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Predator and prey

Researchers hunt for answers to dynamics of Ross Sea ecosystem

There are few places left in the world where the big predators — think lions, tigers and bears (oh my) — still naturally dominate their ecosystem. That’s even true of the world’s oceans — except one.

The Ross Sea, which has been dubbed the Last Ocean, is still relatively pristine. Whales, penguins and seals gather in large numbers every austral summer — a wildlife scene that seems only possible in Pixar cartoons these days.

They feast on a host of prey, from shrimp-like krill to herring-sized silverfish. In turn, these small critters — scientifically classified as zooplankton — graze on the Southern Ocean’s version of flora such as phytoplankton and smaller zooplankton such as copepods.

This intact marine food web offers benthic ecologist Stacy Kim and her colleagues an opportunity to challenge scientific orthodoxy. Researchers have assumed that ecosystems pulse from the bottom up, that it’s the small stuff in the food web that structures the system.

“I don’t think that’s strictly correct,” explains Kim, a researcher at Moss Landing Marine Laboratories, from the lowest floor of the Alfred P. Crary Laboratory at McMurdo Station on a December day last year.

She notes that the bottom-up hypothesis is based on terrestrial ecosystems — but 70 percent of the world is covered by water. In addition, previous ecosystem studies generally began after humans had already rearranged the natural order of things.

“The ecosystems we’ve studied don’t have natural populations of top predators,” Kim says. “Of course, they’re going to have a lesser impact on the food web and ecosystem structure.”

Kim is overseeing a two-year field project to understand the food web interactions between top predators — particularly Adélie penguins, killer whales and minke whales — and the pressures they exert on specific types of prey.

Her team occupies one of several labs attached to a chilly aquarium room containing tanks of hulking Antarctic toothfish and other marine vertebrates and invertebrates. Just a few hundred meters away, down the rocky volcanic slope of Ross Island, the iced-over waters of McMurdo Sound, which opens to the Ross Sea, offer a natural laboratory to test Kim’s hypothesis.

“Out here we have this great opportunity — this Ross Sea opportunity — that is unique to look at these populations of top predators and how they’re influencing things,” Kim says enthusiastically.

In the aquarium room, engineers Eric Stackpole and Laughlin Barker are carrying what appear to be a couple of torpedoes in various states of assembly. In fact, one of the projectile-sized instruments is SCINI. The other is FATTI.

SCINI (Submersible Capable of under Ice Navigation and Imaging) has become one of Kim’s primary tools over the years. A custom-built, remotely operated vehicle (ROV), SCINI carries a couple of cameras that the marine biologist and her team have used to explore the seafloor of McMurdo Sound.

However, on this project, SCINI flies through the water column, pulling its newly built companion, FATTI, for Fluorometer and Acoustic Transducer Towable Instrument. FATTI’s acoustics help find schools of krill and fish. The fluorometer provides a type of flashlight to see phytoplankton, small plant cells, in the water.

The researchers are particularly interested in the pressure that predators put upon a couple of wee-sized marine animals: a species of krill called Euphausia crystallorophias, commonly referred to as crystal krill, and Antarctic silverfish, Pleuragramma antarcticum.

“It’s pathetic the amount of information known about crystal krill. It’s like they don’t exist,” laments David Ainley from his outpost at Cape Royds, a rocky outcrop of Ross Island that’s home to the southernmost breeding colonies of Adélie penguins in the world.

All the glory and research is devoted to crystal krill’s popular cousin, Euphausia superba, a pelagic species that is also a target of krill fisheries that supply the omega 3 supplements market. Crystal krill prefers to hug coastal waters, offering a tasty meal for predators hunting along the receding sea ice edge during summer months.

“We’re hoping to learn more about crystal krill than necessarily penguins,” adds Ainley, who is a co-principal investigator on the project, senior ecologist with ecological consulting firm H.T. Harvey and Associates in California. He and research assistant Jean Pennycook outfit Adélies with satellite tags to track their movements when they leave the colony to forage to feed their hungry, downy-feathered chicks.

Previous research with collaborators Walker Smith of the Virginia Institute of Marine Science and Vernon Asper at the University of Southern Mississippi used an autonomous glider to study the foraging behavior of Adélie penguins from another Ross Island colony at Cape Crozier. The glider also carried acoustic instruments to locate krill and fish.

While good data exist about penguin ecology, less is known about the bigger players in the food web — the whales. That’s where Robert Pitman and John Durban, with NOAA’s Southwest Fisheries Science Center, come into the project.

They are studying minke and killer whales.

“There’s been an increasing realization, not only in the Ross Sea but around the world, that killer whales are important in these food webs,” notes Durban, a population ecologist.

How important is one of many questions to be answered.

Best laid plans

Every year an icebreaker arrives at ice-covered McMurdo Sound. Its job is to break a channel through about 30 kilometers or so of sea ice to reach McMurdo Station and escort cargo and fuel vessels into the small harbor. It doesn’t take long before whales and penguins follow in its wake. In effect, the icebreaker abruptly allows access to new hunting grounds.

The research team has aimed to collect a suite of data about the targeted prey species, as well as phytoplankton and sea ice biota, such as abundance and distribution. The controlled experiment is designed to take snapshots of the food web before, during and after the icebreaker does its work. That should offer insight on how the predators affect prey dynamics.

Ainley’s group in the past has observed that the penguins are forced to make longer foraging trips and dive deeper after the whales have arrived. The biologists wonder if all these predators have chased prey to deeper depths or actually depleted their abundance, making it more difficult for penguins.

“There’s a difference between simply observing what’s going on and putting together all of the pieces so they make sense, and actually doing some sort of manipulative experiment so that you test a hypothesis,” Kim explains.

Unfortunately, in Antarctica, even the best laid plans can go awry. In this case, it wasn’t Mother Nature but ship schedules. The Russian icebreaker Vladimir Ignatyuk didn’t arrive at the ice edge until February, about a month later than normal. By that time, it was too dangerous for the scientists to work on the weakened sea ice.

Plan B: Use the ice edge as a proxy, or substitute, for the ice channel.

Kim and the rest of the SCINI team eventually explored, over the course of the austral summer, a large swath of McMurdo Sound by sending SCINI and FATTI under the sea ice through holes drilled by a gas-powered auger.

The robot and its instrument package made a series of transects away from the ice edge, where the whales and penguins congregate. SCINI and FATTI collected data in areas where the deep-diving animals could and couldn’t reach based on their physiology. The scientists timed their forays around the phytoplankton bloom, when the water turns bluish-green — a phenomenon big enough to see from space.

The fieldwork involved long days on snow machines to reach transect sites. A tent was set up for SCINI’s command center — a couple of laptops and a PlayStation game controller that is used to pilot the robot. Several hundred meters of communication and power cable were spooled and unspooled as the ROV cruises the waters looking for krill and fish.

“We’re controlling SCINI a little bit differently than if we didn’t have anything attached. It’s kind of like pulling a trailer,” notes Barker, as he pilots the instrument through the sub-freezing waters of McMurdo Sound from inside the small pop-up tent with Heather Broadbent, a postdoc with Kendra Daly at the University of South Florida.

Daly, co-principal investigator on the project, and her lab team are the krill experts. They’re also helping collect physical oceanographic data — temperature and salinity — to measure how melting ice might relate to prey distribution.  

“There’s a pretty huge melt signal,” notes Benjamin Saenz, now at the University of Colorado in Boulder. A biologist turned physical ocean modeler, Saenz explains that the changing water properties cause the water to stratify, creating layers of varying temperature and salinity. Does that affect where the krill go?

“I want to know what drives the krill to be where they are,” Saenz says, later echoing Ainley’s comments: “We don’t know much about crystal krill.”

It’s no mystery about what drives the whales to the ice edge: food. But what exactly these large animals are feasting upon is less clear.

For at least one species of killer whale, it’s almost certainly fish of some kind. The killer whales that roam the Ross Sea are primarily a species referred to as Type C, the smallest of at least three different kinds of Antarctic killer whales that Pitman, Durban and colleagues identified in a series of papers. The larger ones are called Type A and Type B.

Type C killer whales, called Ross Sea killer whales by Pitman, have been observed feeding on Antarctic toothfish, a large, long-lived fish that like many of its brethren has special adaptations that allow it to survive in the sub-freezing waters, including a sort of anti-freeze in its blood. Large toothfish also are very fat, which is why killer whales and seals are interested in eating them.

How much Antarctic toothfish account for the killer whale diet is a mystery. Pitman says the silverfish make up about 95 percent of fish biomass in the Ross Sea. Each killer whale requires about 75 kilograms of food per day. That would be a lot of silverfish or just a couple of Antarctic toothfish.

“They’re basic option is they can take a few big fish or they can take an almost unlimited quantity of the little fish,” Pitman notes. “We know they take the big fish sometimes — and that’s all we know.”

The wildcard

The question is not just academic. The whales have competition when it comes to hunting the Antarctic toothfish: humans.

A sanctioned fishery currently operates in the Ross Sea. Conservationists have lobbied to shut it down or dramatically reduce it because so little is known about the ecology of the Antarctic toothfish, which is marketed under the more benign-sounding name of Chilean sea bass.

It’s a topic that riles the soft-spoken Ainley, who argues that his long-term study in penguin population dynamics and their relation to climate effects has been subverted by the impacts the fishery has on the Ross Sea ecosystem. The Antarctic toothfish is itself a top predator, and among its prey are silverfish, also a major prey of the penguins. The fishery is removing a competitor of the Adélies, which are increasing in numbers.

“There’s not a climate signal that explains why the Adélie penguins are increasing. In fact, they should be decreasing based on our previous analyses,” Ainley says. “We’re not studying climate change anymore.”

Ainley says the extent and duration of sea ice in the Ross Sea region is increasing, which should be problematic for the Adélie penguins because it adds to the distance they must travel between breeding colonies and wintering grounds. However, he says, if more silverfish are available due to depleted Antarctic toothfish stocks, then that might offset the challenges associated with sea ice.

(The cause as to why sea ice cover is changing is still being debated, with one long-time hypothesis — blaming ozone depletion in the stratosphere — being contested by other theories and climate models in recent years.)

“The toothfish fishing is the wild card,” Pitman acknowledges. “If it turns out that the Ross Sea killer whale is relying on toothfish heavily, it’s a problem. If not, it’s still important but not as critical.”

The unavailability of the icebreaker channel the first season of the project is also important but not critical. Pitman and Durban, like their SCINI team members, made do with the sea ice edge. The duo flew repeatedly across McMurdo Sound aboard a helicopter, about 30 kilometers from the research station.

These weren’t mere whale watching trips.

Like Ainley, the NOAA researchers outfitted their subjects with satellite tags, small transmitters capable of staying on the whales over multiple months. The tags relay a whale’s location dozens of times per day, as well as record dive depths every 90 seconds.

“That can fill some data gaps pretty quickly,” Durban notes. “Every time [the whale] comes back to the surface the tag transmits its information.”

The researchers (under permit Antarctic Conservation Act permit #2009−013 and Marine Mammal Protection Act permit #14097) use a crossbow to attach the small tag onto the dorsal fin of the whale with a pair of small titanium barbs. They also collect a small tissue sample and photograph each animal.

The biopsy can give them insight into the animal’s diet. Images help the researchers identify and count the whales (they are all individually distinctive, especially from scars) to get more accurate counts of the population.

The latter information is particularly important if Pitman, Durban and their colleagues want to accurately quantify how much biomass the whales are consuming.

“The killer whales are removing as much fish as a large-scale fishery,” Durban notes. “What we’re basically trying to do is a fisheries assessment for killer whales.”

Next season, with the icebreaker slotted to hit the sea ice edge in early January, the team will return with hopes to fish the channel for additional answers about the links within the Last Ocean food web.

“The icebreaker changes things — it changes the balance,” Durban adds.

How that balance changes — who is eating whom, when and where — may become clearer in the coming years.

“I think by creating a better picture of the food web in the Ross Sea and how all the pieces are related, we will provide information that is relevant not only to ecologists but also to natural resource managers,” Kim says.  

NSF-funded research in this story: Stacy Kim, San Jose State University, Award No. 0944747; David Ainley and Grant Ballard, H.T. Harvey and Associates, Award No. 0944694; and Kendra Daly, University of South Florida, Award No. 0944511.

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Curator: Michael Lucibella, Antarctic Support Contract | NSF Official: Peter West, Division of Polar Programs