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

AUGUST 2005 FINAL REPORT
unknown occurrence of pilot whales in waters over the shelf from the coastline to the 100 m
isobath, not including any lagoons. Occurrence patterns are assumed to be the same throughout
the year.
Behavior and Life History—Pilot whales are very social and may be seen in groups of several
individuals to upwards of several hundreds. They appear to live in relatively stable female-based
groups (Jefferson et al. 1993). Pilot whales are often sighted associated with other cetaceans (e.g.,
Bernard and Reilly 1999; Gannier 2000). These are the most frequently reported mass-stranded
marine mammals globally (Nelson and Lien 1996; Mazzuca et al. 1999).
Average age at sexual maturity for short-finned pilot whales is 9 years for females, and 17 years for
males (Bernard and Reilly 1999). The gestation period for short-finned pilot whales is 15 to 16
months, with a mean calving interval of 4.6 to 5.7 years (Bernard and Reilly 1999). Calving peaks in
the northern hemisphere vary by stock (Jefferson et al. 1993).
Pilot whales are deep divers. They can stay submerged for over 10 min; the maximum dive depth
measured is 610 m (Bernard and Reilly 1999). The deepest dives recorded by Baird et al. (2003b) for
tagged short-finned pilot whales were typically 600 to 800 m for 27 min. Pilot whales feed primarily on
squid, but also take fish (Bernard and Reilly 1999). Pilot whales are not generally known to prey on
other marine mammals; however, records from the eastern tropical Pacific suggest that the shortfinned
pilot whale does occasionally chase, attack, and may eat dolphins during fishery operations
(Perryman and Foster 1980), and they have been observed harassing sperm whales in the Gulf of
Mexico (Weller et al. 1996).
Acoustics and Hearing—Short-finned pilot whale whistles and clicks have a dominant frequency
range of 2 to 14 kHz and a source level of 180 dB re 1 µPa-m (Ketten 1998). There are no published
hearing data available for this species.
♦ Northern Elephant Seal (Mirounga angustirostris)
Description—The northern elephant seal is the largest pinniped in the Northern Hemisphere (the
second-largest in the world, after the southern elephant seal Mirounga leonina). It is one of the most
sexually dimorphic mammals, with adult males much larger than adult females (Deutsch et al. 1994).
The northern elephant seal reaches a standard length of up to 2.8 to 3.0 m and weights of 600 to 800
kg (females) and 3.8 to 4.1 m and 300 kg (males) (Stewart and Huber 1993). As males reach
adulthood, they also develop other secondary sexual characteristics. These include the nose being
enlarged into an overhanging proboscis (thus the name, elephant seal) and the development of a
highly cornified and wrinkled chest shield, which often becomes heavily scarred (and therefore,
reddish or pinkish) from fighting with other males of high status. Females and young males lack these
exaggerated characters; their appearance is more similar to that of the related monk seals. The
coloration of the northern elephant seal is simple countershading, with a dark brown back and slightly
lighter belly.
Status—The northern elephant seal population has recovered dramatically after being reduced to
several dozen to perhaps no more than a few animals in the 1890s (Bartholomew and Hubbs 1960;
Stewart et al. 1994). Although movement and genetic exchange continues between rookeries, most
elephant seals return to their natal rookeries when they start breeding (Huber et al. 1991). The
California and Mexican breeding groups may be demographically isolated and are currently
considered two separate stocks (Carretta et al. 2004).
The population size has to be estimated, since all age classes are not ashore at any one time of the
year (Carretta et al. 2004). There is a conservative minimum population estimate of 60,547 elephant
seals in the California stock (Carretta et al. 2004). Based on trends in pup counts, abundance in
California is increasing by around 6% annually, but the Mexican stock is evidently decreasing slowly
(Stewart et al. 1994; Carretta et al. 2004).
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