salar abundance. Temporal variability in capture probability was modelled. Trends in abundance were then estimated from uncalibrated single-pass electrofishing count data and compared with estimates from data calibrated for capture probability. The calibrated data revealed significant declines in S. salar fry (age 0) and parr (age ≥ 1) abundance. However, the trend estimates from the uncalibrated data were positively biased and not significant. Exploration of alternative (realistic) scenarios with different trends in true abundance and capture probability suggests that uncalibrated electrofishing data can provide very misleading estimates of trends. The problem is exacerbated in data where capture probability is low. It is recommended that single-pass and timed electrofishing methods should not be used to assess trends in fish populations without regular (annual) calibration.