By Daniela Abras
It is immensely challenging to try to understand
the mechanisms that move a 15 meter-long and 40 ton organism 9,000 km yearly.
Humpback whales migrate every year from the
feeding grounds of Antarctica to the mating grounds of Brazil. The route, which
is about 4,500 km each way, is made twice a year and typically takes about 2
months going, and 2 months coming back. By including their 4 month stay in
Brazil mating, these whales spend 8 months of the year without food. That’s a
long fast! To accomplish this feat, they need to eat a lot during the 4 months
in Antarctica, and they need to stock up on energy reserves, in the form of body
fat.
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| Map that shows the migratory corridor of the humpback whales between the feeding area in Antarctica, and the main reproductive area on Abrolhos Bank. |
But what do these whales eat? As the adorable
Dory, from Disney/Pixar’s Finding Nemo would say, whales don't eat fish, they
eat krill. Krill are small crustaceans, similar to shrimp, that are about 5cm
long and live in giant clusters (swarms). Krill are the base of the vertebrate
food chain in Antarctica, where most species depend on it, directly or not.
Many species of fish, seals, penguins, and whales prey almost exclusively on
it. Some species, like Orca whales and Leopard seals, prey on fish or penguins.
This is why the food chain in Antarctica has been called by scientists
“krill-dependent.”
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| Krill (Euphausia superba), the main food of Humpback whales in Antarctica, live in large swarms. |
Every year, whales arrive at the Brazilian
coast in July and stay there until November. There are times when the
population arrives slightly earlier in the year and stay longer, but they can also
come later in the season and leave more quickly. In some years, there are more whales
than in others. This started to raise some questions: When they stay in
Abrolhos longer, is it because they fed better? When they leave the bank
earlier than average, is it because of high water temperatures? Or do these
things not influence their behavior at all, and they rely mostly on genetic
programming? What initiates the migration process?
My Master's research focused on these
questions to try to understand the diverse environmental mechanisms influencing
the migratory dynamics of humpback whales. I primarily focused on the
availability of their main source of energy. To do that, I analyzed parameters
such as photoperiod, water temperature in both Abrolhos and in Scotia Sea
(where they stay in Antarctica), and the availability of krill during summer. I
compared this to 7 years of sighting data collected at a fixed location around
the Abrolhos Archipelago. To observe the whales, a piece of topography
equipment with 30X zoom, called a theodolite, was used. For the 5 months the
whales were in Abrolhos, we observed the whales daily, and found that the
population's abundance fluctuates throughout the reproductive season with a
gradual increase in July, followed by the peak in August/September, and then a
gradual decrease, until no more whales were present by the end of November.
Watching whales with the theodolite,
from Abrolhos Archipelago.
The results were more than expected. In years
when there were more krill available, the whales fed more and had greater
energy stores. This allowed them to invest a longer period of time on
reproduction and more whales were seen in Abrolhos. The opposite was also true.
In years with less krill, fewer whales were seen in Abrolhos and their time at
Abrolhos was shortened. The water temperature didn't seem to have significant
influence on their migration, however it assisted in indicating the starting
moment for the migration – the migratory timing.
The most surprising result was related to the
photoperiod (length of daylight in a day). No other research had related the
migratory dynamics with photoperiod, perhaps because scientists thought it was
too obvious. But, sometimes, it's important to understand the obvious! The
photoperiod in Antarctica has a huge difference between summer (18 hours of
light) and winter (6 hours), while in Abrolhos, the difference from summer (13
h) and winter (11h) is far smaller.
Therefore, as my dissertation's conclusion, I
discovered that the humpback whale's migration starts and is influenced by the
sharp lowering of photoperiod when they are in Antarctica. When in Abrolhos,
migration is impacted by the sum of 3 factors: the photoperiod (which is more
steady than in Antarctica), the sea surface temperature (this slightly
increases gradually during the reproductive season) and krill availability
while in Antarctica.
It was difficult to analyze such a high volume
of data, linking different environmental parameters in order to answer all of
my research questions. With these results, we have started to understand
complex migratory dynamics and the importance of krill in the maintenance of
the humpback's population.
If you want to know more about my Master's
dissertation, contact me via email at daniabras@gmail.com
The humpback whale population was almost
driven to extinction in the early 20th century from intensive
commercial hunting. Before commercial whaling, the estimated population was
around 25,000 individuals, but it dropped to about 800 individuals while at the
peak of whaling. After the whale-hunting moratorium in 1986, the population
recovered and is now around 15,000 individuals today! In 2015, humpback whales were officially removed from the endangered
species list in Brazil. This is a victory for the whales as well as for those
of us that have the privilege of watching them arrive annually, in bigger
numbers every time, performing their aquatic ballet. Go meet them! Between July
and November, they are concentrated on the Abrolhos region, but they can also
be seen from the states of Rio Grande do Norte state to Rio de Janeiro.
Want to know more about humpback whales? Visit
the Brazilian Humpback Whale Institute website: www.baleiajubarte.org.br
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| Humpback whale jumping in Abrolhos region. |
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Daniela Abras is from Belo Horizonte, has a
bachelor’s degree in Marine Biology from UFRJ, and has a Masters degree in
Oceanography from USP. She has loved cetaceans since she was 8 years old, when
she did a school project about them. When she was a teenager, she would say
that she wanted to work with whales, but was never taken seriously. In the
early 90s, she heard the famous National Geographic “Whale Songs” vinyl record
and discovered the “Save the whales” project. From all of this obstinacy, her
dream to study and protect whales came to life. She is now a researcher for the
Brazilian Humpback Whale Institute, dedicating herself daily to studying these
magnificent animals.
