Comparison of two point abundance sampling methods to assessyoung roach (Rutilus rutilus L.) microhabitat in the littoral zone of lake Pareloup (France)

Abstract

One of the most frequently used sampling methods used in defining fish microhabitat is the Point Abundance Sampling byElectrofishing (PASE) technique. Nevertheless this method is size-selective and induces escapement behaviour in most fish species.The aim of this study is first to describe a direct visual fish observation method, the Point Abundance Sampling by Scubadiving (PASS), and second, to compare this method with the classical PASE to determine 0+ (i.e. young of the year) roach(Rutilus rutilus,L.) microhabitat in the littoral zone of a mesotrophic lake.The study was undertaken during summer 1998 in lake Pareloup (south-west France). Sampling was performed weekly in tworestricted littoral areas of the lake which presented the same environmental and topographical characteristics aiming to compare0+ roach habitat features obtained using the two sampling techniques (PASE and PASS). The two data matrices obtained (i.e.PASE and PASS) were used to develop microhabitat preference indices for each of the 9 variables as a measurement of habitatuse by the 0+ roach vs. habitat availability.Even though 0+ roach occurrence and abundance were found to be similar with both methods, microhabitat profiles revealeddifferent patterns. A significant microhabitat difference between the two sampling methods was found using the non-parametricstatistical test of Wilcoxon (Z = -4.20, p < 0.01). We can hypothesise that the differences observed between the two samplingdesigns were due to an escapement behaviour of 0+ roach. Unlike PASS, using PASE, fish are located in «shelter habitats» suchas shallow water and dense vegetation. Such behaviour is caused by the environmental disturbance induced by this samplingmethod. The study reveals that PASS appears to be more suitable than PASE for the assessment 0+ fish microhabitat.