170
P.L. Tyack and c.w. Clark
Many of the fish or zooplankton prey of marine mammals are schooling
species and occur in a highly patchy distribution both in time and space.
Marine mammals that feed on them must find small aggregations of prey
within large areas of ocean. Very little is known about how cetaceans find
patches of prey on the order of tens of meters in size at ranges on the order
of kilometers. The deep scattering layer may represent an important concentration of prey for many pelagic cetaceans. This is a layer of organisms
living in the open ocean, which shows a vertical migration on a diurnal cycle.
Hersey and Backus (1962) found that echoes from deep scattering layers
tend to have a strong frequency peak in the range from 2.5 to 25 kHz. Many
cetaceans feed on deep scattering layer organisms, particularly at night
when the layer rises toward the surface. If they are feeding on organisms
that reflect acoustic energy in the 2.5 to 25 kHz range, they might also
benefit from echolocation signals containing energy in these lower frequencies in order to receive echoes backscattered from these targets. The
optimal signal would depend not only upon the resonant characteristics of
the target, which change with depth, but also upon absorption (which favors
lower frequencies) and ambient noise (which usually favors higher frequencies). Echolocation signals designed for longer-range detection of
aggregations of deep prey may be particularly useful as animals are making
decisions about when and where to start a feeding dive.
2.5 Orientation and Exploration of the
Physical Environment
Most work on cetacean echolocation emphasizes high-frequency systems
that can detect small targets out to about 100m (Au, Chapter 9). Yet, as was
just discussed with patches of prey, cetaceans face serious problems for
detecting large targets at longer ranges. Relevant targets might include
other conspecifics (the target could involve the body, dense bones, air-filled
lungs, etc.), inhomogeneities in the ocean, such as fronts, or even the sea
floor. Many cetaceans dive near the sea floor in conditions where it is hard
to see. Even when coastal animals dive in shallow water, the bottom may
be obscured in turbid water or at night. There are many pelagic species that
dive below the depths to which light provides a strong signal. If they dive
deep enough to potentially collide with the sea floor, then they may require
some way to avoid this potential obstacle. A simple depth-sounding sonar
could help solve this problem, and bottom reverberation has been recorded
from a variety of cetacean signals such as the clicks of sperm whales and
songs of humpback whales (Tyack 1997). Many biologists have concluded
that these whale signals are used primarily for communication, but biologists recording these sounds can often easily detect echoes from bottom
reverberation (Tyack 1997). It is important to consider ways in which
the animals themselves might use information from this kind of bottom
P.L. Tyack and c.w. Clark
Many of the fish or zooplankton prey of marine mammals are schooling
species and occur in a highly patchy distribution both in time and space.
Marine mammals that feed on them must find small aggregations of prey
within large areas of ocean. Very little is known about how cetaceans find
patches of prey on the order of tens of meters in size at ranges on the order
of kilometers. The deep scattering layer may represent an important concentration of prey for many pelagic cetaceans. This is a layer of organisms
living in the open ocean, which shows a vertical migration on a diurnal cycle.
Hersey and Backus (1962) found that echoes from deep scattering layers
tend to have a strong frequency peak in the range from 2.5 to 25 kHz. Many
cetaceans feed on deep scattering layer organisms, particularly at night
when the layer rises toward the surface. If they are feeding on organisms
that reflect acoustic energy in the 2.5 to 25 kHz range, they might also
benefit from echolocation signals containing energy in these lower frequencies in order to receive echoes backscattered from these targets. The
optimal signal would depend not only upon the resonant characteristics of
the target, which change with depth, but also upon absorption (which favors
lower frequencies) and ambient noise (which usually favors higher frequencies). Echolocation signals designed for longer-range detection of
aggregations of deep prey may be particularly useful as animals are making
decisions about when and where to start a feeding dive.
2.5 Orientation and Exploration of the
Physical Environment
Most work on cetacean echolocation emphasizes high-frequency systems
that can detect small targets out to about 100m (Au, Chapter 9). Yet, as was
just discussed with patches of prey, cetaceans face serious problems for
detecting large targets at longer ranges. Relevant targets might include
other conspecifics (the target could involve the body, dense bones, air-filled
lungs, etc.), inhomogeneities in the ocean, such as fronts, or even the sea
floor. Many cetaceans dive near the sea floor in conditions where it is hard
to see. Even when coastal animals dive in shallow water, the bottom may
be obscured in turbid water or at night. There are many pelagic species that
dive below the depths to which light provides a strong signal. If they dive
deep enough to potentially collide with the sea floor, then they may require
some way to avoid this potential obstacle. A simple depth-sounding sonar
could help solve this problem, and bottom reverberation has been recorded
from a variety of cetacean signals such as the clicks of sperm whales and
songs of humpback whales (Tyack 1997). Many biologists have concluded
that these whale signals are used primarily for communication, but biologists recording these sounds can often easily detect echoes from bottom
reverberation (Tyack 1997). It is important to consider ways in which
the animals themselves might use information from this kind of bottom