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Deep-Sea Fish Populations Boom Over the Last 15 Years, New Scripps Study Shows Scripps Institution of Oceanography April 3, 2006 Scientists make progress toward understanding mysteries surrounding animals that live in the dark recesses of the oceans The largest habitats on Earth are located in the vast, dark plains at the bottom of the ocean. Yet because of their remoteness, many aspects of this mostly unexplored world remain mysterious. New research led by Scripps Institution of Oceanography at the University of California, San Diego, has produced a rare insight into animal populations in the deep sea. Bailey says the study is a unique glimpse into fish populations undisturbed by human influence. "Scientists have known for years that shallow-water tropical coral reefs are threatened by both warming oceans and chemical changes in seawater caused by the build-up of CO 2 in the atmosphere. Above-average seawater temperatures have caused coral bleaching events throughout the world, and the calcification rates of corals exposed to more acidic conditions in laboratory experiments have shown worrisome declines," says Guinotte. "But now we suspect that this increase in CO 2 will have a detrimental effect on cold-water corals as well. Cold-water reefs are at greater risk than shallow-water reefs because colder waters are naturally less hospitable for coral growth." "This is a rare study of a large marine fish population that doesn't get commercially fished," said Bailey. "Other fish populations have their abundances, body sizes and life histories altered by fisheries activities, so our study probably gives us some information about how fish communities work when they are not driven by human exploitation." The Ecology study follows research published in 2004 by Ruhl and Smith that showed that significant changes in the deep-sea environment were likely driven by changes at the surface of the ocean by El Niño and La Niña events (See "Scripps Researchers Document Significant Changes in the Deep Sea" at: "http://scrippsnews.ucsd.edu/article_detail.cfm?article_num=640). Such oceanographic events, along with longer-term shifting called the Pacific Decadal Oscillation, can bring more nutrients to surface waters. While animals near the surface can rapidly benefit, it can be months to years later for changes to extend to the ocean bottom, leading to a proliferation of bottom-dwelling invertebrate animals that make up some part of the food supply of deep-sea fishes.
Grenadiers eat a range of foods, from the dead bodies of fish and whales to invertebrates such as worms and crustaceans. The most commonly observed animals on the seafloor include sea cucumbers, sea urchins and brittle stars, and these appeared to form part of the grenadiers' diet. The researchers used the abundances of these animals as an indicator of food supply to the fish. Large changes in the abundances of these animals were followed by changes in the numbers of fish, with both groups increasing in number over the 15-year study. The researchers say their results indicate that animals in the deep sea live in an environment in which food supply drives population levels, called a "bottom-up control," rather than a "top-down control" situation in which predator pressure controls prey abundances. "The predominant trend had been that people thought that fish have a powerful effect on their environment, and they drive the changes in everything else," said Bailey, a postdoctoral researcher at Scripps and lead author of the study. "What we've seen is the reverse, that fish are responding to a change in their habitat. We think that a lot of fish communities are fundamentally changed by fishing. Our study is really nice in that we are working on populations that have never been fished, so their population dynamics can be seen being driven by natural processes." Comparing these observations to those for shallow water, the researchers speculate that deep-ocean and shallow-water fish communities' work differently. A possible reason is that the deep ocean is dependent for its food on material falling from the communities nearer the sea surface; this food supply is smaller and less predictable than that available to most shallow-water fish. The effects of this difference on the dynamics of fish communities are not known, and are being explored using mathematical models as the investigators move forward with this project.
The research was supported by the National Science Foundation, the University of California, Scripps Institution of Oceanography and a Marie Curie Outgoing International Fellowship (European Union). About Scripps Institution of Oceanography Scripps Institution of Oceanography, at the University of California, San Diego, is one of the oldest, largest, and most important centers for global science research and graduate training in the world. The National Research Council has ranked Scripps first in faculty quality among oceanography programs nationwide. The scientific scope of the institution has grown since its founding in 1903 to include biological, physical, chemical, geological, geophysical, and atmospheric studies of the earth as a system. Hundreds of research programs covering a wide range of scientific areas are under way today in 65 countries. The institution has a staff of about 1,300, and annual expenditures of approximately $140 million from federal, state, and private sources. Scripps operates one of the largest U.S. academic fleets with four oceanographic research ships and one research platform for worldwide exploration. This is a modified news release from the Scripps Institution of Oceanography.
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