Reduced predator populations lead to algal blooms

Algal blooms are a phenomenon in which algal populations in a marine area proliferate rapidly, creating a water-column shield that blocks sunlight and oxygen. These blooms are usually attributed to rises in nitrogen levels from human agriculture and industrial runoff, which fertilize the algae. But a study in the current issue of Ecological Applications shows that overfishing of top fish predators can also lead to algal blooms. Satellite image of a large algal bloom in the Bering Sea in 1998. After reviewing a year’s worth of data on the Baltic Sea, the authors found that areas with high algal concentrations also had large populations of small fish and small populations of large fish. Specifically, predatory perch and pike shortages resulted in a 50 percent chance of that area experiencing an algal bloom, compared to only a 10 percent chance in normal areas. Experimental studies in which the authors excluded top predators from marine areas corroborated the findings. Lead author Britas Klemens Eriksson of the University of Groningen in the Netherlands says this pattern suggests that the problem is a trophic one: when top predators are scarce, their smaller fish prey become more abundant and eat more invertebrates. These invertebrates feed primarily on algae, so when their numbers are reduced, algae can grow freely. Eriksson says that the key to controlling algal blooms may not be through simply controlling nitrogen, but also by controlling fishing. Said Eriksson in the Nature article: If we want to manage algal blooms effectively, we need to start by taking an ecosystem perspective … we have to restore depleted fish communities. Read the paper in Ecological Applications here (abstract; full-text by subscription), and rest of the Nature article here. Eriksson, B., Ljunggren, L., Sandström, A., Johansson, G., Mattila, J., Rubach, A., Råberg, S., & Snickars, M. (2009). Declines in predatory fish promote bloom-forming macroalgae Ecological Applications, 19 (8), 1975-1988 DOI:...

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Leatherbacks turn up by the tens of thousands

The largest population of leatherback sea turtles in the world has been identified off the coast of Gabon, Africa, and is estimated at somewhere between 15,700 and 41,400 female turtles. This seems to be a big bounceback for the endangered turtles, which are the largest living members of the sea turtle superfamily. This rough estimate was compiled during three nesting seasons between 2002 and 2007, using video to capture footage along Gabon’s 372-mile coastline, in addition to terrestrial monitoring. The study, which appears in Biological Conservation, also identifies the key sites for leatherback nesting, which can be used in assessing and developing management strategies. Beginning about 25 years ago, leatherback populations in the Indo-Pacific oceans were reduced to less than 10 percent of their former size. Although the International Union for Conservation of Nature (IUCN) lists leatherback turtles as critically endangered on a global scale, adequate population assessments across much of the Atlantic, especially along the African coast, are in short supply. In a press release, lead author Matthew Witt of the University of Exeter said: “We knew that Gabon was an important nesting site for leatherback turtles but until now had little idea of the size of the population or its global ranking. We are now focusing our efforts on working with local agencies to coordinate conservation efforts to ensure this population is protected against the threats from illegal fisheries, nest poaching, pollution and habitat disturbance, and climate change.” The study also showed that about 79 percent of nesting in Gabon occurs within national parks and other protected areas. Good news for the turtles, as these areas will be much easier to manage than privately owned lands. Photo courtesy Matthew Witt. Witt, M., Baert, B., Broderick, A., Formia, A., Fretey, J., Gibudi, A., Moussounda, C., Mounguengui Mounguengui, G., Ngouessono, S., & Parnell, R. (2009). Aerial surveying of the world’s largest leatherback turtle rookery: A more effective methodology for large-scale monitoring Biological Conservation DOI:...

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Blue whales picking up where they left off?

New movement patterns may be a sign of good news for blue whales. Blue whales have begun moving around the ocean in ways that strongly resemble their historical patterns before the advent of the whaling trade. A century ago, about 300,000 blue whales existed. But in the early 1900s, humans hunted and killed 99.9 percent of them. The population decimation made them disappear from northern waters. A new paper published online in the journal Marine Mammal Science has shown, however, that 15 individual whales have been spotted both in the waters of southern California and the Pacific Northwest since 1997 — some even as far north as the Gulf of Alaska. But scientists can’t yet be sure if this is in fact a return to historical migration patterns. From the New York Times Dot Earth blog by Andy Revkin: “It’s not yet possible to determine whether the whales are resuming long-abandoned migratory feeding journeys or shifting their patterns to match cyclical shifts in the Pacific Ocean that affect  krill, their dietary mainstay, Jay Barlow, a federal whale biologist and author of the paper, said in a phone interview.” Calambokidis, J., Barlow, J., Ford, J., Chandler, T., & Douglas, A. (2009). Insights into the population structure of blue whales in the Eastern North Pacific from recent sightings and photographic identification Marine Mammal Science DOI:...

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Born at the right time

It’s nice to have some good conservation news every once in awhile, even with caveats. North Atlantic right whales are one of the most endangered species on Earth. These mammals were dubbed by 18th-century whalers to be the “right” whales to catch because they’re huge (up to 70 tons and 55 feet long), stay close to shore, move slowly and have large amounts of baleen and blubber, the latter of which yielded much oil and caused the whales to float when killed. The gentle giants were hunted to extinction around Europe and by 1900 only about 100 known whales remained around North America. Now the whales’ numbers have tripled, and currently 325 whales are known to NOAA scientists, each complete with its own nickname. A concerted effort of international laws and changes in seafaring practices has led to this comeback.  It’s been illegal to kill the whales since 1935, changes in shipping lanes and regulations on ship speeds have reduced collisions and U.S. gear restrictions have limited the number of whales getting caught in fishing lines. Researchers warn, however, that the whale is far from saved. Six whales have turned up entangled in fishing line this year, and estimated 80 to 85 percent of right whales bear a scar from a previous entanglement.  Efforts to preserve the whales can cost hundreds of thousands of dollars each year. But so far, no whales have died from entanglements this year, and the researchers remain optimistic. Read the excellent feature article in this week’s New York Times that chronicles, as they call it, The Fall and Rise of the Right Whale. The article is complete with breathtaking video footage. Photo courtesy Florida Fish and Wildlife Conservation Commission/National Oceanic and Atmospheric...

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