According to a new study of New Zealand blue whale vocalizations, the whales are present year-round in the South Taranaki Bight and their behavior is influenced by the region's environmental conditions.
The findings represent a significant advance in researchers' understanding of the habitat use and behavior of this population of blue whales, which was first identified as genetically distinct from other blue whale populations by Oregon State University researchers less than a decade ago.
Blue Whale To Environmental Conditions
Ishara S.KODIKARA/AFP via Getty Images
"We went from not knowing whether this was a distinct population ten years ago to now understanding these whales' ecology and response to changing environmental conditions," said Dawn Barlow, a postdoctoral scholar in OSU's Marine Mammal Institute, as per ScienceDaily.
The findings can help guide conservation efforts for this blue whale population and its habitat.
Over two years, the patterns and intensity of the whales' calls and songs revealed strong seasonality in their foraging and breeding behavior, and the vocalizations changed in response to environmental conditions such as a documented marine heatwave, according to Barlow.
"Feeding-related calls were reduced during the marine heatwave, reflecting poor foraging conditions at the time," Barlow explained.
However, Barlow noticed changes in vocalizations during the subsequent breeding period, indicating that they put less effort into reproduction after a period of poor feeding conditions.
The findings were recently published in the journal Ecology and Evolution.
Barlow worked as a doctoral student in the Geospatial Ecology of Marine Megafauna Laboratory at Hatfield Marine Science Center in Newport, which is led by associate professor Leigh Torres, who is also a co-author of the new paper.
Blue whales are the largest of all whales, and they can be found in every ocean except the Arctic
Because of commercial whaling in the early 1900s, their populations were depleted, and they are now listed as endangered on the International Union for Conservation of Nature's Red List of Threatened Species.
The habitat of New Zealand whales overlaps with a variety of commercial activities, including oil and gas exploration and extraction, vessel traffic, fisheries, wind energy development, and potential seabed mining.
Torres proposed in 2013 that the South Taranaki Bight, located between New Zealand's North and South Islands, was an undocumented feeding ground for blue whales.
Torres, Barlow, and colleagues were able to demonstrate in 2018 that the population in this region was genetically distinct from other blue whale populations after extensive data collection efforts and the use of multiple lines of evidence.
Previous research was primarily based on observations made by researchers during summer visits to the region.
However, the researchers were curious about the whales' behavior at other times of the year.
They installed five hydrophones, which are underwater microphones, and recorded continuously from January 2016 to February 2018, with only brief breaks to retrieve data every six months.
The hydrophone recordings revealed that the whales' "D" calls were strongly correlated with upwelling oceanographic conditions in the spring and summer.
Upwelling occurs when deeper, cooler water is pushed to the surface; the nutrient-rich water supports krill aggregations that blue whales feed on.
During periods of strong upwelling, the whales' D calls were more intense.
The recordings also revealed that male whale song vocalizations, which are associated with breeding behavior, followed a highly seasonal pattern, with peak intensity in the fall, as per Newswise.
This coincides with previous whaling records' estimates of conception, according to Barlow.
The hydrophone evidence of the whales' breeding behavior and year-round presence in the region can influence the animals' national threat classification status, which impacts management practices, according to the researchers.
Blue whales in New Zealand were previously classified as migrants, but as a result of Torres, Barlow, and colleagues' research, the classification has been changed from migrant to data deficient.
If the whales are reclassified as a resident population, management practices may change, but evidence of breeding in New Zealand is required for that to happen, according to the researchers.
Once the researchers established a link between the whales' behavior and their calls, they were able to examine the calls and behavior in relation to environmental patterns.
They were particularly interested in how the whales' foraging and breeding behavior changed during and after a 2016 marine heatwave.
During the marine heatwave, there were fewer krill aggregations for the whales to feed on, as previously documented by the researchers in a previous study.
The decrease in foraging behavior was correlated with less intense D calls during that time period, and the following breeding season's breeding songs were also less intense.
