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Gray Whale
Eschrichtius robustus
Gray Whale
Science
Nearly three
decades ago, the West Coast Whale Research (now Pacific WildLife
Foundation) pioneered studies of the gray whales off Vancouver
Island that have continued thanks to donor support.
The coastal
dwelling gray whale is the most often encountered whale on the
Pacific Coast of North America and the object of a flourishing
whale-watching industry. Gray whales are distinctive from other
whales by the combination of medium size (for a whale: 10-15m),
mottled gray skin pigmentation, absence of a dorsal fin, and
propensity to come very close to shore.
The eastern
Pacific population, sometimes referred to as the California stock,
calves off the coast of Baja California and most individuals spend
the summer feeding in the Bering and Chukchi Seas. The entire
eastern Pacific population is thought to number about 20,000
individuals, down somewhat from estimates of few years ago of up to
26,000 whales.
Some scientists
suggest the eastern Pacific population has reached its capacity,
noting that large numbers of strandings in recent years may have
been the result of starvation. The average of 41 stranded gray
whales reported between 1995 and 1998, was followed by a large
increase of 283 strandings in 1999 and 368 in 2000. However, the
number of strandings fell to 21 in 2001 and 26 in 2002. Researchers
concluded that no clear explanation could be derived for the
stranded whales but that a common, wide ranging factor was likely
involved and that starvation was the most likely cause (Gulland et
al 2005).
The western
Pacific gray whale population sometimes referred to as the Korean
stock, is composed of just 100 animals and is considered critically
endangered. Western gray whales spend the summer feeding near
Sakhalin Island, in Russia. Their breeding ground is unknown but
suspected to be in the waters off southern China.
Southern Feeding
Group
The gray whales
commonly seen off the west coast of Vancouver Island in summer, as
well as in Boundary Bay and occasionally in other ‘inside’ waters
are part of a population that does not migrate to the northern seas.
This group is comprised of about 200 whales that range from northern
California to southeast Alaska in summer, utilizing the rich feeding
grounds along this coastline.
Research by
Pacific Wildlife Foundation as well as other groups along the coast
has shown that the same individuals return to this section of
coastline each year. Individual whales are identified by the natural
pigment patterns on their sides. Some whales have returned to the
west coast of Vancouver Island each summer for over 30 years. These
whales show a strong site-fidelity to this region.
A recent
investigation of the genetics of this southern group by Drs. Jim
Darling of PWLF and Tim Frasier of St. Mary’s University has
indicated that this population is a distinct genetic entity compared
to the entire herd. This has a number of implications including a
review of current management policy that presumed the southern
whales were just a random subgroup of the overall herd and therefore
replaceable by any member of the 20,000 eastern Pacific gray whale
population.
The summary
points of this study are as follows:
Local gray
whales genetically distinct from overall population
Our recent
genetic study concludes that the “local” gray whales, the Vancouver
Island summer residents (which are part of a southern feeding group
that range between Northern California to southeast Alaska), have a
separate genetic identity from the rest of the eastern Pacific herd
(that spends the summer on feeding grounds in the Bering and Chukchi
Seas).
Offspring adopt
their mother’s home range
The most likely
explanation of this genetic distinction is that mothers bring their
calves to these feeding grounds, and this begins a pattern of
related individuals returning each year to this same section of
coast. This dispersal pattern is called ‘matrilineal directed
fidelity’ or simply, offspring are faithful to the feeding grounds
of their mothers. This finding fits well with other studies that
have shown many individual whales have returned to this region each
summer for decades and that many calves first identified with their
mothers are seen alone in subsequent summers. Matrilineal directed
fidelity has also been described for humpback whales and right
whales, that is, from knowledge to date it is the standard dispersal
pattern for baleen whales.
Local southern
group of 200 whales requires separate management consideration to
the 20,000 northern gray whales.
This discovery
means that the local whales (southern feeding group) do not
represent a random subset, nor can be replaced by, individuals from
the overall population and therefore, this group should be treated
as a separate unit for management purposes. In practice this means
that any examination of potential impacts on this population starts
with a pool of 200 whales (the population estimate of the southern
feeding group) rather than the approximately 20,000 whales in the
eastern Pacific population overall.
Current
management policy needs revision
The most
immediate implication of this study is that the current management
scheme, based on the idea that the entire eastern Pacific herd of
gray whales is just one stock or management unit, needs review. The
current management policy (of the US National Marine Fisheries
Service) has presumed the southern feeding group is simply an
aggregation of individuals with no social structure, which mix
randomly with rest of herd, and are replaceable with any whales in
the remainder of the herd.
Current
‘management’ situation…
The management
of ‘Makah hunt’ is conducted by NMFS in Seattle with input from the
Makah band of the Olympic Peninsula, both of which are participants
in the International Whaling Commission (IWC). This genetic research
was recently presented to the IWC, which concluded that the idea of
the southern feeding group as a separate stock was a ‘plausible
hypothesis’, inferring that it demands further investigation (which
is appropriate). However the evidence is very strong and we would
not anticipate any different outcome with further study. Canada is
not a member of the IWC and apparently has chosen to have no voice
in this issue, even though these whales spend much of their time in
Canadian waters.
Standard genetic
analysis technique
This study in
based on standard techniques that compare mitochrondrial DNA (mtDNA)
haplotypes, and have been used in population genetic studies
worldwide. For more information see Background
Some Background
on the Gray Whale Genetics Study
Researchers
Dr Jim Darling,
resident of Tofino and biologist with the Pacific WildLife
Foundation, has been studying the abundance, behaviour and ecology
of gray whales off Vancouver Island since the mid-1970s. Jim’s work
is based on the identification of individual whales by photographs
of natural markings and repeat sightings of these individuals. Many
of the same gray whales return each summer, some for over for 30
years. Several years ago, Jim began a collaboration with Dr Tim
Frasier a geneticist with St. Mary’s University in Nova Scotia.
Among other things Tim has done extensive genetic work with
endangered right whales in the North Atlantic. The objective of the
collaboration was to investigate the social structure of the local
whales through genetic analyses.
Use of Genetics
in Research
The use of
genetic analyses to assess ‘stock’ definitions and identify
‘management units’, is now a standard technique in wildlife
research. Additionally, genetic analyses are increasingly being
used to assess other aspects of animal biology that are not readily
available through other research techniques, such as parentage and
patterns of relatedness, social structure, and dispersal patterns.
The above
applications are all based on two different sources of DNA that can
be found in animal cells, mitochrondial DNA (mtDNA) and nuclear DNA
(nDNA), with each providing different information about the genetic
legacy of individuals.
1) Nuclear DNA
is the type of DNA that most people think of when thinking of DNA.
This is the DNA that makes up your chromosomes, where you inherit
one copy from your mother and one from your father. There is a vast
amount of information in nuclear DNA, with the average mammalian
genome consisting of ~3 billion base pairs. For population studies,
as well as human forensic cases, we are interested in examining
regions of the nuclear DNA that are highly variable – if we want to
tell individuals apart based on their DNA, then they have to be
genetically different from each other. The types of nuclear markers
that are commonly used today are called microsatellites. These are
highly variable regions of DNA that consist of a short sequence
(~2-4 base pairs) repeated over and over again in tandem.
Microsatellite are found in non-coding regions of DNA, which is why
they can be so variable. By analyzing multiple microsatellite
markers for each individual, we can create a genotype, or genetic
profile, for each individual. This acts like a DNA fingerprint,
where no two individuals will have the same genotype. With this
information, we can address a range of questions, such as parentage,
patterns of relatedness and social structure, and dispersal
patterns. These markers allow us to study the specific genetic
relationships between individuals. For example, these are the
genotypes that would be used in a forensic paternity case, where, by
comparing the genetic profiles of the mother, offspring, and
putative fathers, it would be possible to identify the father.
2) Mitochondria
are structures inside the cell where sugars are broken down into ATP
– the basic unit of energy that is used by our bodies. Therefore,
mitochondria are the “power houses” of our cells, generating the
energy that we need to function. As you might imagine, different
cells have different number of mitochondria in them: cells that need
a lot of energy (e.g. heart cells) have thousands of mitochondria in
each cell, whereas cells that do not need much energy (e.g. hair
cells) only have about a hundred mitochondria in each cell.
Mitochondria have their own DNA – a circular molecule of ~16,000
base pairs. As opposed to nuclear DNA, mitochondrial DNA (mtDNA) is
only passed on from mother to offspring. This different inheritance
pattern is very useful for studies of wild populations, and whales
in particular, because many aspects of whale biology are passed down
from mother to offspring. Therefore, these maternally-inherited
aspects of whale biology should leave a signature in the mtDNA,
whereas no such signature may be present in the nuclear DNA.
For example,
some baleen whale populations have a migration pattern where
individuals are distributed among many discrete summer feeding areas
in higher latitudes, but all individuals congregate on one common
mating ground. Since they represent one mating population, we would
expect the nuclear DNA from all these whales to be relatively
similar. However, if the migration patterns to the different
feeding areas are passed on from mothers to offspring, over many
generations, then we should see differences in the mtDNA between the
different feeding areas, because they represent different maternal
lineages. This is exactly what we did for the gray whale study, and
found that the southern feeding group whales differ from the rest of
the population in their mtDNA, and therefore they represent a
distinct set of maternal lineages.
The results
discussed here are based on analysis of the mtDNA only. We expect
the nuclear DNA/microsatellite analysis will be undertaken in the
next year.
How it Works
When whales are
encountered they are first photo-identified to record the individual
sampled, both to access the sightings history of the whale and avoid
duplicate sampling. A skin sample is collected under permit with a
small dart (shot from a crossbow) that bounces off the whale. The
skin in preserved in vial and sent to genetic lab.
For the analysis
of mtDNA, a short portion (~300 base pairs) of the DNA is sequenced
for each individual. Each unique sequence (series of base pairs) is
given a label – called a “haplotype”. The mtDNA is not as variable
as the nuclear microsatellites, and therefore multiple individuals
may have the same haplotype, or DNA sequence, at the analyzed
region. We then compare the haplotypes from the individuals in each
of the proposed groups to assess how different they are from each
other. For the gray whale analysis, we compared the haplotypes from
the southern resident whales to those from the rest of the
population.
In this study 40
individual whales were sampled from the summer population that
ranges through Clayoquot Sound on Vancouver Island. The haplotype of
each of these individuals was determined. The genetic data obtained
from these samples was presumed to be representative of the southern
feeding group.
The genetic
information from these whales was then compared to haplotypes of
individuals representing the remainder of the herd, and which had
been previously published in scientific literature. These samples
came from whales on the breeding grounds in Mexico and during the
migration. We compared the sample from the local whales to two
different published samples from the herd overall. Statistical tests
were used to compare the type and frequency of the haplotypes in
each of the sample groups.
For the first
test we assessed whether or not the same haplotypes were found in
both sample sets at similar frequencies. The rationale is that if
the southern feeding group is just a random subset of the larger
population, then haplotypes should be found at similar frequencies
in both sample sets. Instead, we found significant differences in
the haplotypes from the two sample sets, indicating that they
represent distinct entities, and that there is not random movement
between feeding locations.
To gain further
insight into this subdivision of the population, we conducted a few
tests to assess how different the two feeding groups are. One
method we used was to estimate a number called the effective
population size (Ne) for both sample sets. Ne
can essentially be thought of as the number of breeders in a
population (the number of individuals that effectively contribute to
the gene pool). If the two sample sets are truly different, then
they should have significantly different estimates of Ne,
whereas if they represent one group, then estimates from each sample
set should converge on the same value. We found that the estimates
of Ne for the two sample sets were very different from
each other, showing that the two groups are demographically
independent (e.g. individuals from one will not replace those in the
other).
Genetic data
also allowed us to estimate the number of migrants between
identified groups of individuals. Previous studies have shown that
populations are demographically independent when the migration rates
between them are < 10%. We estimated the migration rate between the
southern feeding group and the rest of the population to be << 1%,
again indicating that the southern feeding group is demographically
independent.
What’s Next?
This study is a
starting point in the investigation of gray whale genetics and how
it can help us understand the biology of these animals. This study’s
results will help direct further research. For example, in regards
to whale management, this finding begs two key questions: 1) is the
southern feeding group exceptional (in terms of genetic distinction)
or just one example of similar subdivisions that may exist
throughout the northern seas, and 2) are there smaller social units
(than the full southern group) that occupy and utilize more
restricted sections of coast – meaning that the appropriate
management unit may be even smaller than 200 animals comprising the
southern feeding group?
On a broader
biological level this study provides the first hint of social
organization in this species, and raises a number of questions
regarding their behavioral ecology, that is how their social
organization and behavior are tied to local ecology.
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