Nighttime may bring quieter waters to the Salish Sea, but endangered Southern Resident killer whales do not appear to use those hours to make up for feeding opportunities they may lose during the day. That is one of the key findings of new research led by Dr. Marla Holt, a scientist at the National Oceanic and Atmospheric Administration’s Northwest Fisheries Science Center in Seattle. “We investigated the nighttime foraging behavior of our endangered Southern Resident killer whales,” Holt said. “Prior to this study, we had very little information about their nighttime behavior and especially their use of sound for foraging.” The research offers a rare look at what the whales are doing after sunset, when researchers cannot observe them as easily from the surface. Scientists used temporary digital acoustic recording tags, known as D tags, to record the whales’ movements and sounds underwater. The tags are attached with suction cups and can remain on a whale for about 24 hours before detaching. The research team tagged 10 Southern Resident killer whales in the Salish Sea between 2018 and 2021, collecting about 96 hours of data, including 56 hours at night. The tags allowed researchers to listen for different sounds associated with the stages of a hunt. “They produce these very regular, slower clicks at shallow depths,” Holt said. “When they chase prey, when they dive deeper, they increase the repetition rate of those echolocation clicks.” When the whales get close to a fish and attempt to capture it, they produce an especially rapid series of echolocation clicks known as a feeding buzz. “It sounds like a buzz to our ears when they’re homing in on fish acoustically to try to capture them,” Holt said. Researchers compared those different stages of foraging during daylight and nighttime hours. The whales searched for prey and chased fish during both periods. But the researchers found a difference when the whales reached the final stage of the hunt and attempted to capture their prey. Males showed slightly higher feeding buzz activity at night, while females showed the lowest level of buzzing activity, particularly at night. At a specific maximum dive depth used in the researchers’ statistical model, the predicted probability of a female producing a capture related buzz was about 88 per cent during the day, compared with roughly 21 per cent at night. Those are modelled probabilities, not direct measurements of successful fish captures. “We’re not quite sure why we see these lower nighttime prey capture attempts in females,” Holt said. The researchers suggest several possibilities, including differences in the prey females target and the role visual cues may play in successful captures. Females may also use different foraging strategies than males. The research also found the whales generally began echolocating at shallower depths at night, a pattern that is consistent with salmon moving higher in the water column after dark. The findings are particularly interesting because nighttime can bring a reduction in vessel activity and underwater noise. Previous research has shown that vessels and noise can interfere with the whales’ ability to use sound while searching for food. Researchers therefore wanted to know whether the whales might take advantage of quieter nighttime conditions by doing more of their foraging after dark. “We’re not seeing any clear evidence that they are foraging more at night, relative to daytime periods,” Holt said. That does not mean the research shows vessel noise is responsible for the nighttime differences in female prey capture attempts. The researchers say more work is needed to understand how noise, prey availability and nighttime behaviour interact. For the Southern Resident population, those questions matter. The whales are endangered and depend heavily on salmon, particularly energy rich Chinook salmon, for food. Holt said the research highlights the importance of sound to the whales’ survival. The whales face several conservation challenges, including limited prey availability and disturbance from vessels and underwater noise. “There’s that interplay between the two because they must use sound to find their food,” Holt said. The research is based on a relatively small sample, which is an important consideration when interpreting the results. Only six of the 10 tag deployments recorded both daytime and nighttime data, and just three of the tagged whales were female. Holt said that is one reason this type of research takes time. “We’re dealing with small sample size that’s inherent with this type of work,” she said. “You want to make sure that things are done carefully. It takes time.” The researchers’ next steps include looking more closely at foraging efficiency and the relationship between nighttime behaviour and underwater noise. For now, the findings provide a rare glimpse into how Southern Resident killer whales search for and attempt to capture food after dark and suggest that nighttime may not provide a simple way to make up for feeding opportunities lost during the day.
