Humpback whale (Megaptera novaeangliae) populations worldwide have been recovering from whaling, albeit at different rates. A recent global review of their population structure and conservation status under the USA Endangered Species Act led to the recognition of 14 Distinct Population Segments (DPS). In this study, we compare the sighting characteristics for two of these DPSs occurring in the Pacific Ocean that use the same habitat off Central America during their respective winters: the Central America DPS, which migrates from feeding areas off the west coast of North America during the boreal winter and is still considered endangered, and the Southeastern Pacific DPS, which migrates from feeding areas off the Antarctic Peninsula and Chile during the austral winter (also known as Breeding Stock G for IWC management purposes) and is no longer considered endangered. We also compare the characteristics of their breeding habitat in terms of seafloor depth, slope, and sea surface temperature (SST). We conducted non-systematic surveys off northwestern Costa Rica (NCR), southeastern Costa Rica (SCR), and western Panama (PAN) during both the boreal winter (22,866 km surveyed in1996-2003) and the austral winter (6,011 km surveyed in 2000-2007), and made a total of 179 sightings of 305 whales in the boreal winter, and 177 sightings of 364 whales in the austral winter. Overall encounter rates were higher during the austral season than the boreal season (6.06 whales/100 km vs. 1.33 whales/100 km, respectively). Both seasons had a relatively high percentage of groups sighted with a calf, but this percentage was much higher for the austral than the boreal season (57% vs. 28%, respectively), suggesting different demographic parameters for these two DPSs. Although in general whales were seen in similar depth, slope, and SST habitats, significant differences between seasons were found in all three variables. A trend toward shallower depth and steeper slope in the austral winter can be explained by local differences in seafloor characteristics between SCR and PAN, while a trend toward lower SST reflects regionally cooler temperatures during this time of year. Groups with calves had a distinct preference for shallower depth, with most being found at 25 m in the boreal winter and 18 m in the austral winter. Because whales from these two DPSs use the similar habitat during their mating and calving season off Central America, we speculate that factors (natural or anthropogenic) on the feeding areas may be influencing their rates of recovery
Citation:
Rasmussen, K., D.M. Palacios, J. Calambokidis, and G.H. Steiger. 2017. Sighting and environmental characteristics of humpback whale breeding habitat off Pacific Central America: comparison of Northern and Southern Hemisphere populations. IWC Report SC/A17/NP/07 for the Workshop on the Comprehensive Assessment of North Pacific Humpback Whales. 18-21 April 2017. Seattle, WA. 17pp.
Download PDF