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The Ecology of a Stream: A Tale of Balance

See also my Ozark Stream Ecology paper.

A stream is a continuously moving system, whose health is determined by it's balance.  This balance also determines which fish species will be present in the stream, and what they eat (that is-what can an angler catch them with).  Essentially the biggest factors in determining what kinds of fish are in a stream are the streams temperature, size, and turbidity (how muddy it is).  Every type of fish has a preferred temperature/turbidity range  (which also determines how much oxygen is dissolved in the water), and a maximum temperature turbidity range.  See the picture below.

Clear, swift, cool streams usually host Salmonids (trout, whitefish, graylings, salmon).  Slightly slower, warmer, streams host perches (walleye, yellow perch), smallmouth bass, and pikes (pickeral, pike, muskellunge).  Even warmer, slower streams hold catfish, largemouth bass, freshwater drums, and temperate basses (stripers, white bass, yellow bass).  Sluggish, slow, muddy streams, like the lower Mississippi River hold largely largemouth bass, sunfish, catfish, drum, carps, bowfins, and gars.

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Erosion from deforestation, or runnoff from farms and lawns (which carry fertilizers and manure) can turn a naturally cold, clear stream into a cloudy, warm stream, and drastically shift what types of fish will prosper in the stream (This is similar to what has happened to the Columbia River in Oregon).  Likewise, the construction of a dam, with its cold water outflow, may turn a warm water system into a cold water one (the White river in Arkansas).  The life in a stream is a balance of plant life, small insects/molluscs/and crustaceans, and fish: like a three-legged stool. An increase in available light or fertilizer will tilt the three way balance in favor of the plants, while stocking efforts will tilt the balance in favor of the fish.  Bottom line, the middle leg of the small insects/crustaceans/and molluscs will determine the health of the stream, and therefore how many naturally occuring fish will be present.  Too many plants will choak out the middle leg and the fish will starve.  Too many fish and the middle leg will be depleated-making all the fish starve and die.  A good stream has a healthy balance on all three legs. This picture might help a little.

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The amount of water in a stream will also effect how well fish do in it.  An increase in water volume will cool the stream off, while a decrease in water flow will warm it up, shifting the temperature and turbidity (and which fish are present).  An example of this is the Sacremento River in California, where irrigation will at times cause the river stand still, hurting the salmon which spawn in the river, and causing saltwater to creep into the river.


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(and what fish eat!)
The food web for a stream/river has essentially 6 teirs  (each teir mainly eats the one below it, and maybe two below it-teir 5 may eat from teirs 4 and 3).  Teir 0 is the sun, which only the plants and phytoplankton (i.e. microscopic plants like algae) get energy from.  Teir 1 are plants and phytoplankton, Teir 2 are Zooplankton (microscopic animals), Teir 3 are small visible animals-usually freshwater shrimp, freshly hatched fish, small aquatic insects and terrestrial insects and worms that feed in the stream.  Teir 4 consists of fingerling juvenille fishes, small true minnows (such as shiners and chubs), larger crustaceans (crayfish, large shrimps), salamanders, molluscs (clams/mussells), and large predatory aquatic insects (hellgramites, dragonfly larvae). Teir 5 is the layer where most gamefish reside, and the most important to anglers (who, by the way, are on Teir 6). Knowing what animals are on Teir 3 and 4 will help the angler pick lures and baits appropriate for each stream.  Note, the illustrations below are just examples, since every stream has variations that make fishing interesting!   If you are lucky enough to fish where there are fish on Teir 6 (like large Catfish, Sturgeon, Large Salmon, Striped Bass, Large Pikes, and Alligator Gar), then remember that these large predators are vital to the health of the river they live in, and use judgement when keeping one.  Take a peek at the pictures below (cold, mid-warm, and warm) which can all exist on one river (here the Merrimac River (just remove the brookies and salmon) in Missouri), and use them to construct your own mental picture of each stream you fish.
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