Abstract
The Phoenix Archipelago in the Central Pacific is situated in what was once one of the most productive areas for capturing sperm whales (Physeter macrocephalus). These whales were the focal targets of American offshore whalers in the mid-19th century along the equator, an area known as the “on-the-line” whaling grounds. Now, as large-scale Marine Protected Areas (MPAs) have provided protection for marine mammals and their food sources, it is important to have a solid understanding of historical baselines so recovery distributions can be compared with pre-whaling distributions. The Phoenix Islands archipelago contains two large MPAs: the Phoenix Islands Protected Area (PIPA), established by Kiribati in 2008, and the Howland/Baker unit of the Pacific Remote Islands Marine National Monument (PRIMNM), established by the United States in 2009. Using historic whaling records from American whaling vessels operated through the wider Phoenix Archipelago region, we reconstructed information about the presence and distribution of P. microcephalus throughout the 1800s within and around PIPA and the Howland/Baker units of the PRIMNM. Historical data analyzed using ArcGIS showed that sperm whales were present year-round within the study area, which is consistent with 20th century records from the Ocean Biogeographic Information System (OBIS). A Getis Ord Gi∗ hotspot analysis also revealed sighting hotspots within PIPA and near Howland and Baker, suggesting that these two areas may be of long-term ecological importance to sperm whales in the central Pacific. The New England whaling fleet ceased whaling effort in the central Pacific in the late 1800s, and publicly available records since that time are scarce. There has been no modern systematic whale survey ever conducted within the Phoenix Archipelago, though anecdotal accounts and sightings have been compiled over the years. These intermittent accounts suggest that though whale populations have not recovered to pre-whaling baselines, large-scale MPAs may play a role in helping to foster a resurgence of marine mammal populations. As the network of large-scale MPAs continue to grow as part of the commitment to ocean conservation set forth by UNESCO, IUCN, and the UN Decade for Ocean Science, historical baselines will be critical as a “yardstick” to measure population resurgence success for each MPA, and for populations overall.
Highlights
Sperm whales (Physeter macrocephalus) are among the most widely distributed marine mammals in the world (Rice, 1989), feeding primarily in the mesopelagic ocean (Whitehead, 2003) and found often in equatorial waters (Bannister and Mitchell, 1980)
Currents were dominated by zonal flow, with the Equatorial Undercurrent (EUC), North Equatorial Countercurrent (NEC), and the pair of South and North Deep Countercurrents (SDCC and NDCC, known as Tsuchiya Jets) being the principal easterly currents (Figure 3)
detections per unit effort (DPUE) analysis indicates that whales were present throughout the year, with more detections made in the December through February quarter (Figure 6)
Summary
Sperm whales (Physeter macrocephalus) are among the most widely distributed marine mammals in the world (Rice, 1989), feeding primarily in the mesopelagic ocean (Whitehead, 2003) and found often in equatorial waters (Bannister and Mitchell, 1980). During the peak of open-boat whaling efforts in the central Pacific (between 1820 and 1870), some estimates suggest that American whaling vessels may have made thousands of trips through the Central Pacific equatorial region, in the Phoenix Archipelago (Smith et al, 2009) During this period, it is thought that the whaling effort reduced the sperm whale population by as much as 80%, though there is a wide range of variation in these estimates (Whitehead, 2002). Smith et al (2012) compilation of data from these sources combined represents approximately 10% of American whaling voyages, consisting of daily whale encounters recorded by American whalers between 1780 and 1920 This digitization displays spatial distribution of American whaling and the targeted population and enables the possibility of further analysis of the logbook data
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