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

AUGUST 2005 FINAL REPORT
3.1 MARINE MAMMALS
3.1.1 Introduction
More than 120 species of marine mammals occur worldwide (Rice 1998). The term “marine mammal” is
purely descriptive, referring to mammals that carry out all or a substantial part of their foraging in marine,
or in some cases, freshwater environments. Marine mammals as a group are comprised of various
species from three orders (Cetacea, Carnivora, and Sirenia).
The vast majority of the 32 marine mammal species with confirmed or possible occurrence in the
Marianas study area are cetaceans (whales and dolphins). Cetaceans are divided into two major
suborders: Mysticeti and Odontoceti (baleen and toothed whales, respectively). Toothed whales use teeth
to capture prey, while baleen whales use baleen plates to filter their food from the water. Beyond
contrasts in feeding methods, there are also life history and social organization differences between
baleen and toothed whales (Tyack 1986).
3.1.1.1 Adaptations to the Marine Environment: Sound Production and Reception
Marine mammals display a number of anatomical and physiological adaptations to an aquatic
environment that are discussed in detail by Pabst et al. (1999). Sensory changes from the basic
mammalian scheme have also taken place in response to the different challenges an aquatic environment
imposes. Sound travels faster and further in water than in air and is, therefore, an important sense. Touch
and sight are also well developed in whales and dolphins (Wartzok and Ketten 1999).
Marine mammal vocalizations often extend both above and below the range of human hearing;
vocalizations with frequencies lower than 18 Hertz (Hz) are labeled as infrasonic and those higher than
20 kiloHertz (kHz) as ultrasonic. Baleen whales primarily use the lower frequencies, producing tonal
sounds in the frequency range of 20 to 3,000 Hz, depending on the species. Clark and Ellison (2004)
suggested that baleen whales use low frequency sounds not only for long-range communication, but also
as a simple form of echo ranging, using echoes to navigate and orient relative to physical features of the
ocean. The toothed whales produce a wide variety of sounds, which include species-specific broadband
“clicks” with peak energy between 10 and 200 kHz, individually variable “burst pulse” click trains, and
constant frequency or frequency-modulated whistles ranging from 4 to 16 kHz (Wartzok and Ketten
1999). The general consensus is that the tonal vocalizations (whistles) produced by toothed whales play
an important role in maintaining contact between dispersed individuals, while broadband clicks are used
during echolocation (Wartzok and Ketten 1999). Burst pulses have also been strongly implicated in
communication, with some scientists suggesting that they play an important role in agonistic encounters
(McCowan and Reiss 1995), while others have proposed that they represent “emotive” signals in a
broader sense, possibly representing graded communication signals (Herzing 1996). Sperm whales,
however, are known to produce only clicks, which are used for both communication and echolocation
(Whitehead 2003).
Data on the hearing abilities of cetaceans are sparse, particularly for the larger cetaceans such as the
baleen whales. The auditory thresholds of some of the smaller odontocetes have been determined in
captivity. It is generally believed that cetaceans should at least be sensitive to the frequencies of their
own vocalizations. Comparisons of the anatomy of cetacean inner ears and models of the structural
properties and the response to vibrations of the ear’s components in different species provide an
indication of likely sensitivity to various sound frequencies. The ears of small toothed whales are
optimized for receiving high-frequency sound, while baleen whale inner ears are best in low to infrasonic
frequencies (Ketten 1992, 1997).
General reviews of cetacean sound production and hearing may be found in Richardson et al. (1995),
Edds-Walton (1997), Wartzok and Ketten (1999), and Au et al. (2000). For a discussion of acoustic
concepts, terminology, and measurement procedures, as well as underwater sound propagation, Urick
(1983) and Richardson et al. (1995) are recommended.
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