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February 24, 2010, Vol. 16, No. 2
Turtle banner image: Claire Fackler, NOAA National Marine Sanctuaries

In This Issue

Science in the News

Overfishing Causing Smaller Fish to Increase as Predatory Fish Decline

bluefin tuna
Recent research indicates that predatory fish such as Pacific bluefin tuna have declined in number by two-thirds during the last 100 years as a result of commercial fishing. Gerick Bergsma/Marine Photobank

Predatory fish such as cod, tuna and groupers have declined by two-thirds during the past 100 years, while small forage fish such as sardine, anchovy and capelin have more than doubled during the same period, according to University of British Columbia researchers.

Speaking on February 18 at the annual meeting of the American Association for the Advancement of Science (AAAS), Villy Christensen of UBC’s Fisheries Centre described a study in which a team of scientists had analyzed data from over 200 ecosystem models, allowing them to examine more than 68,000 estimates of fish biomass from 1880 to 2007. The study’s goal was to assess the likelihood of a scenario first posited in the journal Science in 2006, that by 2050 the impact of overfishing would be so great that commercial fisheries would collapse completely.

Christensen and colleagues concluded that the future would not be quite that bleak. Marine ecosystems will, however, look decidedly different, they found. “Overfishing has absolutely had a ‘when cats are away, the mice will play’ effect on our oceans,” Christensen said, in a UBC press release to announce the study. “By removing the large, predatory species from the ocean, small forage fish have been left to thrive.”

While the doubling of forage fish amounts to more overall fish production, Christensen cautioned that the lower trophic-level food web is more vulnerable to environmental fluctuations.“Currently, forage fish are turned into fishmeal and fish oil and used as feeds for the aquaculture industry, which is in turn becoming increasingly reliant on this feed source,” Christensen continued. “If the fishing-down-the-food-web trend continues, our oceans may one day become a ‘farm’ to produce feeds for the aquaculture industry. Goodbye, wild ocean!”

Christensen and his colleagues also found that approximately 55 percent of the declines in predatory fish occurred during the past 40 years, with the sharpest decline between 1970 and 1990. However, even as these fish populations have plummeted, fishing capacity has continued to increase, by 54 percent in total between 1950 and 2010, “with no indication of a decrease in recent years.”

Speaking on the same panel at the AAAS meeting, Jacqueline Alder of the United Nations Environment Programme urged that fishing capacity be reduced substantially. “If we can do this immediately, we will see a decline in fish catches. However, that will give an opportunity for the fish stocks to rebuild and expand their populations,” she said.

Source: Christensen, V., et al. 2011. Fish biomass in the world ocean—a century of decline. Manuscript for presentation AAAS 2011, session “2050: Will There Be Fish in the Ocean?” Washington, D.C., February 18, 2011.

Contact: Villy Christensen, UBC Fisheries Centre. E-mail: v.christensen@fisheries.ubc.ca

 

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Science Briefs

Canada’s Polar Bears Feeling Impact of Shrinking Sea Ice

polar bear cub pearing from ice cave
Polar bears in Canada’s western Hudson Bay are likely to give birth to fewer, smaller litters as their sea ice habitat retreats as a result of climate change. Geoff York

Researchers in Alaska have tracked a female polar bear swimming for 232 consecutive hours, during which time she covered 687 kilometers (427 miles) until she finally reached the sea ice of the Arctic Ocean. By the end of the ordeal, the bear had lost 22 percent of her body mass and her yearling cub had apparently died. The researchers note that their finding illustrates the immense cost to polar bears of having to swim for long periods, something many more bears may be forced to do so as sea ice retreats from the shore and diminishes in extent as a result of climate change.


Writing in the journal Polar Biology, George Durner of the United States Geological Survey and colleagues describe capturing an adult female bear and her cub on Alaska's Beaufort Sea coast in late August 2008. Around the adult's neck, the researchers placed a radio collar with a GPS unit.

The bear weighed 226 kilograms (498 pounds), and the yearling weighed 159 kilograms (350 pounds). When the scientists found the bear again, two months later, she weighed just 177 kilograms (390 pounds) and was not lactating; the yearling was nowhere to be seen.

By analyzing the recovered data and overlaying the bear's movements with ice charts, Durner and colleagues deduced that on 25 August (two days after capture and release) she entered the water off the Beaufort Sea coast and swam north for nine days, before finally reaching sea ice. She then spent three days on the ice, another day swimming, and an additional 49 days on the ice before being found again.

They write that in late August, polar bears in the Beaufort Sea region instinctively head north after spending part of the summer ashore, expecting to find sea ice on which they can spend the fall, winter and spring hunting seals. Although the ice edge at that time of year is normally close to shore, it has in recent years been receding as overall summer sea ice extent in the Arctic Ocean region has declined—by, on average, 11 percent per decade since 1979. A 2006 study reported increased sightings of drowned polar bears in open water off Alaska, presumably as the result of having to swim ever-greater distances to reach the ice edge.

A separate study, in the journal Nature Communications, has shown that as sea ice has declined in Canada’s western Hudson Bay, at the southern end of polar bears’ range, the ability of pregnant females to eat enough to gain sufficient energy to give birth to cubs is also diminishing.

Peter Molnar of the University of Alberta and colleagues write that every summer, the sea ice in Hudson Bay melts completely, obliging the bears to come ashore, where they await the ice's return." However, they note, the ice is breaking up earlier in the summer, and polar bears are forced ashore one to two weeks earlier than was the case in the early 1990s. Given predicted temperature increases as a result of greenhouse gas emissions, by mid-century the average date on which bears will come ashore is estimated to be July 1—one month earlier than 20 years ago.

The authors note that, in the early 1990s, 28 percent of pregnant bears in western Hudson Bay failed to produce any cubs. the researchers calculated that, forced to come ashore a month earlier, between 40 and 73 percent of the bears will be unable to raise a litter; furthermore, the average size of the litters born will also decrease. Should sea ice break up fully two months earlier, they estimate that at least 55 percent, and perhaps up to 100 percent, of pregnant females will be unable to give birth.

Not surprisingly, they note that “the litter size predictions provided in this study serve as another indicator that the western Hudson Bay population will probably not remain viable under predicted climatic conditions.”

Sources: Durner, G., et al. 2011. Consequences of long-distance swimming and travel over deep-water pack ice for a female polar bear during a year of extreme sea ice retreat. Polar Biology DOI: 10.1007/s00300-010-0953-2; Molnar, P., et al. 2011. Predicting climate change impacts on polar bear litter size. Nature Communications doi:10.1038/ncomms1183.

Contact: George Durner, United States Geological Survey. E-mail: gdurner@usgs.gov; Peter Molnar, University of Alberta. E-mail: pmolnar@ualberta.ca

Sea Turtle Disease Mystery Solved?

green turtle with tumor-like growths
This green turtle is suffering from fibropapilloma growths on its flippers and head. Researchers in Hawaii believe they have pinpointed the reason why the prevalence of this disease increased suddenly in green turtles in the 1980s and continues at a relatively high level. Lacey Price/Marine Photobank

In the 1980s, researchers were baffled by a sudden spike in fibropapilloma disease in green turtles off Florida and Hawaii. The disease, which had first been identified in 1938 but rarely seen since, manifested itself in tumor-like growths, often around a turtle’s head and neck. Although the growths themselves weren’t fatal, the area in which they grew impaired a turtle’s ability to see and feed, with frequently fatal consequences.

Thirty years later, a paper in the journal PLoS One may have helped provide an answer.

A team of three researchers led by Kyle van Houten of the National Oceanic and Atmospheric Administration (NOAA) examined records of almost 4,000 green turtle strandings along the coast of Hawaii during a span of 28 years. They found that fibropapilloma disease occurred most frequently in juveniles and young adults, the period of their lives during which green turtles return to coastal environments after spending their early years in pelagic waters. Van Houtan and colleagues found that some specific coastal areas had high incidence of the disease, while others had none. Those areas where the disease was found with greater frequency were the ones where agricultural development had resulted in high rates of runoff of nutrients, particularly nitrogen.

That increase in nitrogen in the water stimulated the growth of non-native species of algae that had begun to gain a toehold in the 1950s and, by the 1970s, had so successfully out-competed native species that in some places they now compose more than 90 percent of green turtles' diet. The key to the sudden success of these algae species was that they were able to absorb and store excess nitrogen in the form of arginine, an amino acid that virological studies have shown to be essential to the growth of herpes viruses. Herpes virus, earlier studies had shown, was the cause of fibropapilloma disease.

Van Houten and colleagues therefore conclude that “environmental factors are significant” in promoting fibropapillomas.

Source: Van Houtan, K., et al. 2010. Land use, macroalgae, and a tumor-forming disease in marine turtles. PLoS One. doi: 10.1371/journal.pone.0012900.

Contact: Kyle van Houtan, National Oceanic and Atmospheric Administration. E-mail: kyle.vanhoutan@gmail.com


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Banner image credit: Claire Fackler, NOAA National Marine Sanctuaries