whale strandings

From: Nancy Stevick (Nancy.Stevick@btinternet.com)
Date: Wed Feb 16 2000 - 06:00:24 EST


On 13 February 2000 J Howell wrote:

>I am researching theories of why whales mass strand, particually the theory
>of Geomagnetic navigation. If you have any infomation that could help me I
>would be most appreciative. Other than that, any links you know of which
>could help me would be great. Also, I am researching success rates of human
>help in whale strandings, and scientific thoughts on if we should save
them,
>and why/why not we should help.
>Thanks for your time,
>
>J Howell
>
Dear J Howell,

You are in luck. My husband and colleagues are writing a chapter on marine
mammal migration for a textbook, and they have graciously agreed to let me
send you a section of their draft and bibliography. It follows this
message.

If you want links regarding marine mammal strandings, I suggest that you
check the WhaleNet affiliate list, in particular the New England Aquarium
and the Center for Coastal Studies. They are both very active in the
Northeast Stranding Network and have stranding information on their web
sites.

Good luck with your research. You are asking some difficult questions and
finding satisfactory answers will be challenging.

Best Wishes,
Nancy Stevick

DRAFT -- Do not cite without permission of authors

Navigation
There is no direct evidence for how marine mammals find their way in the
sea. Since most studies are based on natural behaviour rather than on
experimental manipulation, the associations described below do not allow
cause and effect to be established. When a whale is observed to travel along
the continental shelf edge, is it using the bottom topography as a
positional cue, or is the shelf edge simply associated with increased prey?
It is reasonable, however, to assume that marine mammals make use of all
available information and senses. Other animals displaying wide-ranging
movements (e.g. birds) are known to utilize many features [Aidley, 1981
#147; Able, 1996 #175]. Cues potentially available to marine mammals include
celestial bodies, bottom topography, landmarks, currents, temperature and
salinity gradients, odors and tastes, sounds and geomagnetism.

The Sun and Other Celestial Bodies
The sun provides clues to location both through its location in the sky and
through photoperiod. Dawbin (1966) proposed that whales use changing day
length in polar regions as a cue to initiate migration because it changes
more rapidly and predictably during the time of early migration than does
water temperature or other available cues. However, no correlation between
migratory movements and photo-period has been established in bottlenose
dolphin in the western North Atlantic (Barco, 1999). Diurnal patterns of
movement have been recorded in seals and cetaceans (Klinowska 1986b; Stone
et al. 1995; Stone et al. 1998); Stewart 1984, Pauli & Terhune 1987)
suggesting the use of information from the sun, but these movements likely
reflect a response to changes in prey distribution. Unlike birds, marine
mammals are not known to use polarization of light or features of the night
sky to aid movement [Keeton, 1981 #146; Able, 1996 #175].

Geomagnetism
There has long been interest in the possibility that marine mammals could
use the earths magnetic field to aid navigation because of the potential
lack of other stimuli, particularly for pelagic species which inhabit large
expanses of deep open water. Associations have been found between
geomagnetic minima and the locations of live strandings and drive fisheries
(Klinowska 1985; Klinowska 1986a; Kirschvink 1990; Klinowska 1990) and
between the position of migrating fin whales and low geomagnetic gradients
(Walker et al. 1992).

Strandings: a failure of navigation?
Strandings of cetaceans have fascinated humans since at least the days of
Aristotle. Animals that are already dead when they strand are simply the
product of mortality at sea. But live strandings, especially of groups of
animals (so-called mass strandings), raise questions about why animals that
spend their entire lives at sea come to beach themselves ashore. Are
strandings a result of navigation failure? If so, what causes this?
Repeated strandings in particular locations suggest that there is something
in the geology or topography of those sites which contributes to the
strandings. In a detailed study of strandings of cetaceans around the coast
of Britain, Klinowska (1986a) found a highly significant relationship
between live strandings and grid squares where magnetic contours crossed the
coast at angles between 45 and 90 degrees. Klinowska (1986a) made the
analogy that a live stranding is equivalent to a car driver travelling in
the right direction and encountering an unexpected hazard. Similar
investigations in other areas have produced more equivocal results.
Correlation between stranding sites and magnetic minima were also identified
along the east coast of the United States (Kirschvink 1990), while in New
Zealand, no relationship could be found between magnetic fields and
locations of either mass or solitary strandings (Brabyn & Frew 1994).
Gently sloping sandy beaches, often with an adjacent sand spit or peninsula
are common to many mass stranding sites world-wide (Brabyn & McLean 1992).
Other oceanographic and geomorphological characteristics, including complex
topography, nearshore intrusions of deep water, funnel shaped basins,
turbidity, heavy surf, and wind-driven onshore currents have been suggested
to influence strandings [Best, 1982 #53; Brabyn, 1992 #63; Geraci, 1993 #61;
Smeenk, 1993 #154]. Some of these conditions may interfere with echolocation
or give misleading echolocation signals [Brabyn, 1992 #63; Geraci, 1993 #61;
Dudok van Heel, 1966 #155]. In other areas, whale traps may be formed by a
combination of complex channels, extensive tidal flats, strong currents and
high tidal ranges [Best, 1982 #53; Geraci, 1993 #61; Smeenk, 1993 #154].
Parasites that may cause neurological dysfunction have been implicated in
some strandings (Ridgeway & Dailey, 1972). But the lack of information on
the incidence of parasitic and pathogenic infection in cetaceans generally,
makes it difficult to evaluate the importance of these factors (Best 1982;
Geraci & Lounsbury 1993). Offshore species strand much more frequently than
inshore species (Geraci, 1979 #62; Geraci, 1993 #61). This may be because
their previous experience of deep water, pelagic environments gives them
less knowledge of navigating or orienting in waters close to land.
It seems clear that a number of interacting factors contribute to
strandings. The failure of a single sense, or confusion in a single source
of information is unlikely to explain the loss of navigation ability, where
so many other sources of information are available and an integration of all
information is likely to be used. In birds, individuals experimentally
deprived of one sense nearly always compensate and navigate successfully
using alternative cues [Keeton, 1981 #146; Walcott, 1996 #177; Able, 1996
#175].
It is also possible that the causes of strandings are unrelated to
navigation failure. Some tracking studies of previously stranded animals
following successful rehabilitation have shown typical behavior patterns and
a return to typical habitat, indicating no long-term inability to navigate
(Westgate et al., 1998). Helping behavior has been observed in some social
odontocetes, and group cohesion is high (Chapter 11); it is possible that
this could trigger or contribute to a stranding. Other proposals such as
suicide or escape during stress are untestable and difficult to credit
[Best, 1982 #53; Geraci, 1993 #61.

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