Predation in a Patchy Sea

Black-legged kittiwakes are small, cliff-nesting gulls that breed in large colonies, feeding their young with food foraged at sea. (Photo: U.S. Fish and Wildlife Service)
Black-legged kittiwakes are small, cliff-nesting gulls that breed in large colonies, feeding their young with food foraged at sea. (Photo: U.S. Fish and Wildlife Service)

By Lee Anna Sherman

ONE HOWLING, FOG-SHROUDED NIGHT on the Bering Sea, two small boats pitch and roll on a convulsion of waves. From the bridge of the fishing vessel Frosti, marine ecologist Kelly Benoit-Bird is staring hard off to starboard, where a halo of light dances on the slate-gray ocean. She can see the expedition’s second boat, the fishing vessel Gold Rush, lurching wildly in the glow of its deck lanterns. Through her binoculars, she can just make out fisheries ecologist Scott Heppell, her research partner for the expedition, looking back through the salty spray.

Hour after hour, the researchers and their teams ride the gale, each of them keeping a tense eye on the other’s boat as it “thrashes violently in the swell.” At long last, dawn leaks through the mist. The storm is spent. But the trouble isn’t over. For the marine scientists leading this Oregon State University research cruise to the Pribilof Islands — a lonesome archipelago in the shadow of the Arctic Circle — storms at sea are business as usual, typically causing no more than a hiccup on an expedition. Not this time. The storm’s monstrous force has sheared off a critical piece of gear for Benoit-Bird’s two-season study of predator-prey interactions.

“Our acoustic transducers went straight to the bottom,” she notes wryly. “The Bering Sea is unforgiving.”

The scientists were 200 miles from the closest port. As is the norm for deep-ocean researchers, they swallowed their disappointment and improvised, changing their data-collection protocol so the two boats could share the remaining electronic instrument, which processes data from acoustic signals beamed from the boat into the dark, icy waters in search of life.

“I often study processes that happen in the dark,” says Benoit-Bird. “One of the great advantages of using acoustics is that we can observe things not only when there’s no sun, but also at depths where light levels are always low. There are great opportunities to learn new and unexpected things when we make observations at places and times at which others have not.”

A Patchwork of Prey

What drives Benoit-Bird’s work as a “pelagic” (open ocean) ecologist is the enigma of predator-prey synergies at sea. Big picture, she’s searching for the “rules” that predators share as they exploit ocean food sources wherever they live, whether it’s the frigid Arctic Ocean, the temperate Pacific or the balmy South Seas. Why do marine animals congregate where they do? What combination of characteristics makes one school of fish or aggregation of plankton more attractive than another? How do ocean dwellers even find their prey out there in the watery vastness?

Thick-billed murres nest precariously on narrow ledges high above the Bering Sea in the Pribilofs. (Photo: Momatiuk Eastcott / Animals Animals)
Thick-billed murres nest precariously on narrow ledges high above the Bering Sea in the Pribilofs. (Photo: Momatiuk Eastcott / Animals Animals)

For the Bering Sea study, she zeroes in on an unlikely trio of air-breathing pelagic hunters — a marine mammal species and two species of seabird — that breed and feed in this bleak seascape separating Alaska from Siberia. In partnership with the Bering Sea Project — a joint endeavor of the National Science Foundation and the North Pacific Research Board — she has come to the Pribilofs, along with eight other investigators and their teams, to study how northern fur seals (Callorhinus ursinus), black-legged kittiwakes (Rissa triactyla) and thick-billed murres (Uria lomvia) choose their feeding grounds and what those choices might mean for sea life and fisheries management in this remote, rapidly changing ecosystem, home to one of the world’s richest concentrations of life, much of it dependent on sea ice that is disappearing at an alarming rate as the Earth warms.

Notes Benoit-Bird, “This research is critical in places like the Bering Sea where the biology of the system is rapidly showing effects of climate change.”

For each of the three target species — all with offspring waiting on the islands for food — the act of hunting looks quite distinct. Fur seals, insulated in their ultra-thick coats, dive down deep and forage far out to sea, coming back every few days to nurse their pups. Thick-billed murres — the squat, black-and-white birds often called the “penguins of the Northern Hemisphere” — are clunky flyers but expert swimmers, able to dive 500 feet in pursuit of fish, squid and crustaceans to eat themselves, or to bring back whole for their chicks. In contrast, black-legged kittiwakes, a species of gull, can fly like the wind across miles of open ocean to and from the sheer cliffs where their young wait for a meal of whole fish; yet they’re capable of only the feeblest dips beneath the surface of the sea when they forage.

Northern fur seals hunt in the open ocean, returning to the rookery in the Pribilof Islands every few days to feed their young. (Photo: Nature in Stock / Minden Pictures / Yva Momatiuk & John Eastcott)
Northern fur seals hunt in the open ocean, returning to the rookery in the Pribilof Islands every few days to feed their young. (Photo: Nature in Stock / Minden Pictures / Yva Momatiuk & John Eastcott)

“As we studied these three species with very different overall foraging strategies,” she says, “we asked, What is it they share in common? Does that commonality tell us something about the general rules of  predator-prey dynamics? What are the decision-making rules that guide their behavior?”

Her search for answers starts at the bottom of the food chain, the animating bedrock of all marine life. At the base of the chain are uncountable legions of plant-like “phytoplankton” (untethered, single-celled photosynthesizers sometimes dubbed the “grass of the sea”). Next are their animal-like counterparts, the zooplankton (mostly tiny animals with limited mobility that drift with the currents and tides). Comprising the third category are itty-bitty free-swimming creatures called “micronekton” (roughly, “tiny swimmers,” as derived from German and Greek). Then come the so-called “forage fish,” diminutive species like sardines, anchovies and herrings (the ones that often end up at the supermarket, sauced in oil and packed in tin cans), along with juveniles of other fish like pollock and even the plus-size krill that inhabit the Bering Sea. Sardines and their ilk swim in great silvery schools, their mouths wide open to expedite eating the multitudes of planktonic wanderers in their path. When the forage fish are, in turn, devoured by bigger creatures, energy transfers up the food chain. (In the case of baleen whales, the chain is shorter because the whales graze directly on zooplankton.)

The mystery that long has tugged at Benoit-Bird’s imagination is how, in the unfathomable depths and limitless distances of Earth’s oceans, do seabirds, seals and other predators manage to find their most suitable prey? How do they locate the best feeding grounds, places where the energy used in pursuit of food doesn’t exceed the energy gained in consuming it?

To answer these questions Benoit-Bird, winner of a 2010 “Genius Award” from the MacArthur Foundation, extends her senses with technology. She and her collaborators use acoustics, optics, night vision, robotics, radio tagging, net sampling and computer modeling, in addition to direct observation, to locate and analyze clusters of marine organisms such as tiny, transparent, shrimplike krill that otherwise would remain hidden to human observers on the surface. Not satisfied with the limitations of off-the-shelf instruments, she and her team design or modify much of their own equipment in her lab.

Kelly Benoit-Bird uses sophisticated acoustic tools to study marine life. (Photo courtesy of Kelly Benoit-Bird)
Kelly Benoit-Bird uses sophisticated acoustic tools to study marine life. (Photo courtesy of Kelly Benoit-Bird)

One of her tools is the echosounder, a device that sends pulses of sound waves into the briny darkness. When the sound waves bump into something solid, they bounce back to the boat. This “backscatter” gives the scientists clues to what’s out there in the inscrutable sea. Even in the dark or the fog, the acoustic gear lets scientists estimate the vertical depth, horizontal breadth and density of the clusters they call “thin layers” or “patches.” These patches appear to be pivotal, although their etiology is not well understood. “We don’t fully understand the mechanisms that form the layers,” Benoit-Bird says. “It’s a mix of physics and behavior on behalf of the critters.”

However they originate, these layers are pivotal in open-ocean predation dynamics. “A central issue in ecology is the mechanisms underlying the distribution of predators in their habitat,” Benoit-Bird writes in the scholarly journal PLOS One. “Patchiness may be critical for understanding predator-prey relationships in pelagic marine systems.”

It turns out that the seal, the murre and the kittiwake, despite vast tactical differences, use the same overall, patch-related strategy to feed, Benoit-Bird and her team discovered. The mammalian and avian hunters picked patches where prey animals were tightly packed. The scientific terms are “local density” or “local concentration.” For shorthand, she calls them “hotspots.” There was nothing random about the predators’ tactics, no swimming around in hopes of an accidental catch. Rather, the Pribilof hunters were seeking aggregations or “blobs” of fish — what Benoit-Bird describes as “concentrated balls of prey.”

“They will choose a small area where a hundred fish are swimming close together over a large area where thousands of fish are all spread out. It’s not about the thousands of fish. It’s about how tightly packed they are. It’s about how many fish can you get in a single dive before you have to come up for air.”

The Cycles of Time

Benoit-Bird’s Pribilof study focused not only on three predator species, but also on three spatial dimensions — length and width (horizontal) and depth (vertical). But there’s a fourth dimension, one that’s trickier to grab onto in the churning, ever-shifting sea yet critical to understanding predator-prey dynamics. That dimension is time. Benoit-Bird argues that the “snapshots” most scientists capture at sea are only that — momentary glimpses that shift by the time you blink.

Acoustic tools and the animals they are used to study, from zooplankton to marine mammals. (Graphic by Kelly Benoit-Bird and Gareth Lawson)
Acoustic tools and the animals they are used to study, from zooplankton to marine mammals. (Graphic by Kelly Benoit-Bird and Gareth Lawson)

“We can tow a net through one spot and then do it again in the exact same spot and it comes out different,” she says. “That’s because everything is moving.”

The ocean’s perpetual motion — its currents, tides, waves and upwells, confounded by seasonal hurricanes, monsoons and typhoons — are complex enough. The plot thickens further when you add climate-related changes in ocean temperature, oxygen and acidity. This dizzying set of spatial, chemical and physical variables gets even knottier when you try to factor time into the predator-prey picture. How, for example, do the cycles of the moon and the sun affect marine organisms?

“The fourth dimension is crucial in pelagic systems,” she asserts. “That’s because even immobile organisms can be moved by currents and, unlike terrestrial habitats, there are few fixed features where animals can hide or hold fast. Space and time together may underlie the timing of outbreaks such as toxic algae, or blooms of jellyfish and other marine life. They may affect the stability of predator-prey interactions or the competition for resources.”

To begin teasing out the temporal influences, Benoit-Bird voyaged to another island archipelago, this one nearly 3,000 miles south of the Bering Sea. She started by looking at 24-hour migrations of creatures as they move up and down in the water column. Do feeding patterns shift from day to night, dawn to dusk? she wondered. For two summers, she and her team spent three weeks, day and night, counting layers of plankton in the warm seas around the Hawaiian Islands. She identified 200-plus discrete layers of phytoplankton, which shone with a sparkling, indigo fluorescence. She found almost 275 layers of zooplankton, mostly “copepods” (teardrop-shaped crustaceans whose name means “oar-feet”), which reached concentrations as high as 100,000 individuals per cubic meter of water. She also sampled layered concentrations of micronekton and studied their overlap with spinner dolphins (Stenella longirostris), which cooperatively herd patches of prey with their rapid, whirling movements and precision choreography.

“In this Hawaiian system, there is a very short period around dusk each night when almost everything interacts with everything else,” she says. “As the sun goes down and temperatures cool, the winds pick up, and everything starts to mix. Organisms that were strongly stratified horizontally start moving up and down vertically. It goes all the way from the phytoplankton to the zooplankton up through the fish and the dolphins. This 30-minute timeslot is the main time when energy is being transferred from one group to another. We think it could be as much as 90 percent of the daily energy exchange between predators and prey, all driven by the switch from day to night. It’s pretty amazing.”

Swimming the Blues

As powerful as it is, Benoit-Bird’s acoustic gear is only able to narrow down the possibilities of what’s drifting or swimming in each patch. She can get a rough idea of size and biomass, for instance. Pinpointing the exact species requires the team to manually collect samples by towing specially designed nets behind the boats. The trouble with nets is they can mangle fragile zooplankton like “pteropods” (marine snails nicknamed “sea butterflies” for the gossamer “wings” or flaps with which they swim) and juvenile squids, whose big-eyed, bell-shaped bodies are crowned by bursts of delicate tentacles.

That problem is what inspired another OSU marine researcher, Robert Cowen, to design an underwater camera system that takes exquisitely detailed photos of zooplankton without touching them. Cowen, along with colleagues at the University of Miami and the San Diego-based underwater-instrument manufacturer Bellamare, have designed a state-of-the-art system for studying zooplankton in their natural habitat, photographing them in place while they float. In this way, they can be measured, classified and counted digitally instead of manually. Of special interest to Cowen, director of the Hatfield Marine Science Center, are the medium-sized zooplankton or “meso-zooplankton,” which include “ichthyoplankton” (larval fish), whose oceanic lives remain largely a mystery because of their rarity and fragility.

“Our imaging system, towed behind a research vessel, can sample large volumes of water at very high resolution without disturbing the organisms,” he says. “This allows us to quantify their density, size and distribution and observe them in their natural position and orientation within the water column.” Specially designed “shadow-illumination” lighting lets Cowen and his team capture crystal-clear, highly contoured images of the vividly colored and outlandishly shaped creatures for later characterization.

Still another OSU marine scientist is adding new pieces to the predator-prey puzzle — this time on the opposite end of the size spectrum. While Cowen is investigating creatures barely visible to the naked eye, Ari Friedlaender of the Marine Mammal Institute is studying the feeding behavior of blue whales, the largest animals to ever live on Earth. Using their comblike plates of cartilage (“baleen”) to filter thousands of gallons of krill-rich seawater into their massive mouths, blues were long thought to be indiscriminate grazers — randomly swimming around in search of an opportunistic meal — according to Friedlaender. But by radio-tagging blues in southern California, he and his research partners have unveiled the whales’ highly sophisticated foraging habits. By waiting for the densest, highest-quality prey and then feeding in intense sessions of deep, super-consuming lunges, the blues maximize their energy intake while minimizing their energy output.

The implications for the endangered blue whale (and, by extension, other marine predators) are clear, according to Friedlaender. “The decisions these animals make are critical to their survival,” he says. “If they’re disturbed during intense, deep-water feeding, it could have consequences for their fitness, overall health and reproductive viability over time.”

Adds Benoit-Bird: “We’re still trying to learn basic ocean dynamics even as those dynamics are changing rapidly. We need to understand the drivers of predator-prey abundances in marine systems so we can gauge ecosystem resilience, manage fisheries, protect exploited species and predict and mitigate the impacts of climate change.”