Despite being one of the largest animals on the planet, the limited population and vast territory of the North Atlantic right whale make tracking these behemoths challenging. Yet, accurate predictions of their distribution — and preferred habitat at different times of year — is essential for effective conservation.
New research led by scientists at Bigelow Laboratory outlines a modeling approach that incorporates information on the abundance of the whale's favorite zooplankton prey species, considering their actual daily energy needs, or "feeding threshold," to more effectively predict where the right whales congregate at different times of year.
The findings highlight the value of including nuanced and detailed prey information in right whale models to improve their predictive power for management and conservation. They also suggest that secondary prey species may have a more important and complex role in the right whale diet than previously thought.
"This work is exciting because it confirms that realistically representing prey is an important factor in models accurately predicting where right whales gather," said the study's lead author, Camille Ross, a former research associate at Bigelow Laboratory who is now an associate research scientist at the Anderson Cabot Center for Ocean Life at the New England Aquarium. "Our novel approach accounts for the preferences and needs of a hungry whale, highlighting the importance of smaller species in the right whale diet."
"You can't protect whales if you don't know where they are — and they go where the food is," added Damian Brady, a professor of oceanography at the University of Maine's Darling Marine Center and a co-author on the new paper. "This study helps us map that more precisely than ever before."
The study was published in the journal Endangered Species Research and brings together experts on modeling, right whale physiology, and zooplankton ecology from Bigelow Laboratory, the University of Maine's Darling Marine Center, the Anderson Cabot Center for Ocean Life at the New England Aquarium, Duke University, and the NOAA Northeast Fisheries Science Center.
Given the logistical challenges of tracking right whales, species distribution models are important tools for management. Most of these models account for the abundance of the tiny zooplankton whales feed on, like the fatty copepod Calanus finmarchicus. But they do so by using indirect measures, or proxies. For example, NASA orbiters measure the pigment chlorophyll, allowing scientists to estimate the biomass of the plants that zooplankton like Calanus feed on, and then, in turn, approximate the density of those zooplankton.
These indirect proxies, like chlorophyll, are widely available and relatively easy to get from satellite data. In comparison, getting first-hand observations of zooplankton abundance across an ocean basin — given their small size and the depths they inhabit — is next to impossible (though researchers at Bigelow Laboratory are working to develop methods of detecting zooplankton with satellites).
But these proxies are also several steps removed from actual measures of prey abundance. They can be inaccurate for depicting zooplankton presence, and obscure nuances in the whale diet, including the specific species of zooplankton whales prefer.
"Right whales target only a few key zooplankton species, and their feeding habits vary by location and season," Ross said. "Replacing prey proxies with direct prey observations tailored to the whales' foraging requirements has great potential to improve model performance."
In 2023, Ross and some of the same co-authors published the first description of this new approach, interpolating historical observations of Calanus to create a more direct estimate of their abundance relative to how much daily energy a whale needs. In the new paper, the team takes that work further by expanding the approach to two secondary prey species, which right whales appear to rely on in certain places at specific times of year but are less well studied.
Data on the prey species was collected during the NOAA Fisheries Ecosystem Monitoring Survey .
They then incorporated different combinations of these new variables into models to predict the density of right whales in different parts of their foraging habitat. As hoped, including direct estimates of prey availability significantly improved how well the models matched with actual observations by NOAA of right whale movements compared to the models that just included indirect proxies, like chlorophyll.
The researchers also found that the most accurate models included information on the abundance of both Calanus and the secondary prey — but not in the way they expected. Where the abundance of the smaller, less fatty copepod Pseudocalanus was higher, the model actually predicted fewer right whales. That raises many new questions that the researchers hope to tackle on the role of these secondary species in the right whale diet.
Improving predictive tools with this more direct, accurate information on prey will give scientists and managers a more holistic view of right whale habitat and how they're using it. That's essential, Ross said, for being proactive to potential shifts in their behavior as environmental conditions change.
"The key to developing models that will help move things forward is working together with actual users, like NOAA, the Maine Department of Marine Resources, and the fishing and shipping industries," said co-author Nick Record, senior research scientist and director of the Tandy Center for Ocean Forecasting at Bigelow Laboratory. "This more direct approach for including prey information is an important step toward meeting those user needs."
But the value of having better tools for zooplankton distribution goes beyond right whales.
"This paper is specifically focused on a right whale application, but this idea of interpolating zooplankton data from the perspective of the energetic requirements of the predator could be used across marine science," Ross said. "There are other species, like larval lobster, that feed on Calanus, and there's no reason that our method couldn't be extended to those species."
