Marine Resources Assessment for the Marianas Operating ... - SPREP

AUGUST 2005 FINAL REPORT
It should be noted that the sperm whale’s ESA status as endangered is somewhat political, and the
species is actually in no immediate danger of global extinction (unlike some species, such as the
North Pacific right whale, which clearly are). Although many sperm whale populations have been
depleted to varying degrees by past whaling activities, sperm whales remain one of the more globally
common great whale species. In fact, in some areas, they are actually quite abundant. As just a
single example, there are estimated to be about 21,200 to 22,700 sperm whales in the eastern
tropical Pacific Ocean (Wade and Gerrodette 1993).
Habitat Preferences—Sperm whales show a strong preference for deep waters (Rice 1989),
especially areas with high sea floor relief. Sperm whale distribution is associated with waters over the
continental shelf edge, over the continental slope, and into deeper waters (Hain et al. 1985; Kenney
and Winn 1987; Waring and Finn 1995; Gannier 2000; Gregr and Trites 2001; Waring et al. 2001).
However, in some areas, such as off New England, on the southwestern and eastern Scotian Shelf,
or the northern Gulf of California, adult males are reported to quite consistently use waters with
bottom depths less than 100 m and as shallow as 40 m (Whitehead et al. 1992; Scott and Sadove
1997; Croll et al. 1999; Garrigue and Greaves 2001). Worldwide, females rarely enter the shallow
waters over the continental shelf (Whitehead 2003).
Sperm whale concentrations have been correlated with high secondary productivity and steep
underwater topography (Jaquet and Whitehead 1996). Sperm whales are more frequently found in
certain geographic areas, which whalers learned to exploit (e.g., whaling “grounds” such as the
Azores Islands) encompassing 300 to 1,500 km 2 (Townsend 1935). These main sperm whaling
grounds are usually correlated with areas of increased primary productivity caused by upwelling
(Jaquet et al. 1996). Sperm whales in the Gulf of Mexico aggregate along the continental slope in or
near cyclonic (cold-core) eddies (Biggs et al. 2000; Davis et al. 2002). These eddies are mesoscale
features with locally enhanced plankton stocks (Wormuth et al. 2000). Data suggest that sperm
whales appear to adjust their movements to stay in or near cold-core rings (Davis et al. 2000, 2002).
This would demonstrate that sperm whales shift their movements in relation to prey concentrations.
Off the eastern U.S., sperm whales are found in regions of pronounced horizontal temperature
gradients, along the edges of the Gulf Stream and warm-core rings (Waring et al. 1993; Griffin 1999;
Jaquet et al. 1996). It is likely that these habitats are regions where oceanographic conditions are
optimal for the aggregation of prey, such as squid. Waring et al. (2003) conducted a deepwater
survey south of Georges Bank in 2002 and examined fine-scale habitat use by sperm whales. Sperm
whales were located in waters characterized by a SST of 23.2º to 24.9ºC and a bottom depth of 325
to 2,300 m (Waring et al. 2003). In the eastern tropical Pacific, sperm whale habitat use is
significantly related to SST and depth of the thermocline (Polacheck 1987). Gregr and Trites (2001)
reported that female sperm whales off British Columbia were relatively unaffected by the surrounding
oceanography.
Distribution—Sperm whales are found from tropical to polar waters in all oceans of the world
between approximately 70°N and 70°S (Rice 1998). Females use a subset of the waters where males
are regularly found. Females are normally restricted to areas with SSTs greater than approximately
15ºC, whereas males, and especially the largest males, can be found in waters as far poleward as
the pack ice with temperatures close to 0ºC (Rice 1989). The thermal limits on female distribution
correspond approximately to the 40º parallels (50º in the North Pacific) (Whitehead 2003).
Information Specific to the Marianas MRA Study Area—Globally, sperm whales are typically
distributed in waters over the shelf break and continental slope. Sperm whales are expected to
occur seaward of the shelf break into waters with bottom depths analogous to the open ocean
(>200 m) expected in the Marianas study area and vicinity (Figure B-7). Whaling records
demonstrate sightings year-round around the Marianas (Townsend 1935). In some locales,
sperm whales also may be found in waters less than 100 m deep (Scott and Sadove 1997; Croll
et al. 1999). To account for the possibility of encountering this species in shallow waters, there is
a low or unknown sperm whale occurrence from the 50 m isobath to the shelf break (Figure B-7).
This occurrence prediction is based on the possibility of this typically deepwater species being
found in insular shelf waters that are in such close proximity to deep water. As in other areas
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