Richard Musgrove
Whale songs which are rhythmic, structured, haunting, and which echo through vast oceanic distances are the province of these mature mammals, but what do they really mean? Well, some like the North Pacific baleen whale sing more when food is abundant.
It’s been believed they are tied to foraging and reproductive behaviour, migration timing, cultural transmission of behaviour, abundance and distribution.
Dr John Ryan of Monterey Bay Aquarium Research Institute (MBARI) and his team listened to three species of baleen whales — Humpback, Fin and Blue — in the California Current Ecosystem, off the west coast of the US, to see what the songs could tell them about the health of their ecosystem.
During the 6-year study, researchers used hydrophones to record songs; along with visual surveys and skin samples — biopsies — taken using a crossbow. National Oceanic and Atmospheric Administration (NOAA) Fisheries data allowed estimation prey abundance (i.e. krill, anchovies and sardines).
Biopsy-sampling crossbows fire custom-made darts at the side or back of the whale. These go through the skin and a couple of centimetres of blubber, then pop back out — the attached float can be easily retrieved. Biopsy samples were analysed for stable isotopes of carbon 13C and nitrogen (15N), which also allowed researchers to understand changes in prey species and their abundance.
Acoustic monitoring began in 2015, coincidently the peak of a multi-year marine heatwave — which meant researchers could follow recovery of prey species and observe the effects on whale feeding ecology, and song, as the water cooled.
Humpback Whales sang more as prey became abundant after the heatwave, the team concludes. Their songs were detected on 34% of days monitored at the beginning and 74% after 6 years. These observations were tied to more food — dramatic increases in krill then increased anchovy abundance as the ecosystem recovered from the heatwave over the first three years — which also tied in with the biopsy samples taken.
While Humpbacks switched prey, songs of Blue and Fin Whales only increased with rising krill abundance. Krill numbers had fallen by the fifth year of the study, while forage fish increased, says MBARI, which, while positive for Humpbacks, would have forced Blue Whales to feed elsewhere.
Overall, Blue, Fin and Humpback songs rose in parallel during heatwave recovery, Ryan writes, “consistent with the ecosystem-wide impacts of this major environmental perturbation”
Similar changes in Blue Whale foraging behaviour, and reproductive effort, were found during a marine heatwave off the South Taranaki Bight on the West Coast of New Zealand’s North Island.
“Warming waters challenge marine ecosystems in many ways”, says coauthor, Ted Cheeseman, “Marine heatwave conditions like we had in 2015 change the entire picture; adaptable humpback switch foods, while blue whales feed elsewhere; once waters get warm enough, their populations will suffer as will our fishing industries.”
Ryan says: “Surprisingly, the acoustic behaviour provides insights about which species can better adapt to changing ocean conditions.
“Our findings can help resource managers and policymakers better protect endangered whales.”
The paper was published in PLOS 1.
The whale that sings, jumps and swims!
