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Migratory
Bottlenose Dolphin Movements
and Numbers Along the Mid-Atlantic
Coast and Their Correlation
with Remotely Sensed Chlorophyll-a
and Sea Surface Temperatures Along
the Mid-Atlantic coast of
the United States, there
are different sub-populations,
or stocks of bottlenose dolphins.
The bottlenose dolphin, Tursiops
truncatus, has both resident
and migratory stocks. The
focus of this study is the
northern migratory population.
This group of animals moves
north and south along the
coast in response to seasonal
changes. The need for study
arises from this mobile nature.
Determination of the environmental
cues that may be used to
predict the presence or absence
of these animals will aid
in efforts to avoid disturbance
to this protected species.
This stock was also greatly
affected during the 1987-1988
epizootic event that killed
an estimated 50% of the migratory
stock. This disease event
was likely worsened by exposure
to environmental toxins.
The main areas of the field
work, the lower James and
Elizabeth Rivers of Virginia,
are of interest due to their
high toxin loads and frequent
usage by bottlenose dolphins.
The Elizabeth River is largely
developed along its length.
It also has a very high level
of traffic: commercial, military
and recreational. Since
this species represents the
highest level on its food
chain, our hypothesis is
that the movement north represents
can be correlated with the
movements of their prey species.
These prey species are known
to be themselves migratory
with temperature. As a surrogate
for the in situ detection
of the prey species, we feel
that sea surface temperature
(SST) and chlorophyll-a levels
can be used. Both of these
factors can be sensed remotely,
removing the need for local
observations. Sea surface
temperature can serve to
represent the movement of
the prey species, and chlorophyll-a
levels can be used to show
the primary productivity,
and thus the total food energy
available in the ecosystem.
The presence and absence
data on these animals is
then to be compared with
the remotely sensed SST and
chlorophyll-a data. These
data were derived from a
number of sources. MODIS-Aqua
and AVHRR data was obtained
from Goddard Space Flight
Centers Ocean Color web archive.
Additional AVHRR data was
obtained from the Jet Propulsion
Laboratory’s PO.DAAC
Ocean ESIP Tool (POET) website.
Field observations were based
on archives from the Christopher
Newport University Dolphin
Project, and from the Ocean
Biogeographic Information
System (OBIS) archive of
Duke University. The
results of the correlations
show that the critical temperature
in determining the presence
or absence of bottlenose
dolphins is between 16° and
18° C. While there were
two sightings below this
temperature, there were 694
above. A t-test show that
there was a significant (p=0.003)
difference between the mean
temperatures of sighting
and non-sighting efforts.
When compared to the numbers
of animals sighted at the
different temperatures, again
the 16° and 18° critical
temperatures showed up. There
were only 2 animals sighted
below 16°, while there
were 5400 sighted above.
An ANOVA analysis showed
a significant (p<0.01)
difference between the two
temperature ranges when it
came to group size. A t-test
for the mean group size showed
no significant difference
in the sizes of groups between
18° and 28°. While
there was some variation
in the chlorophyll levels
(measured in mg/m3), a t-test
showed no significant (p>0.1)
difference between the means
of sighting and non-sighting
levels. In comparing chlorophyll-a
levels with group size, there
was a significant (p<0.001)
difference, but this was
likely due to the fact that
coastal waters never drop
below moderate chlorophyll-a
levels. Based on these findings,
it becomes clear that in
determining the migratory
movements of bottlenose dolphins
sea surface temperature is
the preferred environmental
variable. |
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