Influence of moon phase on acoustic estimates of the abundance of fish performing daily horizontal migration in a small oligotrophic lake

Abstract

We tested the influence of the moon phase on the relative number of fish performing daily horizontal migrations and we used this information to develop a sampling strategy that accounted for expected effects of the moon phase on fish behavior. Hydroacoustic surveys were performed over 3 days at each moon phase for the months of July and August (from the day before to the day after the new moon, the first and last quarter moon, and the full moon). Dace (Phoxinus eos × Phoxinus neogaeus) migrated from the littoral towards the pelagic zone at dusk and returned to the littoral zone at dawn. Their relative abundance (FRA) in the pelagic zone averaged 0.1 fish echo·m-3during midday and 17.0 fish echoes·m-3at night. The number of dace performing horizontal movements was eightfold larger during new-moon than full-moon nights. Simulations indicated that sampling fish populations six times during a 2-month period (once at full, new, and quarter moon; once during the day and once during the night) provided the most accurate estimates of FRA.