82
Marine Mammal Physiology: Requisites for Ocean Living
(Figure 4.4; McDonald and Ponganis 2012; Meir et al. 2009). On ascent, the reduction in pressure causes the lung volume to increase, reducing the partial pressure of O 2 , and potentially
reversing the O 2 gradient across the lung, causing a rapid drop in alveolar PO 2 and resulting hypoxemia (Figure 4.4; McDonald and Ponganis 2012; Meir et al. 2009). Marine mammals
hyperventilate before and after deep dives (Kooyman 1989). Such behavior could increase
susceptibility to shallow water blackout if they dive on hypocapnic blood PO 2 levels.
Some phocid species, for example, the Weddell seal, harbor seal, and elephant seal
have been documented to be extremely hypoxia tolerant, which may be an adaptation to this
problem (Elsner et al. 1970; Kerem and Elsner 1973; Meir et al. 2009). Unlike phocids, otariid
seals are thought to be more reliant on their lung O 2 stores (Kooyman 1973). Continuous
arterial PO 2 loggers attached to California sea lions have shown a relatively deep depth of
alveolar collapse (225 m). Alveolar collapse potentially mitigates the potential for shallowwater blackout as cessation of gas exchange preserved a reservoir of O 2 that supplemented
blood O 2 during ascent (McDonald and Ponganis 2012). In contrast, during the relatively
shallow (less than 160 m) dives of Antarctic fur seals, animals were found to exhale during
the ascent portion of dives (Hooker et al. 2005). This behavior appeared to be associated
with maintenance of lung compression. Since these dives were shallower than the likely
depth of alveolar collapse, they could potentially result in extreme lung O 2 depletion and
severe hypoxemia. Exhaling on ascent was suggested to mitigate the risk of shallow-water
blackout by maintaining compressed alveoli and minimizing diffusion between lungs and
blood during ascent (Hooker et al. 2005). A similar effect was previously observed during
measurement of blood N 2 levels in a forced dive of a harbor seal. The seal exhaled during
decompression from a simulated dive to 130 m in a hyperbaric chamber, which resulted in
delaying the removal of N 2 from its blood until it surfaced and breathed (Kooyman et al.
1972). Thus, exhalation can maintain blood gas concentrations during ascent and help prevent drops in arterial PO 2 that may cause unconsciousness.
Whether shallow-water blackout is a problem for cetaceans and whether they have
methods to mitigate this are unknown. However, given that shallow-water blackout is
more likely to be a problem for active inhalation divers, diving to depths shallower than
alveolar collapse, we would expect that some species of cetaceans would be vulnerable.
4.3 Tools/methods used for this field
4.3.1 Animal-attached dive recorders
Crucial to our understanding of the exposure of marine mammals to pressure has been the
development of the animal-attached dive recorder. Although some information about diving behavior can be obtained viewing animals remotely—observing dive times or following
animals sub-surface by sonar, long time-series information on diving behavior necessitates
the use of animal-attached instruments (Ropert-Coudert and Wilson 2005). The earliest of
these instruments were simple capillary tube manometers (Scholander 1940). The tubes were
welded closed at one end and dusted internally with dye that was easily soluble in water. The
intrusion of water as the air within the tube was compressed showed the maximum pressure achieved (Scholander 1940). In some cases, multiple rings could be identified (Kooyman
1965), or addition of a radioactive bead that followed the meniscus could record time spent
at depth (Wilson and Bain 1984). However, such instruments, although benefiting from their
small size, were unable to provide the resolution achieved by time-depth recorders.
The earliest time-depth recorders linked measurement of pressure to a moving needle
which recorded directly as a trace on a 60 min kitchen timer with a smoked glass disc
Marine Mammal Physiology: Requisites for Ocean Living
(Figure 4.4; McDonald and Ponganis 2012; Meir et al. 2009). On ascent, the reduction in pressure causes the lung volume to increase, reducing the partial pressure of O 2 , and potentially
reversing the O 2 gradient across the lung, causing a rapid drop in alveolar PO 2 and resulting hypoxemia (Figure 4.4; McDonald and Ponganis 2012; Meir et al. 2009). Marine mammals
hyperventilate before and after deep dives (Kooyman 1989). Such behavior could increase
susceptibility to shallow water blackout if they dive on hypocapnic blood PO 2 levels.
Some phocid species, for example, the Weddell seal, harbor seal, and elephant seal
have been documented to be extremely hypoxia tolerant, which may be an adaptation to this
problem (Elsner et al. 1970; Kerem and Elsner 1973; Meir et al. 2009). Unlike phocids, otariid
seals are thought to be more reliant on their lung O 2 stores (Kooyman 1973). Continuous
arterial PO 2 loggers attached to California sea lions have shown a relatively deep depth of
alveolar collapse (225 m). Alveolar collapse potentially mitigates the potential for shallowwater blackout as cessation of gas exchange preserved a reservoir of O 2 that supplemented
blood O 2 during ascent (McDonald and Ponganis 2012). In contrast, during the relatively
shallow (less than 160 m) dives of Antarctic fur seals, animals were found to exhale during
the ascent portion of dives (Hooker et al. 2005). This behavior appeared to be associated
with maintenance of lung compression. Since these dives were shallower than the likely
depth of alveolar collapse, they could potentially result in extreme lung O 2 depletion and
severe hypoxemia. Exhaling on ascent was suggested to mitigate the risk of shallow-water
blackout by maintaining compressed alveoli and minimizing diffusion between lungs and
blood during ascent (Hooker et al. 2005). A similar effect was previously observed during
measurement of blood N 2 levels in a forced dive of a harbor seal. The seal exhaled during
decompression from a simulated dive to 130 m in a hyperbaric chamber, which resulted in
delaying the removal of N 2 from its blood until it surfaced and breathed (Kooyman et al.
1972). Thus, exhalation can maintain blood gas concentrations during ascent and help prevent drops in arterial PO 2 that may cause unconsciousness.
Whether shallow-water blackout is a problem for cetaceans and whether they have
methods to mitigate this are unknown. However, given that shallow-water blackout is
more likely to be a problem for active inhalation divers, diving to depths shallower than
alveolar collapse, we would expect that some species of cetaceans would be vulnerable.
4.3 Tools/methods used for this field
4.3.1 Animal-attached dive recorders
Crucial to our understanding of the exposure of marine mammals to pressure has been the
development of the animal-attached dive recorder. Although some information about diving behavior can be obtained viewing animals remotely—observing dive times or following
animals sub-surface by sonar, long time-series information on diving behavior necessitates
the use of animal-attached instruments (Ropert-Coudert and Wilson 2005). The earliest of
these instruments were simple capillary tube manometers (Scholander 1940). The tubes were
welded closed at one end and dusted internally with dye that was easily soluble in water. The
intrusion of water as the air within the tube was compressed showed the maximum pressure achieved (Scholander 1940). In some cases, multiple rings could be identified (Kooyman
1965), or addition of a radioactive bead that followed the meniscus could record time spent
at depth (Wilson and Bain 1984). However, such instruments, although benefiting from their
small size, were unable to provide the resolution achieved by time-depth recorders.
The earliest time-depth recorders linked measurement of pressure to a moving needle
which recorded directly as a trace on a 60 min kitchen timer with a smoked glass disc