Behavior Human impacts

Killer Consequences: How boats affect the behavior of endangered killer whales

Reference: Holt, Marla M., et al. “Effects of vessel distance and sex on the behavior of endangered killer whales.” Frontiers in Marine Science 7 (2021): 1211.

The resident killer whales

Imagine you are about to eat your favorite meal. You go in to take a bite, and suddenly, someone starts loudly banging pots and pans and staring at you very intently. You would probably lose your appetite. A similar situation can occur when boats, including whale watching and fishing vessels, get too close to killer whales who are trying to feed. The boats’ motors make noise that can damage or interfere with the whales’ hearing, sonar signals may disrupt the whales’ attempts to locate food using echolocation, and the surface of the water may not seem safe for the whale to emerge from to breathe.

The resident killer whales in the San Juan Islands. These whales are very well documented in this area, and are the famous subjects of many whale watching expeditions. They are so well known that each whale has a name, and can be identified by their various unique markings. Image by Ingrid Taylar via Flickr.

In the San Juan Islands, located off the coast of Washington State, this very issue is affecting the population of killer whales that live there. These whales are considered “residents” of the island’s waters, meaning they do not travel widely as other killer whale populations might do. They stay very close to the waters off the San Juan Islands, sometimes venturing a bit north or south off the west coasts of America and Canada. In the summer however, they generally tend to stay put.

To complicate matters, these resident killer whales are endangered, with just 74 whales remaining as of last year. This number is not sustainable for the population long term, and if they are to survive, increased conservation efforts are needed.

Motivated by this problem, a group of researchers based at the Northwest Fisheries Science Center sought to study how the presence of boats affects foraging (a feeding attempt) in these killer whales. The scientists placed tags on the whales, allowing them to see where whales were in the water column, and pick up sounds the whales were making as they ate, searched for prey, or travelled. The team also observed how killer whale behavior changed based on how close boats were, and if their responses differed between male and female whales.

How do boats affect whale behavior?

A ferry approaching a killer whale in Bluemill Sound near Great Britain. When vessels pass so close to whales, the noise from them can interfere with the whales’ ability to communicate with their fellow whales and feed. Boats can also pose a physical danger to the whales if they get too close. Image by Mike Pennington via geograph.

Researchers found two main trends in their data that did not spell good news for the killer whales. The first was that when boats were close to whales, they tended to make feeding dives less often. A boat was considered “close” if it was about 400 yards—a little over three football fields—or less away. If a boat approached during a feeding dive, whales would often swim away or make shallower dives to look for a different food item. The whales clearly demonstrated their sensitivity to human activity.

The second piece of bad news concerned differences between how male and female whales reacted to the presence of nearby vessels. Males would more often transition from deep water feeding dives to shallow dives, while females were more likely to simply swim away. The researchers attributed this difference to several factors. For one, females are usually in charge of the juvenile whales and may be more likely to leave an area to protect their young if they perceive a threat. For another, females tend to make feeding dives closer to shore than males, and so there is less space available to maneuver away from a threat or chase prey. This lack of space could cause them to give up on a foraging event and move to a new area more quickly when faced with noise pollution or other boat-related disruptions.

Whale, what can we do to help?

Given that this population of killer whales is endangered, the negative effects that boats have on killer whale foraging could have devastating consequences. For females, the problem is more dire. Because they are smaller than males, female dive attempts are more energetically costly. Thus, if nearby boats cause female whales to abandon their feeding dives, these whales are more likely to end up malnourished, which can affect the health of their calves as well.

A mother-calf pair, from the resident population in the San Juan Islands. Because boats can negatively affect female whales more so than males, they also have the potential to endanger calves, who rely on their mothers greatly in the first two years of life. Image by NOAA Fisheries West Coast via Flickr.

In thinking about ways to lessen the effect humans are having on this population, a good first step would be to legally increase the distance boats, including whale watching vessels, must stay away from these killer whales—whales were much more likely to forage when boats stayed more than 400 yards away—and find effective ways to enforce such laws.

Even if you don’t happen to have any legal authority in the San Juan Islands, there are still ways you can help! The San Juan Islands Visitor Bureau has many resources on ways to learn about and protect these killer whales (including adopting your own individual whale!), who are an extremely important part of their island community.


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