New Studies Highlight Concern Over Rising
Jellyfish populations appear to be increasing along the West Coast and in the Bering Sea and scientists studying the phenomenon are concerned because jellyfish may feed on the same plankton species targeted by herring, sardines and anchovies, juveniles salmon and other fishes.
Compounding the situation, the scientists say, is that there are few predators for adult jellyfish.
"A few birds and fish will eat the jellies in their larval or juvenile stages," said Richard D. Brodeur, a NOAA biologist and adjunct professor in the College of Oceanic and Atmospheric Sciences at Oregon State University. "But once the medusae reach a certain size, not much eats them."
Newly published studies by Brodeur, OSU oceanographer Lorenzo Ciannelli and others are looking at the link between climate change and jellyfish populations and they have found this relationship is complex. The prevailing school of thought has been that as ocean waters warm, jellyfish populations will increase. But they have discovered that food sources, reproduction dynamics and ocean currents all play a role in jellyfish populations.
In a paper just published in Progress in Oceanography, the scientists describe a steep increase in jellyfish populations in the Bering Sea through the 1990s, peaking in the summer of 2000. But during the years of 2001 through 2005, when scientists recorded some of the warmest temperatures ever in the Bering Sea, jellyfish populations declined.
"They were still well ahead of their historic averages for that region," said Ciannelli, an assistant professor in OSU's College of Oceanic and Atmospheric Sciences. "But clearly jellyfish populations are not merely a function of water temperature."
One key to learning more about jellyfish expansion has been Ciannelli's work looking into the organisms' complex life cycle. Adult males release their sperm into the water column and fertilize the eggs that female adults have released. From each fertilized egg, a larva is produced that attaches itself to a rock or some other solid surface and produces a polyp. These polyps reproduce asexually and eventually the young medusae detach themselves and begin the life cycle anew.
The researchers' preliminary findings suggest that warmer ocean waters may enhance the stage where polyps transform into colonies, but that hypothesis is based on lab work, not field research. The reason, Ciannelli says, is that polyps are notoriously difficult to locate because of their small size.
"We think that higher temperatures lead to a higher metabolic rate and faster division of cells," he said. "It accelerates the whole system. But finding polyps in the Bering Sea is like trying to do research on the dark side of the moon."
Ciannelli and his colleagues are funded by the National Science Foundation to better understand how these polyps are distributed. One hypothesis is that there is a single unique source that produces the small jellyfish in the Bering Sea and their expansion is a product of currents. An alternative theory is that the jellyfish are using pockets of warm water to establish new colonies, which would be consistent with global warming scenarios, he said.
"What we're trying to figure out is where the energy of the food web is going," Ciannelli pointed out. "If it is going to the jellyfish, which are eating the plankton, it creates an overall sink because they have few predators. It is diverting the energy of the ocean from the pelagic to the benthic system."
Scientists have begun looking more closely at food sources for jellyfish off the West Coast of the United States and their findings are surprising. In a paper published in the April 2008 issue of the Marine Ecology Progress Series, a team of scientists including Brodeur quantified diet and predation rates for large jellyfish from an upwelling region in the northern California Current. They found that in an area north of Cape Blanco, Ore., abundant populations of jellyfish ate an average of one-third of all the euphausiid - a type of zooplankton -- eggs available each day. Consumption of other taxa reached 10 to 12 percent of the standing stocks.
On the other hand, copepods, important components of the marine food web, were consumed at relatively low levels -- less than 1 percent a day. Lead author on that study was Cynthia L. Suchman, who conducted her research out of OSU's Hatfield Marine Science Center Hatfield Marine Science Center in Newport, Ore., where Brodeur works.
Few scientists are conducting long-term jellyfish studies and the authors suggest that zooplankton studies and predation impacts by jellyfish should be incorporated into long-term studies and ecosystem models. "Unfortunately," Brodeur said, "there hasn't been a great deal of funding for jellyfish studies, so we don't know as much as we should about their impact."
Trawl surveys by Brodeur and his colleagues found that the spatial overlap between jellyfish and most pelagic fishes, including salmon, was relatively small. But in a forthcoming article in Marine Biology, the researchers point out that the overlap with "planktivorous" fishes that consume copepods and euphausiid eggs -- including Pacific sardines, the northern anchovy, Pacific saury, and Pacific herring -- was considerable. These prey species also are critical to the diets of salmon and other species in the ocean.
"We've been collecting data now for about nine years and it appears, at least on a preliminary basis, that when cold water regimes are prevalent, jellyfish numbers increase," Brodeur said. "During the warmer years, when food sources are scarcer, there may be fewer jellyfish, but they grow quickly -- whether because of elevated metabolic rates or less competition, we don't know."
This summer Brodeur will be involved in a series of cruises off the Oregon coast to sample jellyfish populations and see what effect this year's cold-water La Nina phenomenon may have had.
"It won't be a good sign for the ecosystem if we get a lot of jellies out there," he said.
Their research has been supported by the National Science Foundation, NOAA and the National Marine Fisheries Service
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