A TOWED-DIVER SURVEY METHOD FOR MESOSCALE FISHERY-INDEPENDENT ASSESSMENT OF LARGE-BODIED REEF FISHES

Coral reef fishes are ecologically and economically important, particularly the largest species and individuals, which often include top predators, as they show disproportionate fecundity and generally make up the basis of nearshore fisheries. Large-bodied coral reef fishes can be difficult to sample using conventional survey methods, because they are often relatively rare, have comparatively large home ranges, and can exhibit behaviors that influence their sightability. We describe a towed-diver survey that can be used to assess populations of these fishes in which a pair of SCUBA divers maneuver towboard dive planes and record the abundance and size of large-bodied (> 50 cm TL) reef fishes while being towed at slow speed behind a small boat for 50 min. The resulting biological data are georeferenced, allowing for location-based comparison with habitat and environmental data. Comparisons with conventional belt transect surveys within the same reef and habitats reveal similar archipelagic patterns, but show some key differences. Encounter rates and statistical power are generally higher using the towed-diver method, and taxa that show behavioral responses to SCUBA divers exhibit different abundance and distribution patterns with each method. The scale of the towed-diver survey allows for more complete and representative coverage of the shallow-water reef environment, resulting in improved large-scale estimates of the density and spatial distribution of large-bodied reef fishes. As a result, the towed-diver method provides managers and scientists with an additional tool for assessing the distribution and status of large-bodied reef fish stocks.