How do we know what humpback whales eat? Insights from a whale researcher’s toolkit

Dr. Reidy [right] and field crew, Jared Towers, Simon Pidcock, in Port Renfrew to access Swiftsure Bank. Photo credit: Steve Pearce.

You can use soundwaves to “see” what the whales are eating. 

In a study published in the Canadian Journal of Fisheries and Aquatic Sciences, Dr. Reidy used active acoustic prey mapping to see how humpback whales interact with prey in different habitats. Using a scientific echosounder called an Acoustic Zooplankton and Fish Profiler (AZFP) installed onto the Juanes lab’s field boat, Reidy sent soundwaves into the water at three different frequencies every two seconds. These signals rebounded off fish and zooplankton and were recorded.

Because the echo from fish and zooplankton differs strongly with frequency, Dr. Reidy’s use of multifrequency allowed her to differentiate between the two types of prey. Her findings showed that humpback whales favour areas with more zooplankton than fish in the feeding areas she surveyed between 2018 and 2020.

There are other—more hands-on—ways to figure out what humpback whales are eating

Whale research isn’t always glamorous – but someone’s got to do it! By collecting floating whale poop and examining it using a microscope or genetic tools (e.g., DNA metabarcoding), scientists can figure out what prey the whale ate.

In another study, Dr. Reidy, whale-watching crews, and University of Victoria researchers did just that – they collected fecal samples from humpback whales using pool skimmers or buckets after a defecation event. Samples were transferred to Ziploc bags for transport to the lab to examine any hard parts (e.g., fish bones) and to vials of ethanol for DNA analysis by collaborating scientists at the Hakai Institute, Dr. Matt Lemay and Dr. Rute Carvahlo. The most common prey detected in the poop of humpback whales foraging off southern Vancouver Island were planktonic krill (Euphausia pacifica) and the following species of fish: Pacific herring (Clupea pallasii), hake (Merluccius productus), and eulachon (Thaleichthys pacificus).

Dr. Reidy deploying the Acoustic Zooplankton and Fish Profiler (AZFP) echosounder. Photo credit: Jessica Qualley

You can also dive with whales – virtually!

To get a closer “look” at how the southern-feeding humpback whales behave underwater, Dr. Reidy analyzed acoustic data with information from tags attached to whales with a suction cup. The tags recorded how fast the whales moved and how deep they dove, offering a detailed view of their foraging strategies in southern B.C.’s Juan de Fuca Strait. By integrating these insights with fecal analysis, Dr. Reidy built a comprehensive picture of how and where these animals feed.

Just like the scientists, animals also listen for sounds in the sea. Humpback whales listen for the vocalizations of their conspecifics (fellow humpback whales) and low-frequency sounds made by prey, such as schooling Pacific herring. How clearly a humpback whale can hear sounds produced by other whales and prey can be affected by underwater noise caused by human activity. An Ocean Noise Strategy is in development for Canada, however, multiple delays in the release of this strategy have raised concerns.

Foraging humpback whales in the Juan de Fuca Strait. Photo credit: Rhonda Reidy

Humpback whales are increasingly involved in vessel strikes and entanglement in fishing gear in coastal areas. Meeting and maintaining recovery conservation objectives will require long-term, quantitative information on subsurface prey resources that appear to be key indicators of humpback whale habitat use.

If you’re also an early-career aquatic researcher, follow the Canadian Journal of Fisheries and Aquatic Sciences  (@cjfas) on social media to be the first to know about early-career research opportunities!

Header image: Photo of the Reidy Field Team at Blackfish Sound. Photo credit: Christie McMillan.