Fisheries

Introduction

Fisheries are areas where fish are caught. Fisheries supply a large fraction of the world's food, but are under severe stress today from over fishing. Climate change may add to the stress on ocean fisheries by changing the planktonic food web that is the basis of the marine food chain. Changes in fisheries are associated with damage to coral reefs by water heating and ocean acidification, and other means.

Fish populations are also shifting to different ranges because of warming water, with resulting changes in marine ecology and fishery populations. Climate warming affects freshwater fisheries by reducing dissolved oxygen, increasing stratification (layering of deep water), and shifting the patterns of rainfall that supply lakes and rivers with water.

Historical Background and Scientific Foundations

Humans have harvested fish for thousands of years. However, not enough fish could be harvested to injure large fish populations until the twentieth century, when mechanized ships in large fleets began to ply the seas. Most fish are captured at sea: in 2005, 7.6% of all fish were caught in freshwater, 9.5% were raised on freshwater fish farms (aquacultured), 7.1% were aquacultured at sea, and 75.7% were captured wild at sea. Sixty-nine percent of the total fisheries output was used for human consumption, the remainder for other purposes (for example, pet food). Fish provide 2.6 billion people with 20% or more of their dietary protein. About 15% of the world animal protein supply is from fish.

The world wild-fish catch increased 4.5-fold from 1950 (19 million metric tons) to 2000 (90 million metric tons). However, the global catch leveled off in the early 2000s and has declined slightly since, probably passing its all-time historic peak. There are few fish stocks that are not already being intensively exploited, and many fish populations are declining. Some fishing fleets are subsidized by governments, which encourage over fishing, and in most fisheries there is either a struggle to fill catch quotas before others do or to simply catch every fish possible.

Such practices lead to what fisheries experts call “the race for fish.” The race for fish is causing fish abundance to decline in many fisheries, with consequent structural changes to ecosystems, loss of jobs, and the breakdown of fishing communities. Aquaculture can also stress wild fisheries, as the food supplied to captive fish (e.g., shrimp) often consists of small, wild fish caught in indiscriminate fine-meshed driftnets that reduce populations of commercially valuable species by harvesting their young along with adults of small, noncommercial species.

Fish populations fluctuate over decadal time scales (10 or more years), in some cases centennial time scales; fish populations in areas that are geographically separated often vary at the same time, showing the influence of climate. Freshwater fish populations also vary in response to climate changes, which affect water temperatures, dissolved oxygen content (the warmer the water, the less the oxygen), and the amount, timing, and location of precipitation supplying water to rivers and lakes.

Impacts and Issues

Anthropogenic (human-caused) climate change adds stress to fishery ecosystems that are already under historically unprecedented stress from over fishing. The most general result of modern anthropogenic global climate change is warming, which has several effects. First, temperature affects the species mixture and abundance of plankton. Plankton are tiny animals (zooplankton) and green plants (phytoplankton) that float in the upper layers of bodies of water. Phytoplankton play an ecological role similar to that of green plants on land, capturing the energy of sunlight and passing it up the food chain to larger organisms. Almost all oceanic fish are dependent on plankton either directly (as filter feeders, for example, the tiny shrimp called krill) or indirectly (as predators on smaller fish).

Plankton productivity—the amount of plankton for a given area of water surface—is altered by climate variations. Satellite observations show that plankton productivity decreases as sea surface temperature increases: warming of surface waters decreases upwelling of deeper waters, which contain nutrients that phytoplankton need. Decreased upwelling occurs because the warmer that the surface water is, the less dense it is, and the more stably it floats on deeper, cooler waters. Warming therefore decreases planktonic productivity, which inevitably decreases populations of animals higher up in the food chain, such as fish. Such decreases can happen naturally, as for example during the El Niño/Southern Oscillation climate cycle, when the climate and surface waters of the central Pacific warm. It also occurs as a result of anthropogenic global warming. Some studies forecast a possible shift to permanent El Niño conditions as a result of anthropogenic warming.

A general effect of global warming involves migration toward the Poles by animal and plant species, both in water and on land, as species track the climate zone in which they are adapted by evolution to thrive. In the Northern Hemisphere, both fish and plankton species have already been observed to shift their ranges northward. In the North Sea, for example, where waters warmed by 1.9°F (1.05°C) from 1977 to 2001, a study by A. L. Perry and colleagues published in 2005 showed that the ranges of 15 out of 36 fish species had shifted northward by distances ranging from 30 mi (48 km) to 250 mi (403 km).

Other effects on fisheries may be less direct. For example, general circulation models (computer simulations of global climate) predict that with the doubling of atmospheric CO₂, which is possible within the next century, freshwater discharge from the Mississippi River into the Gulf of Mexico might increase by 20%. The Gulf of Mexico is the site of the world's largest and most severe oxygen-depleted zone, sometimes called a dead zone because no fish can live in it. Agricultural chemicals in the river water are the cause of the dead zone, already up to 7,700 mi²(20,000 km²) in size. Increased river discharge is expected to enlarge the dead zone, affecting food webs in the Gulf and probably decreasing fish populations.

Increased stratification—the stabilization of warmer surface waters, with less mixing of nutrient-rich deeper waters up to sunlit layers where phytoplankton can exploit both nutrients and sunlight—is an important climate impact on some large freshwater fisheries. For example, the world's second-largest lake, Lake Tanganyika in Africa, is home to hundreds of species of fish and is a major freshwater fishery, supplying a harvest of 165,000–200,000 metric tons of fish per year, 15–40% of the protein intake of the inhabitants of the four countries that border the lake, namely Burundi, Tanzania, Zambia, and Congo. Since the 1970s, catches of the two primary species of the fishery (both members of the clupeid family, which includes herrings and sardines) have declined by 30–50%, though the lake is not over fished.

Until the early 2000s, the decline was attributed to some unknown environmental factor. This factor was finally shown to be climate warming. Steady atmospheric warming at Tanganyika since 1950 has caused the surface layer of the lake to warm, decreasing mixing of that layer with cooler, nutrient-rich waters from the depths. As of 2003, plankton density in Lake Tanganyika was less than one third than it had been 25 years earlier. Since plankton are the basis of the marine food chain, fish stocks declined along with plankton, and were 30% smaller in 2003 than they were 80 years before. Over the next 80 years, the climate in Tanganyika's area is predicted to warm by a further 2.3–3°F (1.3–1.7°C). This is likely to further increase stratification, reduce mixing, deplete plankton, and shrink the fishery, with possible consequences for human well-being in the surrounding areas.

Primary Source Connection

One potentially catastrophic effect of global climate change is a predicted change in Earth's hydrologic (water) cycle, including an increase in freshwater runoff. This source states that climate models indicate increased freshwater runoff in most of the world's major river systems. Greater discharge of freshwater will result in increased hypoxic zones in oceans, causing major disruptions in coastal ecosystems. Hypoxic zones are coastal waters with depleted oxygen content due to fertilizers and other sediments that are carried by freshwater runoff. Hypoxic zones are sometimes referred to as “dead zones” because the oxygen content is so low that the waters cannot support life. One of the largest hypoxic zones in the world is in the Gulf of Mexico where the Mississippi River discharges. The Gulf of Mexico hypoxic zone covers an approximate area of 22,000 km².

CLIMATE, HYPOXIA AND FISHERIES: IMPLICATIONS OF GLOBAL CLIMATE CHANGE FOR THE GULF OF MEXICO HYPOXIC ZONE

Introduction

There is a growing consensus among scientists that human activities, which have increased atmospheric concentrations of carbon dioxide (CO₂) by one-third during the last 100 years, may be responsible for an increase in global Earth's temperatures. This so-called “global warming” theory is not without challengers who argue that scientific proof is incomplete or contradictory, and that there remain many uncertainties about the nature of climate variability and climate change. Nevertheless, global temperature averages increased by almost 1°C during the last 150 years, and further temperature increase seems probable. General circulation models (GCMs) forced by enhanced greenhouse gas concentrations have projected a global temperature increase of 2 to 6°C over the next 100 years. An increase in global Earth's temperature of 2 to 6°C would likely produce an enhanced global hydro-logic cycle that would be manifested in altered freshwater runoff. This hypothesis is supported by several lines of evidence, including “paleofloods,” decadal trends in the freshwater runoff and GCM's scenarios.

In the United States, there is historic evidence suggesting that a change in climate enhances the frequency of extreme flood events. An analysis of a 5000-yr old geological record for the southwestern United States suggested that floods occurred more frequently during transitions from cool to warm climate conditions. Apparently, modest changes in climate were able to produce large changes in the magnitude of floods. Additional evidence in support of the above hypothesis came from a 7000-yr old record of over bank floods for the upper Mississippi River tributaries. Approximately 3300 years ago, an abrupt shift in flood behavior occurred, with frequent floods of a magnitude that now recurs every 500 years or more. Also, an analysis of the data collected by the U.S. Geological Survey indicated statistically significant increasing trends in monthly streamflow during the past five decades across most of the conterminous United States. These results seem to support the hypothesis that enhanced greenhouse forcing produces an enhanced hydrologic cycle. One of the GCM studies has examined the impact of global warming on the annual runoff of the 33 world's largest rivers. For a 2xCO₂ climate, the runoff increases were detected in all studied rivers in high northern latitudes, with a maximum of +47 %. At low latitudes there were both increases and decreases, ranging from +96% to -43%. Importantly, the model results projected an increase in the annual runoff for 25 of the 33 studied rivers.

The northern Gulf of Mexico, which receives inflows of the Mississippi River—the eighth largest river in the world, is one of the coastal areas that may experience increased freshwater and nutrient inputs in the future. According to a GCM study referenced above, the annual Mississippi River runoff would increase 20% if the concentrations of atmospheric CO₂ doubles. This hydrologic change would be accompanied by an increase in summer and winter temperatures over the Gulf Coast region of 2°C and 4°C, respectively. A higher runoff is expected during the May-August period, with an annual maximum most likely occurring in May. While there are no other GCM estimates of the Mississippi River runoff, this result is in agreement with a projected 2xCO₂ increase in rainfall over the Mississippi River drainage basin.

Here we review probable implications of climate change for the Gulf of Mexico hypoxic zone, focusing on two areas: (1) coupling between climate variability, freshwater runoff of the Mississippi River, and hypoxia in the coastal northern Gulf of Mexico, and, (2) potential implications of global climate change for coastal fisheries in the hypoxic zone. In this analysis we use our previously published physical biological model and extensive long-term data sets collected at station within the core of the Gulf of Mexico hypoxic zone.

WORDS TO KNOW

EL NIÑO/SOUTHERN OSCILLATION: Global climate cycle that arises from interaction of ocean and atmospheric circulations. Every 2 to 7 years, westward-blowing winds over the Pacific subside, allowing warm water to migrate across the Pacific from west to east. This suppresses normal upwelling of cold, nutrient-rich waters in the eastern Pacific, shrinking fish populations and changing weather patterns around the world.

PHYTOPLANKTON: Microscopic marine organisms (mostly algae and diatoms) that are responsible for most of the photosynthetic activity in the oceans.

UPWELLING: The vertical motion of water in the ocean by which subsurface water of lower temperature and greater density moves toward the surface of the ocean. Upwelling occurs most commonly among the western coastlines of continents, but may occur anywhere in the ocean. Upwelling results when winds blowing nearly parallel to a continental coastline transport the light surface water away from the coast. Subsurface water of greater density and lower temperature replaces the surface water and exerts a considerable influence on the weather of coastal regions. Carbon dioxide is transferred to the atmosphere in regions of upwelling.

Coupling Between Climate and Hypoxia

Climate change, if manifested by increasing riverine freshwater inflow, may affect coastal and estuarine ecosystems in several ways. First, changes in freshwater inflow will affect the stability of the water column, and this effect may be enhanced due to changes in sea surface temperatures. Vertical density gradients are likely to increase, that could decrease vertical oxygen transport and create conditions in the bottom water favorable for the development of severe hypoxia or anoxia. Second, the concentrations of nitrogen (N), phosphorus (P), and silicon (Si) in riverine freshwater inflows are typically an order of magnitude higher than those in coastal waters. The mass fluxes of riverine nutrients are generally well-correlated with integrated runoff values. Consequently, the nutrient inputs to the coastal ocean are expected to increase as a result of the increasing riverine runoff, which could have an immediate effect on the productivity of coastal phytoplankton. Third, the stoichiometric ratios of riverine nutrients, Si:N, N:P and Si:P, may differ from those in the coastal ocean. Increased freshwater inflow, therefore, may also affect coastal phytoplankton communities by increasing or decreasing a potential for single nutrient limitation and overall nutrient balance. Thus, it appears that there is a plausible link between global climate change and the productivity of river-dominated coastal waters.

Changes in the areal extent of hypoxic (<2 mg O₂1^-1) bottom waters provide a representative example of the riverine influence on coastal productivity processes. The northern Gulf of Mexico is presently the site of the largest (up to 20,000 km²) and most severe hypoxic zone in the western Atlantic Ocean. Hypoxia normally occurs from March through October in waters below the pycnocline, and extends between 5 and 60 km offshore. During the drought of 1988 (a 52-year low discharge record of the Mississippi River), however, bottom oxygen concentrations were significantly higher than normal, and formation of a continuous hypoxic zone along the coast did not occur in midsummer …. The opposite occurred during the Great Flood of 1993 (a 62-year maximum discharge for August and September), when the areal extent of summertime hypoxia doubled with respect to the average hydrologic year. Hypoxia in the coastal bottom waters of the northern Gulf of Mexico develops as a synergistic product of high surface primary productivity, which is also manifested in a high carbon flux to the sediments, and high stability of the water column. Likewise, the 1993 event was associated with both an increased stability of the water column and nutrient-enhanced primary productivity, as indicated by the greatly increased nutrient concentrations and phytoplankton biomass in the coastal waters influenced by the Mississippi River….

Conclusions

Projections of general circulation models suggest that freshwater discharge from the Mississippi River to the coastal ocean would increase 20% if atmospheric CO₂ concentration doubles. A higher Mississippi River runoff would be accompanied by an increase in winter and summer temperatures over the Gulf Coast region of 4.2°C and 2.2°C, respectively. This is likely to affect the global oxygen cycling of the northern Gulf of Mexico, which is presently the site of the largest (up to 20,000 km²) and the most severe coastal hypoxic zone (<2 mg O₂1^-1) in the western Atlantic Ocean. Model simulations suggest a close coupling between climate variability and hypoxia, and indicate a potential for future expansion of the Gulf's hypoxic zone as a result of global warming. In simulation experiments, a 20% increase in annual runoff of the Mississippi River, relative to a 1985–1992 average, resulted in a 50% increase in net primary productivity of the upper water column (0–10 m) and a 30-60% decrease in summertime subpycnoclinal (10–20 m) oxygen content within the present day hypoxic zone. Those model projections are in agreement with the observed increase in severity and areal extent of hypoxia during the flood of 1993. Because of large uncertainties in the climate system itself, and also at different levels of biological control, it is difficult to predict how climate change may affect coastal food webs. Nevertheless, future expansion of the coastal hypoxic zones would have important implications for habitat functionality and sustainability of coastal fisheries.

Dubravko Justi?, et al .

justi?, dubravko, et al. “climate, hypoxia and fisheries: implications of global climate change for the gulf of mexico hypoxic zone.” proceedings of the 2000 louisiana environmental state of the state conference, 2006. < http://www.epa.gov/msbasin/taskforce/2006symposia/146justice.pdf> (accessed november30, 2007).

See Also Biodiversity; Oceans and Seas.

BIBLIOGRAPHY

Books

Cushing, D. H. Climate and Fisheries. New York: Academic Press, 1982.

Periodicals

Behrenfeld, Michael J. “Climate-Driven Trends in Contemporary Ocean Productivity.” Nature 444 (December 7, 2006): 752–755.

Ficke, Ashley D., et al. “Potential Impacts of Global Climate Change on Freshwater Fisheries.” Reviews in Fish Biology and Fisheries 17 (November 2007): 581–613.

Hilborn, Ray, et al. “State of the World's Fisheries.” Annual Review of Environmental Resources 28 (2003): 359–399.

Lehodey, P., et al. “Climate Variability, Fish, and Fisheries.” Journal of Climate 19 (October 2006): 5009–5030.

Perry, Allison J., et al. “Climate Change and Distribution Shifts in Marine Fishes.” Science 308 (June 24, 2005): 1912–1915.

Roessig, Julie M., et al. “Effects of Global Climate Change on Marine and Estuarine Fishes and Fisheries.” Reviews in Fish Biology and Fisheries 14 (June 2004): 251–275.

Web Sites

“State of the World's Fisheries and Aquaculture 2006.” Food and Agriculture Organization of the United Nations, June 22, 2007. < ftp://ftp.fao.org/docrep/fao/009/a0699e/a0699e.pdf> (accessed November 19, 2007).

Larry Gilman