Abstract
The in situ avoidance reaction of vendace, Coregonus albula L., in relation to continuous artificial light in terms of a scaring device was tested by split‐beam echosounding. The tests were carried out in the mesotrophic Bigge Reservoir (Germany) in November 2007 and 2008 using a Simrad EK 60 echosounder (120 kHz). An underwater lamp with light turned on and off was lowered to 30 m water depth at four sampling sites. No reaction of vendace was observed for the sampled references (light off), but strong avoidance behaviour by swimming mainly downwards was observed when the light was turned on. The light avoidance reaction was confirmed by the volume backscattering strength ( S v ) and calculated fish densities (fish ha −1 ). The mean fish densities for the samples carried out with light turned on were lower (268 fish ha −1 , ±409 SD, n = 32) than for the references with light turned off (5028 fish ha −1 , ±2317 SD, n = 56). The related median values (33 and 3726 fish ha −1 , respectively, n = 88) differed significantly (Mann–Whitney test, P =