Changes in Ross Sea environment, fishery cause demographic shift in species

Photo Credit: Nate Biletnikoff/Antarctic Photo Library

Adélie penguins float on an iceberg near Cape Crozier on Ross Island. Cape Crozier is home to one of the largest colonies of Adélies in the world. Scientists hypothesize its growth may, in part, be to shifts in the number of marine species in the Ross Sea due to pressures caused by a fishery that captures commercially available Chilean sea bass.

Population pressures

By Peter Rejcek, Antarctic Sun Editor

Posted February 18, 2011

 

The penguin colony at Cape Royds on Ross Island in Antarctica is the southernmost breeding ground for any penguin in the world — seen by humans as an extreme place to raise chicks.

Still, the colony had grown to about 4,200 breeding pairs before 2001, one of the highest populations for that location in historic records dating back to 1907, according to David Ainley , senior ecologist at a SF Bay Area ecological consulting firm, H.T. Harvey and Associates . Ainley is the principal investigator for a long-term project trying to understand factors behind the population dynamics and trends at Royds and other colonies in the Ross Sea region. 

But then giant icebergs calved off the Ross Ice Shelf in 2000, locking in hundreds of additional square kilometers of sea ice beginning in 2001. That meant a long trek for the Adélies from Cape Royds to the open ocean to get food for their chicks.

The population crashed. In 2006, the dam of ice broke and open water appeared again farther south. Two years later, when The Antarctic Sun checked in with Ainley, the Royds colony was just “treading water,” though enjoying good reproductive success. He thought the young adults would start returning to the colony in the foreseeable future.

And, indeed, for the second straight year, there was a large influx of young birds (three to four years old) at the end of the breeding season. Ainley said that while last year’s “invasion” of young adults did contribute to the breeding population this year, the colony was further diluted in 2010 by the late arrival of former breeders.
That left the breeding population at Royds at about 1,400 pairs, its lowest total since 1970, when it started to recover from uncontrolled tourism in the 1960s. Since then, the location of the colony has been an Antarctic Specially Protected Area , a designation under the Antarctic Treaty system that controls access to the site.

One factor in the decline may be the lack of younger birds returning to the colony to try breeding, according to Ainley. Young adults will often set up nests around the exterior of the colony as they “practice” breeding skills.

Photo Credit: Melanie Connor/Antarctic Photo Library

Scientist Art DeVries weighs an Antarctic toothfish in November 2002, four years before the Ross Sea fishery began.

Photo Credit: Donald LeRoi/Antarctic Photo Library

A scientist observes killer whales in the Ross Sea. Researchers have reported that occurrence frequency of orcas has dropped since a fishery moved into the region.

Those nests serve as a sort of shield for the successful breeders against predatorial skuas, a clever species of seabird that uses all manner of trickery to steal chicks and eggs. A favored ploy is for the skuas to operate in pairs: One pulls an adult’s tail, while its partner pulls an egg out of the nest while the penguin is distracted.

“The skuas are just having a field day. They’re like people: They don’t manage their resources very well; they don’t believe in rainy days,” Ainley said. “There’s actually large areas of Royds now that are totally vacant of penguin nests where there were nests before the iceberg.”

Fishy business

Meanwhile, other colonies in the region have grown tremendously, buoyed to a limited extent by penguins from Royds that abandoned their colony in the tough iceberg years.

Cape Crozier on Ross Island now boasts an estimated 230,000 breeding pairs, which could place it at the top of the list for the largest Adélie colony in the world. That’s up by nearly 50,000 breeding pairs in the last decade before the icebergs moved in. On nearby Beaufort Island, the colony has expanded from 40,000 to 55,000 breeding pairs.
But immigration from Royds cannot explain the rapid expansion of these other colonies, Ainley said.

Instead, he suspects that the Adélie populations are skyrocketing because they face less competition from another predator in the Ross Sea food web — the Antarctic toothfish (Dissostichus mawsoni), known to seafood consumers as Chilean sea bass. Both prey extensively on the Antarctic silverfish (Pleuragramma antarcticum) in waters over the Ross Sea continental shelf.

Ainley believes a toothfish fishery that operates in the Ross Sea — authorized by the Commission for the Conservation of Antarctic Marine Living Resources (CCAMLR) , which oversees fishing in the Southern Ocean — may be taking too big of a bite out of the D. mawsoni population.

“Just as when thousands of food-competing Antarctic minke whales were removed from the wintering area of Ross Sea penguins during the 1970s, Adélies are exhibiting a spurt of colony growth not easily explained by climate change,” Ainley said.

He and others have argued in the scientific literature that too little is known about the life history of the toothfish, a late-maturing, slow-growing, long-lived species that can grow up two meters long. He said it was unwise to allow the fishery to operate without learning more about what the limits may be to its footprint.
The fishery began in the 1996-97 austral summer. In less than a decade, scientists who had been successfully capturing and releasing Antarctic toothfish in McMurdo Sound for research since the 1970s could no longer find any specimens.

Concurrently, Ross Sea killer whales, which prey on toothfish, have decreased in occurrence frequency, according to a paper in 2009 in the journal Aquatic Mammals by Ainley, Grant Ballard and Silvia Olmastroni. A staff scientist at PRBO Conservation Science , Ballard is a co-principal investigator on the Ross Sea penguin population dynamics study with Ainley and Katie Dugger at Oregon State University  (and collaborators Phil Lyver and Melanie Massaro of Landcare Research New Zealand).

Ainley noted that silverfish-eating emperor penguins (Aptenodytes forsteri) have also increased their presence in McMurdo Sound. “[It’s] the only part of the Ross Sea where these sorts of systematic observations are possible owing to the logistics available from the U.S. Antarctic Program ,” Ainley said.
Is the Ross Sea fishery to blame for the apparent shift in populations of whales, penguins and Antarctic toothfish?

“You can’t discount it, even though there are a lot of people who want to,” said Ainley, who is concerned that the fishery may skew data collected by researchers such as himself studying the effects of climate change on the marine ecosystem.

“You’re either studying climate change or you’re studying fish depletion, so what are you going to study?” he asked.

NSF-funded research in this story: David Ainley, H.T. Harvey and Associates, Award No. 0944411 ; Grant Ballard, PRBO, Award No. 0944141 ; and Katie Dugger, Oregon State University, Award No. 0944358 . 

Source