Boat traffic has bigger effect on female orcas than males, raising reproduction concerns: study
Researchers in Washington state have found that the impact nearby boats have on endangered southern resident killer whales isn’t equal between males and females.
A study published in the latest edition of the journal Frontiers in Marine Science shows that female orcas are more likely to stop foraging for food when vessels are close — a finding that raises further concerns about the dwindling southern resident population’s ability to find the required food to reproduce.
“I was surprised to see a bigger effect in females, but when you think about it more, it kind of makes sense,” said Marla Holt, lead author on the paper and research wildlife biologist with the National Oceanic and Atmospheric Administration.
Holt reasoned that female orcas have smaller bodies and less capacity for deeper dives, and they’re more likely to be associated with younger orcas, which have even less ability to make deep dives.
Holt and her team carried out the study from 2010 to 2014, using multi-sensor tags temporarily attached to orcas by suction cups. The tags used accelerometers and magnetometers to track movements and dives, as well as hydrophones to record vessel noise and the sounds killer whales use to track their prey.
Identifying the different types of sounds the animals make when they hunt helped the researchers determine whether they were using echolocation to find possible prey in the water below, or whether they were nearing the fish and capturing them — a behaviour that includes such rapid clicks that it sounds like a buzz to the human ear.
“Both males and females actually made fewer dives, and they spent less time in dives involving deep foraging and prey capture,” said Holt, adding that “nearby” vessels included any that were within 400 yards (366 metres).
But that effect was more significant in females.
“The fact that we’re finding when vessels are close to females, that they forgo foraging, is very significant, because those lost calories can have cascading effects on a female’s ability to support reproductive efforts,” she said.
In the waters off southern B.C., an interim order bans boats within 400 metres of killer whales. Holt said according to Washington state law, vessels must stay 274 metres away from the sides of orcas and 366 metres away if they’re in front of or behind them. U.S. federal law only requires 183 metres from an orca’s side.
According to Holt, one of the goals of this research is to provide a scientific basis for clear rules in the different jurisdictions that people can easily follow.
The total number of southern resident killer whales, which live in the Salish Sea in B.C. and Washington state, is believed to be 74, after two new calves were born in September.
Holt says the primary factors affecting the recovery of the endangered group are contaminants, the availability of prey like chinook salmon, and disturbances by vessels.