The impacts of global warming such as wetland loss, salinity changes, and higher temperatures are all likely to affect finfish and shellfish in the coastal zone. Coastal marshes and mangroves are the primary nursery grounds; most of the reproduction occurs in the part of the wetlands that are within about 50-100 feet of the open water. Those species that are either reproduce in coastal wetlands or spend their entire lifetimes in an estuary. As sea level rises and inundates wetlands, the initial effect will be the increase in total area of marsh to which fish have access. Hence, sea level rise may initially increase the production of these species. As sea level continues to rise, however, the loss of marsh and mangroves accelerates; and eventually most or all of the wetlands in an area are replaced by open bodies of water. Thus, in the long run, an accelerated rise in sea level would decrease production of these species. Approximately 50% of ocean fish, including crustaceans (shrimp) and molluscans spend most of their lives in the ocean, but spawn in estuaries. As climate warms, these fish might be able to migrate to other areas, as long as an estuarine environment remains available. However, in the long-run rising sea level may substantially diminish the critical coastal habitats. A number of non-commercial marine species are also vulnerable to the inundation and erosion of coastal habitat. The horseshoe crab lays its eggs on sandy estuarine beaches; turtles nest on small sandy islands and sand spits, which could be eroded by rising sea level. Increased salinity from rising sea level or increased drought; warmer temperatures could leave estuaries too warm for some of the current species; lower dissolved oxygen content resulting from warmer water could also lead to fish kills.
The costs of adaptation to the natural disaster caused by global warming make fish harvesting less profitable, increased costs of insurance and/or rebuilding, shortened the duration of fish harvesting at sea, increased risk of accidents and reduced viability of fish harvesting as livelihood options for the poor. This negative condition reduced opportunities and diversity of rural livelihoods, decreased ability to plan seasonal livelihood activities and increased risks in agriculture in near future.
Scientists generally expect fish on the high seas to be less affected by global warming than coastal and inland fisheries. Warmer temperatures are likely to enhance fishing in many areas. Overall biological activity is greater at higher temperatures, more food is available, fish grow faster, and they reproduce at a younger age. However, this expected increase in fisheries from warmer temperatures may be partly offset, by a decline in the timing and latitude of upwelling. Decline in upward flow of nutrients cause destruction in food web and disturb the distribution of pelagic fisheries. Warming of upper layer of ocean cause pole ward shifting of plankton and fish species and thereby changes in the time of phytoplankton bloom. Reductions in phytoplankton caused by warmer sea temperatures have devastating effects on fish predators, such as seabirds and marine mammals, at the top of the food web. But even based on the effects of temperature alone, it seems likely that fish will become smaller and harder to find, and valuable cool and cold water species will begin to be replaced by more adaptable, less valuable warm water fish. Another impact of global warming changes ocean currents which affect the recruitments and abundance of juvenile fish species. Many fish that cannot find a local solution are already heading towards the poles as the water becomes too warm. A report of WWF (2005) acclaimed that even a slight increase in global temperature is expected to shift the ranges of many economically valuable fish, including:
- Pacific and Atlantic salmon – unusually warm years have already led to poor Pacific salmon harvests in the southern part of Atlantic Ocean of their range.
- Cod, plaice, and halibut – these ground fish are expected to become scarce in US and southern Canadian waters, and cod are likely to disappear from the southern North Sea, one of their main spawning areas.
- Trout, whitefish, and bass – suitable habitat for these and more than 20 other cool and cold water fish in the USA is expected to decline by as much as 50% due to the effects of global warming.
Because of reduction in availability of fish species many fisheries resources will permanently shift location as water temperatures rise. Large, commercial fleets that can follow the fisheries may not be as strongly affected as local, small-scale fishermen, who will have to adapt their gear and methods, travel further, and longer to continue providing enough food for their families and local markets.
In addition to global warming, carbon dioxide emissions cause another, less well-known but equally serious and worrying phenomenon: ocean acidification. Carbon dioxide, the gas most responsible for the Earth’s increase in average global temperature, will affect nature’s ability to sustain life both above and below the waves. Acidification of seawater could eventually lead to oceans incapable of fostering life for mammals, fish, and even gastropods; those organisms which grow calcium carbonate shells for protection. The carbon dioxide system ranging from the potential outcomes of the increase in dissolved inorganic carbon dioxide in the oceans, to the increase in hurricane intensity scientists feel may occur over the next hundred years. Key marine organisms, such as deep-water corals and pteropods (shelled pelagic molluscans) will be profoundly affected by this phenomenon during the years to come. The impact of such is a reduction in pH on calcifying organisms. Pteropods (pelagic marine mollusks) and deep-water corals, both playing essential roles in their respective ecosystems, live in areas that will be among the first to be affected by ocean acidification. If the key organisms one day disappear, their natural predators will be forced to migrate elsewhere for food sources. This could have large-scale effects on the availability of fish and carbon saturation in oceanic regions. Again, Coral reefs have proved to be particularly sensitive to warming. As surface temperatures have risen in recent years, many reefs have bleached, and with further increasing temperatures over longer periods the corals die.
Global warming itself may be increasing the frequency of El Nino. El Nino results from changes in atmospheric pressure in the Pacific Ocean and is associated with higher sea surface temperatures and sea levels, lower nutrients, and increased intensity of storms and storm surges. El Nino has wreaked havoc in stocks of sardines and anchovies in Peru, marine iguanas and kelp forests off California.
- Broad, K., Pfaff, A.S.P. and Glantz, M.H. (1999) Climate Information and Conflicting Goals: El Nin˜o 1997-98 and the Peruvian Fishery. Public Philosophy, Environment, and Social Justice. Carnegie Council on Ethics and International Affairs, New York.
- Bruno, J.F., Selig, E.R., Casey, K.S. et al. (2007) Thermal Stress and Coral Cover as Drivers of Coral Disease Outbreaks. PLoS Biology 5, e124.
- Edwards, M. and Richardson, A.J. (2004) Impact of climate change on marine pelagic phenology and trophic mismatch. Nature 430, 881–884.
- IPCC (2001a) Climate Change 2001: Impacts, Adaptation & Vulnerability, Contribution of Working Group II to the Third Assessment Report of the Intergovernmental Panel on Climate Change, Vol. Cambridge University Press, Cambridge.
- Lehodey, P., Alheit, J., Barange, M. et al. (2006) Climate variability, fish and fisheries. Journal of Climate 19, 5009–5030.
- Perry, A.L., Low, P.J., Ellis, J.R. and Reynolds, J.D. (2005) Climate change and distribution shifts in marine fishes. Science 308, 1912–1915.
- Reid, P.C., Johns, D.G., Edwards, M., Starr, M., Poulin, M. and Snoeijs, P. (2007) A biological consequence of reducing arctic sea ice cover: arrival of the Pacific diatom Neodenticulata seminae in the North Atlantic for the first time in 800,000 years. Global Change Biology 13, 1910–1921.
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