Diurnal Abundance Patterns of Mediterranean Fishes Assessed on Fixed Transects by Scuba Divers

Abstract The effect of diurnal activity patterns and water depth on fish censuses has received little attention from researchers. In this study, we censused 18 species of Mediterranean fish off the coast of Corsica, France, by scuba dives during daylight hours on fixed transects, at depths of 0–3, 3–6, and 6–9 m. Daylight hours were divided into five 3-h time periods between 0500 and 2000 hours. Overall abundance (i.e., all species combined) was relatively constant between 0800 and 1700 hours, but was lower at other times, even when nocturnal species were included. Although overall abundance remained relatively constant throughout daylight hours, there were significant differences in depth for 15 of the 18 species, despite the narrow range of depths studied. Therefore, depth should be an integral part of any Mediterranean transect census and should be considered as potentially important in other waters. On our transects, any time of day from 3 h after sunrise to 3 h before sunset was suitable for general ...