Wanda Poteat, age 14, of Gastonia, No. Carolina, for her question:
Does stagnant water kill fishes?
It is true that certain fishes die when dumped into stagnant water. And, as some of us know, a pet goldfish suffers when the water in his tank becomes stale. But some species can survive in stagnant water and a few species can survive even through a waterless drought.
Most fishes prefer the turbulent waters of swift streams or tossing waves. The teeming fishing grounds off Newfoundland and Peru are regions where cool ocean currents stir up the salty sea water. There is a reason why fishes are friskier in moving water, especially where the surface water comes in contact with the air.
Fishes, like all animals, need oxygen to keep going. They must take their oxygen dissolved in water, and the water gets most of its dissolved oxygen from the air. The spray from ocean waves absorbs gases from the air, so do swift streams and breezy lakes. Even the surfaces of smooth water absorb oxygen from the air.
But under certain conditions, a stretch of still water may lose most of its dissolved oxygen. If there are fishes present, they cannot get enough oxygen to remain active. If there is no oxygen at all in the still, stagnant water, the fishes die from suffocation.
Our watery world is home to more than 30,000 different fishes and each species is adjusted to life in its own special niche. Most species live in the salty seas, though some are at home near the sunlit surface, some near the tidal shores and others out in deep water.
Most freshwater fish soon perish in saltwater. But many other species alternate between saltwater and freshwater during their life cycles. Eels are born at sea, live most of their lives in freshwater streams and return again to the sea. Salmon are born in freshwater streams and spend most of their lives in the sea. These lively tourists would not survive in stagnant water.
The electric eel is at home in the muddy, sluggish streams of the Amazon. Here oxygen is in short supply. Many other species lead lazy lives in still water where oxygen is scarce. But no fish can survive in stagnant water where there is no oxygen at all.
Each species is used to its own niche and as a rule the frisky types live in turbulent water where there is plenty of oxygen. You would expect to find plenty of dissolved oxygen along the shores, where the water is tossed by the tides. But tides are changeable sometimes this water contains more oxygen and sometimes less. Amazing, as it may seem, the fishes who live in these neighborhoods are adjusted to these changes. They are more active when oxygen is plentiful and their bodies are adjusted to change with the rhythm of the tides.