Predictive Models of Cetacean Densities in the Hawaiian Islands EEZ, 2020

To develop improved and updated species distribution models (SDMs) and to update cetacean stock abundance estimates for waters of the US Hawaiian EEZ, sighting data from the Hawaiian Islands Cetacean and Ecosystem Assessment Survey (HICEAS) 2017 were combined with previous line-transect survey data collected within the study area to create a modeling database spanning the period from 2002 to 2017. The majority of these data were from the two previous HICEAS efforts, the first in 2002 and the second in 2010. In contrast to previous modeling efforts that included survey data from a broader region of the central Pacific Ocean, the current SDMs were built only with survey data collected within waters of the Hawaiian EEZ. Habitat models were developed to derive spatially-explicit estimates of species density specific to the Hawaiian EEZ based on previously established methods that allow for the incorporation of segment-specific estimates of detection probability. Potential habitat variables included bathymetric depth, distance to islands, and a suite of dynamic surface and subsurface outputs from an ocean circulation model. The habitat-based models of cetacean density developed in this study represent an improvement over previous models because they more accurately account for variation in detection probabilities, provide finer-scale density predictions (~9km x 9km grid resolution), and better account for uncertainty in the resulting study area abundance estimates. In addition, they include dynamic subsurface variables that were not available for the previous models. Further, increases in sample sizes allowed for the development of a new habitat model for Risso’s dolphin (Grampus griseus).

These models have been used by the Navy and others to assess potential impacts on cetaceans in compliance with U.S. regulations such as the Endangered Species Act and Marine Mammal Protection Act, which require species specific assessments of the number of animals potentially affected by their activities.