Diver towed GPS to estimate densities of a critically endangered fish

Abstract Spotted handfish are a critically endangered anglerfish. Monitoring of the species has, in the past, been undertaken via underwater visual census (UVC) parameterised with fixed length 100 m × 3 m strip transects measured with reel lines. However, the sparse distribution and cryptic nature of handfish resulted in low statistical power to track populations through time. To achieve reasonable confidence of detecting change ~ 40 transects are required for density estimates, making the logistical constraints of monitoring onerous. We trialled a new survey method for UVC at one study site with a known local population of fish, replacing reel transects with variable length strip transects parameterised with a diver towed Global Positioning System (GPS) float. From 18 transects we determined a density of 24 (± 3.5) handfish per hectare, but bootstrapping suggested little improvement in precision occurred after 8 transects. Modelling these results to previous estimates at the site indicated that our GPS float approach required many fewer transects and dives to determine 50% increases or decreases in fish densities. This improvement was partly due to the better sampling efficiency; with 3 times the average search area per transect but was also a result of longer transects being a better solution for monitoring a sparse population. Our results returned a normal sample distribution of fish counts with few zeros and many multiple observations, which differed to the original approach which was heavily skewed towards zero observations per transect. However, at very low population densities even this more robust solution will require many samples to detect change. Our GPS parameterized surveys also improved the functionality of observing fish behaviour as we could accurately plot fish allowing for detailed investigation of distributions. When fish were observed they were also bi-laterally digitally photographed to record their unique natural markings. By synchronising the photographs timestamp with the GPS clock, we were able to geo-locate individual recaptures and our preliminary results suggests net movements were limited. This new approach increases the logistical feasibility for monitoring spotted handfish and may have applications for other sparsely distributed benthic species.