Global climate change
Since the industrial revolution of the late 1800s, atmospheric concentrations of “greenhouse gases” – mostly carbon dioxide, methane, chlorofl ourocarbons, and nitrous oxide – have increased substantially as a direct result of human activity. Sunlight passes through the atmosphere, heats the planet, and this heat is radiated back to space as infrared energy. The greenhouse gases act as a blanket, trapping the infrared radiation, heating the earth even further. No one questions the process; without greenhouse gases, the average temperature on earth would be about – 18°C, or about 33° colder than at present. Nor is there disagreement over the fact that greenhouse gases are increasing in the atmosphere at a rate of about 1–10% annually due to fossil fuel and wood burning, deforestation, cattle grazing, rice growing, and industrial pollution. What remains unknown is what effect this continued increase will have on regional and global climate. Average temperatures have increased about 0.5°C over the past century. If current trends of greenhouse gas production continue, most climate modelers predict that average temperatures will rise another 3°C over the next century, which is 10 times the rate at which the earth warmed after the last glacial advance (Ramanathan 1988; Smith 1990).
Of major concern are the likely climatic effects of this temperature increase and how they will be distributed (IPCC 2007a, 2007b, 2007c, 2007d). Altered wind direction and intensity and changes in the freeze–thaw cycle have been predicted. Vagaries of ocean currents and cloud behavior will undoubtedly lead to greater warming in some regions and even cooling in others. Similarly, rainfall patterns will shift, making some regions wetter, others drier. One likely result of temperature increase will be an increase in sea levelof about 0.3–0.7 m due to thermal expansion of oceanic water and melting of polar ice caps. The postulated consequences for fishes of such a change are potentially dramatic.
Temperature increases are likely to affect many aspects of fish biology. Metabolic processes are evolved responses to long-term thermal regimes characteristic of different climatic regions (Portner & Knust 2007; Temperature relationships). Alterations in thermal regime can affect the kinetics of such processes. Increased temperatures are a threat because fishes often live close to their critical thermal maxima (e.g., Magnuson & DeStasio 1997), because oxygen solubility is reduced at higher temperatures at the same time that metabolic requirements increase, and because many pollutants are more toxic at higher temperatures (Roessig et al. 2004).
Fishes respond to temperature changes by altering metabolic processes, reproduction, behavior, and distribution. Sex determination in fishes can be sensitive to thermal alteration, with different species producing unequal numbers of males or females in response to elevated temperatures (Devlin & Nagahama 2002; Determination, differentiation, and maturation). Gonadal development and germ cell viability are also temperature sensitive (Strüssmann et al. 1998). Timing of reproduction is highly sensitive to seasonal temperature cycles, via effects on precipitation and freeze–thaw cycles. In the northern hemisphere, lake freezing has occurred about 10 days later than 150 years ago (Magnuson et al. 2000), and in parts of Europe, snows melt 1–2 months earlier than 50 years ago, reducing spring floods and disrupting fish migrations and spawning. In Estonia, spawning migrations and timing of several freshwater fishes (Pike, Ruff, Bream, Smelt) have advanced on average 12–28 days from historical values (Ahas 1999; Ahas & Aasa 2006).
The latitudinal and altitudinal distribution of many fish species is determined by water temperature. Ultimately, species ranges can be altered via extensive dispersal, or population collapse can occur where suboptimal conditions cannot be avoided. Shifts in distribution of commercial and non-commercial marine species have been observed in the North Atlantic, where bottom temperatures increased 1°C between 1977 and 2001 (Perry et al. 2005). Among 36 species assessed, two-thirds moved northward or deeper toward cooler waters over that period. Such large-scale changes can have multiple, serious impacts on community structure, ecosystem function, and recovery of depleted fisheries (Murawski 1993). Elevated temperatures often prevent cold water species from occurring at lower latitudes and elevations. The temperature dependence of some species squeezes them into seasonally reduced habitat space, such as Striped Bass in the southern portions of their range (see Thermal preference). Continued elevated temperatures would be potentiallylethal (Coutant 1990; Power et al. 1999).
Many other impacts of elevated global temperatures can be anticipated (see McGinn 2002). Sea level rise will flood coastal marshes. Coastal wetlands, mangroves, and saltmarshes are major nursery grounds for numerous fish species. Vegetation loss due to flooding has several ecological consequences. The food webs of coastal marshes depend on vegetation as both a source of and a physical trap for detritus, and vegetation also provides spawning substrates, physical refugia for juvenile fishes, and substrates for prey. Marshes and their defining flora and fauna could disappear from many coastal areas (Kennedy 1990; Meier 1990).
Most global climate models predict less pronounced climatic changes at low latitudes. However, tropical animals tend to have relatively narrow climatic tolerances compared to high-latitude species and may therefore be more vulnerable to slight deviations from normal conditions (Stevens 1989). Coral reefs, already stressed by periods of slight temperature elevation, will be devastated by higher temperatures and accompanying stresses such as acidification (Hoegh-Guldberg 1999; Hoegh-Guldberg et al. 2007). Coral reefs globally are declining due to a number of local impacts, but global climate change, especially global warming, has already affected reefs throughout the tropics. Reef-building (hermatypic) corals generally exist in water close to their upper thermal limits. Increases of only a few degrees cause coral bleaching (loss of symbiotic algae) and death. Strong El Niño–Southern Oscillation (ENSO) events in 1982–83 and 1998 killed 50–100% of the corals in many areas, often as a result of average temperature rises of no more than a degree (Goreau et al. 2000; Glynn et al. 2001; Guzman & Cortes 2007). As the corals died, algae spread and covered all surfaces, followed by erosion and physical collapse of the limestone.
These alterations to the basic, underlying biological and physical structure of the reef have had far-reaching impacts on the fish assemblages. Where coral death exceeded 10%, more than 60% of fish species declined in abundance, with losses strongest among species that relied on live coral for food and shelter. Abundances among herbivorous and detritivorous species increased initially, but even these groups declined as reef erosion progressed. Overall fish diversity declined in direct response to the amount of coral lost, and prospects for long-term recovery are poor given projected trends in climate (Garpe et al. 2006; Wilson et al. 2006a; Feary et al. 2007). El Niños are expected to intensify as the climate warms, portending further, widespread reef degradation (Timmermann et al. 1999; Lesser2007). The implications for reef fish diversity, reef fisheries, marine protected area design, and the economics of small tropical nations are immense (Soto 2001; Bellwood et al.
2004). Of related importance to human welfare, bleached corals appear to provide an enhanced surface for the growth of dinofl agellates, including the ones responsible for ciguatera poisoning among humans that eat coral reef fishes.
Impacts on seasonal phenomena
Phenological (seasonal) cycles are likely to be disrupted, especially spawning periods that are timed to deliver larvae into regions of high productivity. Such productivity, which is driven by ocean currents and upwellings, has already been disrupted (Gregg et al. 2003; Schmittner 2005). Some global climate models indicate major shifts in ocean currents and upwelling patterns as a result of global warming. Such changes may alter or intensify the ENSO phenomenon, which has a substantial influence on major oceanic and coastal food webs (see The open sea). Some models predict weakening of major low-latitude currents such as the Gulf Stream and Kuroshio currents, reduction in nutrient-transporting eddies of these currents, and reduced upwelling off the western coasts of South America and Africa. Climate determines the vertical and horizontal distribution of ocean currents, and altered currents could affect the distribution and production of pelagic species that make up 70% of the world’s fisheries (Bakun 1990; Francis 1990; Gucinski et al. 1990). Timing of reproduction, particularly in migratory fish, would undoubtedly be disrupted. Migrations of anadromous salmonids are timed to take advantage of increased flows and cold water temperatures associated with snowmelt. Genetically determined migration times would be decoupled from altered melt cycles. Decoupling of phenological relationships will affect trophic interactions, alter food web structure, and produce ecosystem-level changes (Harley et al. 2006). Temperate marine environments may be particularly vulnerable because the recruitment success of fishes depends on synchronization with pulsed planktonic production (Edwards & Richardson 2004).
Variations in the frequency and severity of climatic extremes of drought, flood, and cyclonic force storms are also predicted. Storms and attendant floods wash young fish out of appropriate habitats, and can dilute high salinity, nearshore regions with fresh water, lessening their value as nursery grounds for larvae and juveniles. Altered rainfall patterns are expected to intensify droughts. Droughts would also affect water-stressed areas such as deserts and their already imperiled fish species, aridify areas that now have intermittent rainfall, and lead to contraction of the habitat space available for many species. Droughts also cause shifts in the distribution of estuarine habitats, because sea water typically intrudes farther up river basins during periods of low rainfall. Drought conditions would also exacerbate human impacts on fish habitat by reducing stream flow, elevating temperatures, and increasing pollutant concentrations.
Increased evaporation or decreased rainfall would decrease river flows and lake levels, causing wetlands to disappear and water tables to decline. The volume of cool water in many lakes would shrink, especially in summer. Cool water species whose ranges extend into warmer regions, such as Brook Trout, would be excluded from lower portions of streams during the summer. A few degrees of warming could be catastrophic for fishes that live near their critical thermal maxima because groundwater temperature is strongly dependent on air temperature (Power et al. 1999). Many stream fishes in the southwestern USA find temperatures above 38–40°C lethal. When temperatures in southern rivers exceed these limits, heat-related deaths occur, as they do with salmonids on the West coast at even lower temperatures (NRC 2004a). A 3°C temperature rise would potentially exterminate 20 species of fishes endemic to the southwest (Matthews & Zimmerman 1990). Warming would contract the geographic ranges of Arctic species, pushing the southern edge of their ranges northward (e.g., IPCC 2001).
Global warming also means that some warm water species would benefit from an increase in available habitat space at northerly latitudes, and some cool water species would gain access to higher altitudes and latitudes that are currently too cold to inhabit (Magnuson et al. 1990; Magnuson 2002). However, these shifts would dramatically alter assemblage relationships, with unknown consequences (Mandrak 1989). Cold water species will probably be both replaced and displaced by warm water species, especially invasive generalists, accelerating the process of faunal homogenization. Any “gains” would be offset by an overall loss of genetic and species diversity, especially because climate appears to be changing too quickly for genetic change to keep pace. New species will not have time to evolve to take the place of those that cannot adapt (IPCC 2001). A likely reduction in biodiversity is a serious, potential, negative impact of climate warming.
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