Identification of humpback whale Megaptera novaeangliae wintering habitat in the Northwestern Hawaiian Islands using spatial habitat modeling

Abstract

During the winter months, humpback whales Megaptera novaeangliae prefer warm and shallow regions, and these habitat preferences have been linked to reproductive status and suc- cess. Central North Pacific humpback whales winter in the main Hawaiian Islands (MHI), with peak densities occurring in late March. The Hawaiian Archipelago extends over 1800 km to the northwest of the MHI and includes a number of islands and atolls referred to as the Northwestern Hawaiian Islands (NWHI). The NWHI likely provide wintering habitat for humpbacks in the North Pacific, but have rarely been surveyed during winter. We modeled the extent and spatial location of humpback wintering habitat across the Hawaiian Archipelago, using General Bathymetric Chart of the Ocean (GEBCO) 1 min bathymetry data and synoptic and January–March averaged sea surface tempera- ture (SST) grids (advanced very high resolution radiometer AVHRR) according to previously pub- lished habitat envelopes. Using these data, we produced polygons identifying areas shallower than 200 m and warmer than 21.1°C as potential wintering habitat. We then compared these habitat esti- mates with the locations of visual and acoustic detections of humpbacks during a March 2007 pilot survey across the NWHI. The spatial analysis predicted approximately 21900 km2 of wintering habi- tat in the Hawaiian Archipelago, with 2/3 (14700 km2) occurring in the NWHI. Nine sightings of humpback whales (n = 19) occurred during the 15 d cruise, including 3 groups with small calves or exhibiting breeding behaviors. All sightings occurred in warm, shallow water at or within our pre- dicted habitat regions. Acoustic detections of humpbacks followed the same pattern. The results of our habitat analysis and survey observations document for the first time the existence of extensive wintering habitat used by humpback whales in the NWHI. These novel but simple methods are applicable for predicting wintering locations of humpbacks in other regions, and our results should be incorporated into assessment and recovery efforts for humpback whales in the North Pacific.