Ellen Chenoweth: salmon hatcheries are not the best buffet for Baleen whales

In our latest post, Ellen Chenoweth from University of Alaska Fairbanks introduces her work ‘Confronting assumptions about prey selection by lunge-feeding whales using a process-based model’, discusses how the apparently easiest foraging strategy is not always the optimal and presents the diverse set of project she and her colleagues are currently running.

About the Research

Baleen whales are generalist and innovative predators. They grow to their enormous size by lunging through and engulfing not only the fish but the water they are swimming in as well.  The whales then force the water out of their mouths through their unique baleen sieve while trapping the prey inside.  They deploy this method to target a wide variety of prey from layers of tiny swarming krill more than 100 meters deep to much larger, quicker, herring tightly packed in shape-shifting patches.  In 2008 in Southeast Alaska, humpback whales were seen targeting a new, and particularly valuable, prey for the first time: hatchery reared juvenile salmon. 

Ellen Chenoweth uses a prey mapping robot to measure the distribution of hatchery salmon after a release at Hidden Falls Hatchery in Alaska.
Ellen Chenoweth uses a prey mapping robot to measure the distribution of hatchery salmon after a release at Hidden Falls Hatchery in Alaska.

Salmon hatcheries are artificial nurseries for salmon that give them a head start in life.  As juveniles–some as small as the size of your finger–these fish are released from rearing pens to fend for themselves in the ocean. If they survive and grow into adult salmon, they help support the Alaskan fishing fleet that foots the bill.  Of course, most don’t make it.  The staff are mostly resigned to watching many of these tiny fish get picked off one-by-one by seals, birds and larger fish that are attracted to the release site. However, it was alarming for them to see a humpback whale scooping their former dependants up by the very-large mouthful.  Surely these whales were getting an easy meal at the fishing fleet’s expense.

For my dissertation, I set out to discover if these few whales were brilliant entrepreneurs on the cutting edge of a new and costly (to fisheries) whale trend, or misguided eccentrics exploring a tactical dead end.  We took the common optimal foraging approach of assuming that the best prey for whales is that which allows them to capture the most energy quickly with the least amount of effort. We used this metric to compare hatchery salmon to against typical whale prey.  We realized that the mathematical model we were developing could be usefully applied to any baleen whale that feeds using the same method and any prey patch, typical, novel or even hypothetical, including those that may result from changes in climate.  In addition, this method allowed us to put some basic assumptions about baleen whale foraging to the test.

About the Paper

Our mathematical model requires data obtained from observing humpback whales feeding in the wild on many different types of prey and measuring the characteristics of the prey and behaviours of the whales.  Gathering this data required heading out to sea in Alaska to tag foraging whales and record their movements as well as observing, capturing and analysing their prey.  In total we analysed 21 different whales observed feeding in 13 distinct foraging situations.  This information was then incorporated into a single equation to calculate a prediction of the net energy a whale can gain per minute of feeding on a particular prey patch.

Humpback whale feeding on juvenile salmon at Hidden Falls Hatchery. Photo by Monique Anderson. NOAA Fisheries Permit 14122
Humpback whale feeding on juvenile salmon at Hidden Falls Hatchery. Photo by Monique Anderson. NOAA Fisheries Permit 14122

In science, we maintain a preference for the simplest explanation, even though we know that the world is rarely simple.  This project took stock of often simplifying assumptions about whale foraging and played them out to see if they are supported mathematically, revealing where additional important complexities may exist.  For example, our model predicted that not all newly released juvenile salmon are “easy” prey.  While one species of juvenile hatchery-released salmon was predicted to provide whales with high net energy gain compared to the typical prey, another species tended to spread out in the water more, making it more challenging for whales to get enough fish per lunge for an energy surplus. More broadly though, whales don’t seem to be gaining enough energy in many foraging situations.  Is that because they are scraping by and biding their time most of the year and then feasting during a few exceptional foraging situations? Or did we underestimate the importance of subtle ways whales manipulate their more challenging prey into submission?  These results suggest exciting areas of future research that will impact our understanding of baleen whale adaptation and resilience.

About the Author

I was raised in Michigan and have been interested in the world’s largest predators since we studied them in my kindergarten class. In college I majored in biology and loved my courses in physiology, ecology and evolution, which continually brought my attention back to whales.  Whales’ terrestrial past and marine adaptations make them fascinating examples in all these disciplines, while the difficulty of studying them in the wild means there remain many fascinating open questions.  After my freshman year of college, I got a summer job as a housekeeper in Glacier Bay National Park and volunteered for a day with the Humpback Whale Monitoring Program.  The park’s biologists encouraged me to apply for a student technician position and two summers later they hired me in my first science job that happened to be on the edge of unbelievably stunning wilderness.

Still in Alaska a decade and a half later, I currently work as a Research Advising and Mentoring Professional and an Affiliate Faculty at the University of Alaska Fairbanks, but I am based remotely (or should I say even more remotely) at the UAS Sitka Whale Lab.  I love the variety of work I get to do including teaching courses, informal science education, mentoring students, volunteering for the marine mammal stranding network, conducting fieldwork and analysing data.

This paper in Functional Ecology with my co-authors is my proudest scientific contribution.   It was the most time consuming and ambitious project of my PhD, but one that engaged my curiosity during the entire process.  I am excited to continue energetics and modelling research, particularly as it relates to ecosystem adaptation to climate change and human-cetacean interactions.

Currently I am excited about collaborating on a Virtual Whale Necropsy Project. I am developing an open-source lesson around external and internal anatomy of a 4-D scanned dead stranded humpback whale that will allow students–or any curious individual–to look inside a real whale for clues as to a its cause of death. Check out the UAS Sitka Whale Lab website for updates on this project https://uas.alaska.edu/research/whalelab/.

As the founding director of the Rural Alaska Students in One Health Research (RASOR) program, I work with a team to match high school students that live in small isolated coastal communities in Southeast Alaska with local tribal research mentors.  Students do original, local and culturally relevant research at the interface of environmental and human health while earning college credit. Find out more about our collaborators and student research projects at https://rasor.alaska.edu/.

I enjoy kayaking and backpacking trips in Alaska’s different environments.  Currently I spend a lot of time training my new puppy, Booster (like the rocket), to be a very good boy.

Read the article in full here.

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