One of the most frequently used sampling methods used in defining fish microhabitat is the Point Abundance Sampling by
Electrofishing (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 Scuba
diving (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 two
restricted littoral areas of the lake which presented the same environmental and topographical characteristics aiming to compare
0+ 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 habitat
use by the 0+ roach vs. habitat availability.
Even though 0+ roach occurrence and abundance were found to be similar with both methods, microhabitat profiles revealed
different patterns. A significant microhabitat difference between the two sampling methods was found using the non-parametric
statistical test of Wilcoxon (Z = -4.20, p < 0.01). We can hypothesise that the differences observed between the two sampling
designs were due to an escapement behaviour of 0+ roach. Unlike PASS, using PASE, fish are located in «shelter habitats» such
as shallow water and dense vegetation. Such behaviour is caused by the environmental disturbance induced by this sampling
method. The study reveals that PASS appears to be more suitable than PASE for the assessment 0+ fish microhabitat.