Acoustic survey tracks whale population trends along the coast of Southern California

Posted on June 29, 2015 by Bob Berwyn: Blue whale numbers holding steady; fin whales increasing… 

Staff Report: FRISCO — A new acoustic survey in Southern California coastal waters is helping researchers track whale populations.The data analyzed by the Scripps Institution of Oceanography at UC San Diego suggests that blue whale numbers are holding steady, while the number of fin whales is increasing.Both species are often seen in the Southern California Bight, the curved region of California coastline with offshore waters extending from San Diego to Point Conception (near Santa Barbara, Calif.), but little is known about their use of the area, where ever-increasing ship traffic has raised concerns about collisions between whales and boats.To learn more, researchers with the Scripps Marine Bioacoustics Lab and Scripps Whale Acoustic Lab  set out specialized recording devices on the seafloor, tracking whale vocalizations  from 2006-2012.The findings were described in the journal Endangered Species Research. The study was supported by the Office of Naval Research, and provides the first detailed view into the spatial use of Southern California waters by blue and fin whales, the two largest cetacean species in the world. Both are classified as endangered species.

Scripps marine acoustician Ana Širović found that blue whale calls were more commonly detected at coastal sites and near the northern Channel Islands, while fin whale calls were detected further off shore, in central and southern areas.

“I think it’s an interesting difference in trends because both of the species were subject to whaling earlier in the twentieth century, and now they’re clearly responding differently,” said Širović.

The acoustic data and overall trends outlined in this study are consistent with visual observations from another Scripps-led study. Širović said the parallel findings between the two studies as evidence that passive acoustics can be used as a powerful tool to monitor population trends for these large marine mammals.

“I think it’s very exciting that we see the same trends in the visual and acoustic data, because it indicates the possibility of using acoustics to monitor long-term trends and changes,” she said, adding that the new study suggests there is a resident fin whale population in the area.

The seasonal recordings of blue whale calls reinforces what’s already known about their migration from the waters off the coast of Mexico and Costa Rica, arriving in Southern California in late spring to forage through the fall.

The leave in early winter, but researchers aren’t certain where they go next. Although researchers have studied blue and fin whales for years, Širović said both species are particularly mysterious, and scientists still don’t know some basic information about them, such as their mating system or breeding grounds.

The Southern California Bight is a highly productive ecological territory for many marine animals due to strong upwelling of cold water, but researchers have not found any evidence that blue or fin whales are breeding there.

The productivity of the coastal region also makes it a hotbed for human activity, with large cities onshore and ships, commercial fishing vessels, and other human impacts ever-present in the water. Since fin whales generally live further offshore, Širović posits that they might have a slight advantage over blue whales, which tend to inhabit areas where there is more ship traffic–increasing their chances for ship strikes.

“It seems that for fin whales, things are probably improving,” she said Širović.

“For blue whales, it’s a little bit harder to tell. There is a question right now as to whether their population has grown to its maximum capacity, because there are many lines of evidence showing that their population is not growing currently,” she said. “So the question remains, is it because that’s just what their population size can be maximally, or are there factors that are keeping them from growing further?”

Širović hopes that future studies can help identify why there is this difference in population trends of blue and fin whales. Now that she and her colleagues have taken a first look at the broad trends of the two species, they want to dig deeper and look into environmental drivers and other factors and features that may be causing some of the spatial distribution patterns and long-term changes of the whales.