5 Easy Pieces: The Impact of Fisheries on Marine Ecosystems (The State of the Worlds Oceans)

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One study even predicts that foraminifera from tropical areas will be extinct by the end of the century. The shells of pteropods are already dissolving in the Southern Ocean , where more acidic water from the deep sea rises to the surface, hastening the effects of acidification caused by human-derived carbon dioxide. Like corals, these sea snails are particularly susceptible because their shells are made of aragonite, a delicate form of calcium carbonate that is 50 percent more soluble in seawater.


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One big unknown is whether acidification will affect jellyfish populations. In this case, the fear is that they will survive unharmed. Jellyfish compete with fish and other predators for food—mainly smaller zooplankton—and they also eat young fish themselves. Plants and many algae may thrive under acidic conditions.

These organisms make their energy from combining sunlight and carbon dioxide—so more carbon dioxide in the water doesn't hurt them, but helps. Seagrasses form shallow-water ecosystems along coasts that serve as nurseries for many larger fish, and can be home to thousands of different organisms.

Under more acidic lab conditions, they were able to reproduce better, grow taller, and grow deeper roots—all good things. However, they are in decline for a number of other reasons—especially pollution flowing into coastal seawater—and it's unlikely that this boost from acidification will compensate entirely for losses caused by these other stresses.

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Some species of algae grow better under more acidic conditions with the boost in carbon dioxide. But coralline algae , which build calcium carbonate skeletons and help cement coral reefs, do not fare so well. Most coralline algae species build shells from the high-magnesium calcite form of calcium carbonate, which is more soluble than the aragonite or regular calcite forms.

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One study found that, in acidifying conditions, coralline algae covered 92 percent less area, making space for other types of non-calcifying algae, which can smother and damage coral reefs. This is doubly bad because many coral larvae prefer to settle onto coralline algae when they are ready to leave the plankton stage and start life on a coral reef.


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  4. One major group of phytoplankton single celled algae that float and grow in surface waters , the coccolithophores , grows shells. Early studies found that, like other shelled animals, their shells weakened, making them susceptible to damage. But a longer-term study let a common coccolithophore Emiliania huxleyi reproduce for generations, taking about 12 full months, in the warmer and more acidic conditions expected to become reality in years. The population was able to adapt, growing strong shells. It could be that they just needed more time to adapt, or that adaptation varies species by species or even population by population.

    While fish don't have shells, they will still feel the effects of acidification. Because the surrounding water has a lower pH, a fish's cells often come into balance with the seawater by taking in carbonic acid. This changes the pH of the fish's blood, a condition called acidosis. Although the fish is then in harmony with its environment, many of the chemical reactions that take place in its body can be altered.

    Just a small change in pH can make a huge difference in survival. In humans, for instance, a drop in blood pH of 0. Likewise, a fish is also sensitive to pH and has to put its body into overdrive to bring its chemistry back to normal. To do so, it will burn extra energy to excrete the excess acid out of its blood through its gills, kidneys and intestines.

    It might not seem like this would use a lot of energy, but even a slight increase reduces the energy a fish has to take care of other tasks, such as digesting food, swimming rapidly to escape predators or catch food, and reproducing.

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    It can also slow fishes growth. Even slightly more acidic water may also affects fishes' minds. While clownfish can normally hear and avoid noisy predators, in more acidic water, they do not flee threatening noise. Clownfish also stray farther from home and have trouble "smelling" their way back. This may happen because acidification, which changes the pH of a fish's body and brain, could alter how the brain processes information. Additionally, cobia a kind of popular game fish grow larger otoliths —small ear bones that affect hearing and balance—in more acidic water, which could affect their ability to navigate and avoid prey.

    While there is still a lot to learn, these findings suggest that we may see unpredictable changes in animal behavior under acidification. The ability to adapt to higher acidity will vary from fish species to fish species, and what qualities will help or hurt a given fish species is unknown. A shift in dominant fish species could have major impacts on the food web and on human fisheries. But to predict the future—what the Earth might look like at the end of the century—geologists have to look back another 20 million years.

    The main difference is that, today, CO 2 levels are rising at an unprecedented rate— even faster than during the Paleocene-Eocene Thermal Maximum. Researchers will often place organisms in tanks of water with different pH levels to see how they fare and whether they adapt to the conditions. They also look at different life stages of the same species because sometimes an adult will easily adapt, but young larvae will not—or vice versa. Studying the effects of acidification with other stressors such as warming and pollution, is also important, since acidification is not the only way that humans are changing the oceans.

    In the wild, however, those algae, plants, and animals are not living in isolation: So some researchers have looked at the effects of acidification on the interactions between species in the lab, often between prey and predator. Results can be complex. In more acidic seawater, a snail called the common periwinkle Littorina littorea builds a weaker shell and avoids crab predators—but in the process, may also spend less time looking for food. Boring sponges drill into coral skeletons and scallop shells more quickly. And the late-stage larvae of black-finned clownfish lose their ability to smell the difference between predators and non-predators, even becoming attracted to predators.

    For example, the deepwater coral Lophelia pertusa shows a significant decline in its ability to maintain its calcium-carbonate skeleton during the first week of exposure to decreased pH. But after six months in acidified seawater, the coral had adjusted to the new conditions and returned to a normal growth rate. There are places scattered throughout the ocean where cool CO 2 -rich water bubbles from volcanic vents, lowering the pH in surrounding waters. Scientists study these unusual communities for clues to what an acidified ocean will look like.

    Researchers working off the Italian coast compared the ability of 79 species of bottom-dwelling invertebrates to settle in areas at different distances from CO 2 vents. For most species, including worms, mollusks, and crustaceans, the closer to the vent and the more acidic the water , the fewer the number of individuals that were able to colonize or survive.

    Smithsonian Ocean

    Algae and animals that need abundant calcium-carbonate, like reef-building corals, snails, barnacles, sea urchins, and coralline algae, were absent or much less abundant in acidified water, which were dominated by dense stands of sea grass and brown algae. Only one species, the polychaete worm Syllis prolifers , was more abundant in lower pH water. The effects of carbon dioxide seeps on a coral reef in Papua New Guinea were also dramatic, with large boulder corals replacing complex branching forms and, in some places, with sand, rubble and algae beds replacing corals entirely.

    One challenge of studying acidification in the lab is that you can only really look at a couple species at a time. To study whole ecosystems—including the many other environmental effects beyond acidification, including warming, pollution, and overfishing—scientists need to do it in the field. Scientists from five European countries built ten mesocosms—essentially giant test tubes feet deep that hold almost 15, gallons of water—and placed them in the Swedish Gullmar Fjord. After letting plankton and other tiny organisms drift or swim in, the researchers sealed the test tubes and decreased the pH to 7.

    Now they are waiting to see how the organisms will react , and whether they're able to adapt. If this experiment, one of the first of its kind, is successful, it can be repeated in different ocean areas around the world. If the amount of carbon dioxide in the atmosphere stabilizes, eventually buffering or neutralizing will occur and pH will return to normal. This is why there are periods in the past with much higher levels of carbon dioxide but no evidence of ocean acidification: But this time, pH is dropping too quickly. Buffering will take thousands of years, which is way too long a period of time for the ocean organisms affected now and in the near future.

    So far, the signs of acidification visible to humans are few. But they will only increase as more carbon dioxide dissolves into seawater over time. What can we do to stop it? In , carbon dioxide in the atmosphere passed parts per million ppm —higher than at any time in the last one million years and maybe even 25 million years. The "safe" level of carbon dioxide is around ppm, a milestone we passed in Without ocean absorption, atmospheric carbon dioxide would be even higher—closer to ppm. The most realistic way to lower this number—or to keep it from getting astronomically higher—would be to reduce our carbon emissions by burning less fossil fuels and finding more carbon sinks, such as regrowing mangroves , seagrass beds , and marshes, known as blue carbon.

    If we did, over hundreds of thousands of years, carbon dioxide in the atmosphere and ocean would stabilize again. Even if we stopped emitting all carbon right now, ocean acidification would not end immediately. This is because there is a lag between changing our emissions and when we start to feel the effects. It's kind of like making a short stop while driving a car: The same thing happens with emissions, but instead of stopping a moving vehicle, the climate will continue to change, the atmosphere will continue to warm and the ocean will continue to acidify.

    Carbon dioxide typically lasts in the atmosphere for hundreds of years; in the ocean, this effect is amplified further as more acidic ocean waters mix with deep water over a cycle that also lasts hundreds of years. It's possible that we will develop technologies that can help us reduce atmospheric carbon dioxide or the acidity of the ocean more quickly or without needing to cut carbon emissions very drastically. Because such solutions would require us to deliberately manipulate planetary systems and the biosphere whether through the atmosphere, ocean, or other natural systems , such solutions are grouped under the title "geoengineering.

    The main effect of increasing carbon dioxide that weighs on people's minds is the warming of the planet. Some geoengineering proposals address this through various ways of reflecting sunlight—and thus excess heat—back into space from the atmosphere. This could be done by releasing particles into the high atmosphere , which act like tiny, reflecting mirrors, or even by putting giant reflecting mirrors in orbit! However, this solution does nothing to remove carbon dioxide from the atmosphere, and this carbon dioxide would continue to dissolve into the ocean and cause acidification. Another idea is to remove carbon dioxide from the atmosphere by growing more of the organisms that use it up: Adding iron or other fertilizers to the ocean could cause man-made phytoplankton blooms.

    This phytoplankton would then absorb carbon dioxide from the atmosphere, and then, after death, sink down and trap it in the deep sea. Even public parks are not what nature created over the eons of time, working with wind and wave and sand. And so, besides public parks for recreation, we should set aside some wilderness area of seashore where the relations of sea and wind and shore—of living things and their physical world—remain as they have been over the long vistas of time in which man did not exist.

    Carson , p. The authors thank Adrian Kitchingman for his work on the maps presented here as figures 2 — 3 , and their colleagues in the Sea Around Us Project for discussions. Support from the Pew Charitable Trusts is gratefully acknowledged; D. National Center for Biotechnology Information , U. Published online Jan This article has been cited by other articles in PMC. Abstract This contribution, which reviews some broad trends in human history and in the history of fishing, argues that sustainability, however defined, rarely if ever occurred as a result of an explicit policy, but as result of our inability to access a major part of exploited stocks.

    Wars of extermination This dynamic obviously mimics the successive wars of extermination humans conducted on land, against large mammals and other animals. Open in a separate window. Fishing down marine food webs Two measures that can be easily estimated from fisheries landings have shown themselves to be highly indicative of the status of the underlying ecosystems, and thus could be used for such monitoring: Table 1 Some contributions demonstrating the occurrence of FD using locally disaggregated datasets, following the original presentation of this phenomenon by Pauly et al.

    Impacts on food security The masking effect of geographical expansion would not have worked, however, were it not for a tightly integrated global market resulting from the relaxation of investment regulations, the opening of international banking and advances in telecommunications, capable of compensating through imports from the Southern Hemisphere, for the shortfall in meeting the increasing demand for fish in the Northern Hemisphere owing to increased recognition of the benefits of eating seafood and increasing affluence. What needs to be done Given the FD and related trends discussed here, it appears rather urgent to now implement the reforms long proposed by most fisheries scientists and economists: Acknowledgments The authors thank Adrian Kitchingman for his work on the maps presented here as figures 2 — 3 , and their colleagues in the Sea Around Us Project for discussions.

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