- Global Warming increasingly threatens fish. Greenhouse
gases released mainly by humans burning coal, oil
and natural gas have led to sharp rise in mean global
temperatures over the last 50 years.
- Temperatures are expected to rise 1.4-5.8o C
more by the end of the century.
- While slightly warmer water may not sound so bad
to many of us, its effect on fish and aquatic ecosystems,
and ultimately on the global food supply and economic
stability could be severe.
- Fishes expected to grow more slowly if food supply
does not increase as temperature rise. Even if there
is more food to eat, it may not be enough to satisfy
the ravenous appetites of warmed-up fish. Experiments
in Toolik Lake, Alaska (US), suggest that a warming
of 3oC will double the food supply for arctic trout,
but their metabolism will increase so much that
young trout are still expected to starve by the
end of their first year.
- 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 fish will experience
an "oxygen squeeze" as the climate warms
- 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.
- Warmer fish tend to mature more quickly, but
the cost of this speedy lifestyle is often a smaller
body size. Ninety percent of aquatic animals like
fish raised in warm water end up smaller than their
peers raised at cooler temperatures.
- Many fish will have less offspring as temperatures
rise, and some may not be able to reproduce at all.
- 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. This could have bad effects.
When fish in the Gulf moved deep in 1993, 120,000
sea birds starved to death, most likely because
they could not dive deep enough to catch their relocated
prey. Many fishes that cannot find a local solution
are already heading towards the poles as the water
becomes too warm. Even a slight increase in global
temperature is expected to shift the ranges of many
economically valuable fish.
- As water warms up, many parasites and microbes
that cause fish diseases grow faster and become
more virulent. Parasites in cooler climates are
more likely to survive the winter and produce multiple
generations of offspring each year, so more fish
may become infected. 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.
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