Predators in the Prey Patch

Predators in the Prey Patch
Marine Mammal Research Unit

Northern fur seals at night

New insights into the complex relationship between fur seals and pollock

A multidisciplinary team of scientists tracked the foraging patterns of northern fur seals to understand how they interact with walleye pollock, an important fish stock. What the researchers discovered may have significant implications for how the commercial pollock fishery is managed, and how declining populations of northern fur seals are protected.

Alaska’s remote Pribilof Islands are home to the largest breeding population of northern fur seals and some of the largest populations of seabirds in the North Pacific Ocean. Nourished by an apparent abundance of walleye pollock and other prey—the declines of seabird and fur seal populations breeding on the Pribilofs has scientists puzzled. Is there more to the relationship between these predators and their prey than meets the eye?

Kelly Benoit-Bird at work

Observing thick-billed murres

To answer this and other questions, a diverse team of researchers participated in the Bering Sea Project — a large, interdisciplinary ecosystem study funded by the North Pacific Research Board — to gather critical data about top predators and their prey. Two of the inter-related studies, led by oceanographer Dr. Kelly Benoit-Bird, were recently published in the journals PLOS One and Marine Ecology Progress Series. They present a new understanding of how northern fur seals, thick-billed murres and black-legged kittiwakes behave with respect to their prey.
Three Top Predators
“We conducted field studies in the Bering Sea over two summers,” says Benoit-Bird. “In the first study, we wanted to determine the specific ‘hot spots’ where northern fur seals and two species of seabirds foraged for prey, and how they lined up with juvenile pollock in those areas. All three predators sought out small, dense schools of juvenile fish that aggregated within a larger prey field. We found that what matters to them is not the overall number of fish in a region, but how and where fish aggregate on a small scale.”

Sea gull

The small-scale distribution of prey, or patchiness, is a much better predictor of a predator’s success than an overall concentration of prey, says Benoit-Bird. In other words, a predator is more likely to find food if it swims from one dense patch of prey to another — like a baseball player running the bases, she says — than if it swims through one large, diffuse aggregation.
Benoit-Bird notes that many management practices take into account the differences between regional aggregations of fish — but often do not consider the patchiness within them. “For predators, we now know that how prey is distributed is critical,” she says, “especially when three key predators with very different foraging strategies give us the same message. It’s a much stronger statement.”
Fur Seal Foraging
The second study focused on the foraging behavior of northern fur seals, employing innovative tracking tags that logged each seal’s movements in three dimensions, multiple times per second. This provided Benoit-Bird and her colleagues with a sophisticated set of behavioral data that could be plotted against the location of pollock prey patches.

northern fur seal

Northern fur seal resting

“We learned a lot about the foraging behavior of northern fur seals, and some of it is really surprising,” says Benoit-Bird. “For example, previous studies that analyzed scat (feces) on Bogosloff Island showed that fur seals don’t eat very much pollock, but their foraging movements in the water are really focused on juvenile pollock. This tells us that they’re using juvenile pollock as a basis for foraging even while they’re searching for other things to eat, like squid or mesopelagic fish.”
Using her baseball analogy, Benoit-Bird describes the fur seals as swimming between “bases” of pollock, but perhaps doing their most important eating between the bases. “To me, this suggests that while pollock may not be their preferred food, this strategy provides a basic safety net if they can’t find anything better to eat.”
A New Way of Seeing
Benoit-Bird’s job as a scientist is to find the story in the data. At first, the data she and her colleagues collected told a confusing story. “We weren’t finding predator-prey relationships that made any sense,” she says. “We had to go back to the data and let the animal’s behaviors guide us to the right answer. The exciting part was saying to them, ‘Tell us what matters, tell us what you’re looking at, what was it that made you pick this spot?’ and then having them teach us a new way of seeing them.”


Juvenile walleye pollock

The Bering Sea Project enabled Benoit-Bird and her colleagues to study how multiple species interact in the same space and time, offering an unprecedented opportunity to collect and analyze multidisciplinary sets of data. This collaborative effort is lending new insights into the interactions between marine predators and their prey that could have profound implications for fish and wildlife management.
“We’re working with agencies responsible for assessing pollock stocks in the Bering Sea to optimize their survey protocols, so that they can accomplish their current objectives while taking into account new information from our science,” Benoit-Bird says. “Our work can potentially help them collect new information that we can analyze over time, giving better insights into long-term trends like regime shifts in ocean climate. We can’t go backward in time to the last oceanic regime shift, but we can think about how to move forward.”


Prey patch patterns predict habitat use by top marine predators with diverse foraging strategies.
Benoit-Bird, K. J., B. C. Battaile, S. A. Heppell, B. Hoover, D. Irons, N. Jones, K. J. Kuletz, C. A. Nordstrom, R. Paredes, R. M. Suryan, C. M. Waluk and A. W. Trites. 2013.
PLoS ONE Vol 8(1):e53348.
Spatial coherence between predators and prey has rarely been observed in pelagic marine ecosystems. We used measures of the environment, prey abundance, prey quality, and prey distribution to explain the observed distributions of three cooccurring predator species breeding on islands in the southeastern Bering Sea: black-legged kittiwakes (Rissa tridactyla), thick-billed murres (Uria lomvia), and northern fur seals (Callorhinus ursinus). Predictions of statistical models were tested using movement patterns obtained from satellite-tracked individual animals. With the most commonly used measures to quantify prey distributions - areal biomass, density, and numerical abundance - we were unable to find a spatial relationship between predators and their prey. We instead found that habitat use by all three predators was predicted most strongly by prey patch characteristics such as depth and local density within spatial aggregations. Additional prey patch characteristics and physic al habitat also contributed significantly to characterizing predator patterns. Our results indicate that the smallscale prey patch characteristics are critical to how predators perceive the quality of their food supply and the mechanisms they use to exploit it, regardless of time of day, sampling year, or source colony. The three focal predator species had different constraints and employed different foraging strategies – a shallow diver that makes trips of moderate distance (kittiwakes), a deep diver that makes trip of short distances (murres), and a deep diver that makes extensive trips (fur seals). However, all three were similarly linked by patchiness of prey rather than by the distribution of overall biomass. This supports the hypothesis that patchiness may be critical for understanding predator-prey relationships in pelagic marine systems more generally.
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Foraging behavior of northern fur seals closely matches the hierarchical patch scales of prey.
Benoit-Bird, K. J., B. C. Battaile, C. A. Nordstrom and A. W. Trites. 2013.
Marine Ecology Progress Series 479:283-302.
Marine prey often occur in hierarchical mosaics whereby small, high-density patches are nested inside of larger, lower density aggregations. We tested the extent to which the foraging behavior of a marine predator (northern fur seal Callorhinus ursinus) could be explained by the hierarchical patch structure of a dominant prey species (juvenile walleye pollock Theragra chalcogramma) in the eastern Bering Sea. Comparing the movements of satellite-tracked fur seals with ship-based acoustic surveys of prey revealed that fur seals did not randomly search for prey, but instead showed deviations in the distribution of step-lengths (distances between their foraging patches) corresponding to the distances between aggregations of prey. Scales of prey distribution varied between Bering Sea shelf and deep-water slope habitats, while spatial scale distributions of fur seals showed corresponding changes, indicating that their search strategies were not innate patterns decoupled from the environment. Fur seals tended to avoid the smallest prey patches in both shelf and slope habitats. They also avoided prey patches that were separated by large distances. Fur seals responded to several levels of prey patchiness simultaneously, resulting in strong correlations between predator and prey over the entire range of aggregation scales observed in juvenile pollock. Our results indicate that, despite having a varied diet, fur seal foraging paths were defined by juvenile pollock aggregations. The presence of hierarchical, scale-dependent aggregation in both predator and prey provides new insights into fur seal behavior and a means to predict the dynamics of their interactions with prey.

keywords     Patchiness, Spatial scale, Predator–prey, Foraging behavior, Hierarchical, Northern fur seal, Juvenile walleye pollock
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