Fish are more sensitive to temperature than many animals because they cannot maintain a constant body temperature and their body is exactly the same temperature as the water they are swimming in. Different species can live in very cold or very hot water, but each species has a range of temperatures that it prefers, and fish can’t survive in temperatures too far out of this range. When fish encounter water that is too cold for them, their metabolism the chemical engine that drives their body slows down and they become sluggish. As the surrounding water warms up, their metabolism speeds up they digest food more rapidly, grow more quickly, and have more energy to reproduce. But fish need more food and more oxygen to support this higher metabolism. Warmer fish tend to mature more quickly, but the cost of this speedy lifestyle is often a smaller body size. They also hatch much earlier (and smaller), and reach sexual maturity earlier. Many fish will also have less offspring as temperatures rise, and some may not be able to reproduce at all. Climate change has contributed to a drastic decline of the fishery over the last few years because of increased water temperatures and decreased rain (which fusses critical nutrients from the land).
If there is not enough food, all of a fish’s available energy goes to fuelling its high metabolism, and less energy is available for growth and reproduction. Rainbow trout, salmon, whitefish, and perch are all expected to grow more slowly if food supply does not increase as temperatures rise. Even if there is more food to eat, it may no be enough to satisfy the ravenous appetites of warmed-up fish. To make matters worse, fish may not have enough oxygen to breathe as the water grows warmer. Fish filter oxygen from the water they are swimming in, but the amount of oxygen dissolved in water decreases as temperatures rise. So many fishes will experience an “oxygen squeeze” as the climate warm up, they will need more oxygen to support their elevated metabolisms, but may not be able to get it from the warmer, oxygen-poor water around them. And some unlucky fish may be killed by even a slight rise in temperature. Many tropical species already suffer near-lethal temperatures during the hottest part of the day. An increase of 1-2°C could cause massive fish kills, especially in tropical aquaculture ponds or shallow pools of the Amazon and Mekong rivers. Ponds and lakes supporting lots of fish also experience critically low levels of oxygen overnight, when aquatic plants stop releasing oxygen through photosynthesis. As these waters warm, the combination of increased metabolic demand and reduced dissolved oxygen could lead to lethal oxygen shortages. The fish that stick around warmer water will also have to deal with new species showing up on their doorstep. Many areas have been colonized by new species as water has warmed in the last few decades, and invasions are likely to increase. WWF (2005) reported that, in the north-western Mediterranean, which is traditionally cooler than nearby areas, new species such as the ornate wrasse, dusky grouper, and Senegalese sole have become common in recent decades, and tropical fish like the blunt head puffer have been spotted for the first time. Naturally, when fish find themselves in hot water, they head out in search of cooler locales. As global temperatures rise, some fish may be able to shift locally by moving deeper or by heading upriver towards cool headwaters. So migratory behaviour also changes forever with the change in migratory route, time of migration, grounds of recruitments etc.
A warm water welcome for diseases and toxins. As water warms up, many parasites and microbes that cause fish diseases grow faster and become more virulent. And as harmful microbes and parasites become stronger and more numerous, fish whose immune systems are already stressed by warm water, low oxygen, and crowding, become even more susceptible to diseases and parasites. Massive fish die-offs due to toxic algae and the risk of human illness from eating poisoned fish will also increase as temperatures climb.
Warmer water increases the toxicity of pollutants, and as fish pump more water through their gills to meet increased metabolic needs, they also collect more pollutants. While warmer fish can flush out the extra load of some types of toxins, cadmium and lead levels remain higher in high temperatures, and fish in warmer water accumulate mercury more rapidly. Mercury poisoning is already a major economic problem for fisheries in Canada, Japan, and Scandinavia, and poses a significant public health risk.
Some of the most severe impacts of global warming (at least from a fish’s perspective) may be related to changes in climatic variables other than temperature such as changes in rainfall and evaporation that alter lake levels and river flow. Higher water temperatures may have critical implications for inland fisheries. Freshwater species of closed water bodies that are geographically isolated that cannot migrate to cooler waters as temperatures rise may be stuck in hot water. Migration is impossible from many isolated lakes and wetlands, and many major river systems worldwide run from east to west, making pole ward migration impossible. Native fishes are already living near their thermal tolerance limits in some of the hottest free-flowing water on earth. Most fish species tolerate seasonal variations in temperature, but have limits to how hot or cold the water they can find more hospitable. Global warming change the Chemical composition of the water: The amount of oxygen in the water may decline, while pollution and salinity levels may increase and alter the availability of food for some fish species. Lakes that remain stratified longer tend to have more blue-green algae, which produce toxins harmful to fish, their prey and humans who consume the fish. Fish die-offs and human illness can also be caused by ciguatera fish poisoning, in which fish are poisoned by tiny organisms called dinoflagellates. In Bangladesh as temperatures warm up in the River Ganges and Brahmaputra colder water could generally be found by swimming upstream, but smaller rivers and lakes generally do not cover as wide a range of temperatures. Warming of upper layer cause changes in zooplankton composition which causes the potential mismatch between the prey (plankton) and predators (fish species). Rising of sea level caused by global warming intruded saline into freshwater habitats also. Permanent high water temperature brings changes in physiology, sex ratio of fish species and alters timing of spawning. As a result, warmer temperatures could make these waters entirely uninhabitable for some of the fish species which reduces the desired production of target species. Higher water temperatures lead to lower levels of dissolved oxygen in closed water body. With the 70ºF warming that could eventually occur, majority of rivers will have oxygen concentrations below the level necessary to support most fish. Increased warming could lead to the extinction of up to 20 freshwater species that are found nowhere else in the world. Another problem that shown by global warming is the changing levels of precipitation where rainfall decreases, more possibility of droughts or floods, damage to productive assets (fish ponds, weirs, rice fields, etc.) ensures and less predictable wet/dry seasons.
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