BY BJORN CAREY, Stanford University
For millions of years, blue whales have cruised the world’s oceans
with hardly a care, their sheer size making them largely free from
predator attacks. The downside to being the largest animals in history,
however, is that the species was never pressured to evolve defensive
behaviors.
Now, the first direct observations of blue whales attempting to avoid
cargo ships suggest that this lack of an evasive response might make
the whales particularly susceptible to deadly collisions.
“It’s not part of their evolutionary history to have cargo ships
killing them, so they haven’t developed behavioral responses to this
threat,” said Jeremy Goldbogen, an assistant professor of biology at
Stanford’s Hopkins Marine Station, and the senior author on the study.
“They simply have no compelling response to avoiding these dangerous
ships.”
The study, published in Endangered Species Research, could help
improve methods to protect blue whales and other marine animals from
deadly ship collisions.
Collisions with ships are a major threat to whales and pose a
significant threat to the recovery of some endangered populations.
Efforts to reduce collisions have mostly involved placing speed limits
on ships passing through busy whale habitats or rerouting shipping
channels around these areas altogether.
However, a critical piece of information needed to make these
decisions and evaluate their effectiveness is currently lacking: direct
knowledge of how whales behave once they sense an oncoming ship.
To fill that gap, Goldbogen and colleagues from several academic
institutions headed to Long Beach, California, home of one of the
busiest shipping ports in the world and also a hotspot for blue whales.
Just a few miles offshore, the continental shelf drops off and there is a
huge upwelling of nutrients that attract krill, a favorite food of blue
whales.
The scientists used suction cups to adhere GPS (global positioning
system) and dive-logging units to blue whales, and then tracked their
movements for 24-hour periods. The scientists then cross-referenced this
data with boat traffic, including the tonnage and speeds of ships
passing through the area.
In this first run of the experiment, the researchers observed 20 ship
passages with nine individual whales, at distances ranging from 60
meters to more than 3 kilometers. In each of these instances, the whales
exhibited behavior similar to the “startle response” that scientists
observe during the tagging process, in which the whales essentially
“play dead.”
“Blue whales have a subtle and not very convincing ability to get out
of the way of oncoming ships,” said Goldbogen. “Instead of diving,
where the animal kicks tail up and goes down vertically, they just sink
horizontally. This results in a slow dive and leaves them susceptible to
ship strikes.”
A whale must dive 30 meters below the surface to escape the suction
created by a ship’s propeller. In the study, the whales sank at about a
half a meter per second and showed no evidence for swimming laterally to
avoid the ship. In most cases, this was barely fast enough to get out
of the ship’s way.
This is just the first step in figuring out the behavior of whales in
the context of heavy shipping traffic, Goldbogen said. The research
team is already planning a second round of tests in which the GPS units
will remain attached to the whales for several weeks, and will extend to
species such as humpback whales. With more data about both whale
behavior and the frequency of near misses, Goldbogen hopes to be able to
make a compelling recommendation to pleasure boaters and the shipping
industry for how to minimize the risk of collisions.