Coral reef research programs in Hawaii primarily use diver-based underwater visual censuses in ≤30 m depths to assess roving predator populations between the Main (MHI) and Northwestern Hawaiian Islands (NWHI). As a probable consequence of survey biases, results from some methods imply remarkably top-heavy trophic pyramids that potentially inflate the scale of differences between remote and populated regions. Other limitations include the absence of predator information in > 30 m depths. To better understand regional differences, we compared shallow-water roving predator abundances and estimated predator length-frequencies between 2 diver-based visual assessment methods (stationary point count [‘SPC’] and towed-diver) and 2 video sampling techniques (unbaited and baited remote underwater stereo-video systems: RUVS and BRUVS). We also surveyed 30−100 m (‘mesophotic’) roving predators using RUVS and BRUVS. As with diver-based assessments, RUVS and BRUVS sampled considerably more roving predators in the NWHI versus the MHI, with patterns remaining consistent between methods. However, the NWHI:MHI scales of difference for RUVS and BRUVS tended to be substantially lower than for diver surveys. The largest discrepancies were recorded for the giant trevally Caranx ignobilis, where NWHI:MHI abundance ratios varied by > 2 orders of magnitude between diver SPC and all other methods. Although our results corroborate substantially higher roving predator densities in the NWHI, this study demonstrates that the application of different methods can result in strikingly dissimilar predator estimates. Continued assessments among survey techniques, coupled with the inclusion of mesophotic surveys, remain vital to improving understanding of predator populations, providing information that is properly aligned with management and conservation needs.
Methods and Technology
USA - Hawaii