Counting coral reef fishes: Interaction between fish life-history traits and transect design

Underwater visual censuses are the most commonly used methods to estimate the density of reef fish populations and assemblages. One basic assumption is that the observer will always detect fish in the same way from one sampling unit to the next, implying that, on average, the spatial distribution pattern of fish abundance or occurrence remains the same from one transect to the next (H0). The present work tested H0 using data from 730 transects covering two regions (New Caledonia and French Polynesia), 604 species and 504 000 fish. Within transect variations in reef fish abundance and occurrence were studied according to site factors (region, reef type), life-history traits (adult size, home range, schooling behaviour, color, pattern, swimming speed, level in the water column, inquisitiveness, crypticity), and observations characteristics (distance of observation, size of the observed fishes, number of fishes within an observation, observer identity). Two general trends were detected: 1 — at the start of transects, both fish occurrence and abundance were higher than the values expected under H0; 2 — a similar trend was also observed at the end of transects, but at a much lower magnitude. These effects were observed with varying degrees of magnitude for all regions, reef types and observers, varied significantly according to three life-history traits (size, home range, and behaviour), but were not influenced by species richness or abundance. These results indicate that datasets gathered from transects of various lengths cannot be pooled without correction. They also shed light on some of the known differences between transects and point counts.