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FOR ANGLERS and other fish-ites
Like a stream, a healthy lake or pond, from the largest freshwater sea (like the Great Lakes) to the smallest backyard fishpond exists in a complex balance.   In fact, for a still (or near still) body of water, there are three balances, like a three legged stool, that must be maintained: water, oxygen, and energy.  Each balance will affect the other two: not enough water in a small pond in the summer means the water gets warmer, which means it can hold less oxygen, which means few large fish can survive, which means the animals that the fish eat may multiply, which means the plants the small animals eat my multiply, etc.  If you don't understand yet, that's OK because more explanation and pictures follow below, and will help detail how different fish live in different types of water, and what can help or hurt fish populations. Why do I, as an angler care? Other than the basic ecological responsibilities all humans should have, THE BETTER YOU UNDERSTAND THE FOOD WEB AND THE BALANCES THAT EFFECT IT, THE BETTER YOU WILL UNDERSTAND HOW TO CATCH THE FISH THAT MUST LIVE IN THE FOOD WEB!

The Water Balance

(Click here to jump to the illustration) (Click on image to see full size)
A lake or river by definition holds water.  The amount of water it can hold (and also at what temperature/depth) determines what kinds of fish can live in it.  Changes in the amount of water in a lake or pond also favor or hinder different parts of the food web.  Lakes and ponds receive all of their water from rain, either directly by the rain falling on the lake, or via feeder streams/rivers, springs, and drainage.  Primarily, water leaves the lake by either outflow into streams (or below dams), seepage into the ground, or (biggest loss usually) by evaporation into the air.  In areas that are cold enough, lakes and pond can lose water during the winter months to ice, which is mostly recovered in the spring when the ice thaws (some water is lost due to sublimation-evaporation directly from the ice). The amount of water in a lake effects fish in several ways, most importantly: oxygen holding capability, lake temperature, acid/base balance, and swimming/breeding room.  The basic rule: bigger is better.  An increase in water level, as long as nothing bad is brought in with the water, is good for the fish (but this is harder on the angler).  In fact, many species depend on flooding water levels to obtain additional food  (like the carp which raid flooded corn fields), and to reach their spawning grounds.  The more water there is, the more oxygen the water can hold (which means the more pounds of fish it can support), the more stable the water's average temperature is, and the more freedom fish have to seek out areas they are comfortable in, and the more stable the lake's pH level is.   A loss of water in a lake can crowd fish together, let the sun warm the water up, and take away some of the lake's capability to hold oxygen.  Is every loss in water bad? No, it is a common practice on some reservoirs to lower the lake level in the winter to manage aquatic plant growth, and then raise the water in the spring to raise nitrogen levels (from the decomposed plants that were exposed in the winter).  Is every increase in water level good? No, if harmful contaminants are brought into the lake by the water, such as acid rain, or silt from erosion, the lake can suffer.   The bigger is better rule is just a general rule.  See the energy and oxygen balances to see how the water effects these.

(Click on image for full size)

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The Oxygen Balance
(click here to jump to the illustration)

While fish must have water to swim in, they also must have oxygen dissolve in the water to breathe.  While any water, no matter how stagnant holds some oxygen (the surface of the water absorbs some oxygen from the atmosphere above it), fish holding oxygen levels come from either an inflow of oxygen holding water (such as a stream), from artificial aerators (such as in an aquarium or hatchery), or from plants (microscopic and macroscopic).  As stated above in the water balance, the more water a lake has, the more oxygen it can hold (it oxygen capacity).  A second factor in oxygen capacity is the lake's temeprature - the cooler the water, the more oxygen is has the potential to hold.  The more oxygen the water can hold the more mass in animals (fish included) it can support.  Note that a big fish needs far more oxygen then a smaller fish of the same species, and if oxygen in a lake become critical, the big fish are the first to die.  While animals like fish consume oxygen, so do bacteria which decompose dead matter (and turn it into nutrients), and sometimes even plants.
What about the plants?  Thanks to the wonder of photosynthesis, plants in the water (microscopic plants=phytoplankton for future reference) can use sunlight to get oxygen (O2) from carbon dioxide (CO2).  However, there is a downside to plants, especially if there are too many microscopic ones.  While plants do add oxygen to the water if there is light, with out light plants consume oxygen just like an animal.  Large plants like duckweed and eelgrass are very efficient at making oxygen (thanks to their complex make-up), and just as efficient at using it, but single celled plants like green algae, can consume almost as much oxygen as they make.  Most anglers have at one time or another seen a green slime covered lake and noticed a distinct lack of large fish (exceptions: Carp, lungfish, bowfins, and gars which have ways of breathing air in a pinch).  What happened?  If there are too many algae (putting the lake out of balance), at night or on cloudy days, the algae will consume nearly all the oxygen in the water, suffocating the fish.  The phenomena can also happen to even healthy balanced lakes under extreme circumstances.  Some small, shallow northern lakes will freeze over, which in itself is OK, but every once in a while the ice becomes covered with a thick layer of snow, which cuts off light to the plants, and therefore stops oxygen production, also killing fish (a large& deep enough lake will have enough oxygen capacity to support most fish through these times).

How do plants get out of balance? A couple of ways: too few plant eaters (zooplankton, grass carp, etc.) or too much fertilizer (drainage from lawns, gardens, golf courses, farms, cattle pastures, and pig pens are some culprits here).  This phenomena is also explained in the energy balance below.

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The Energy Balance
 (click on image to see full sized)

The third 'leg' of the 'stool' is the Energy Balance.  If either of the other two balances are out of whack (water and oxygen), it will affect the energy balance.  It is the energy balance that is the easiest for humanity to upset, and it is the most delicate of the balances, for it is the balance that moves with the life of the lake.   The energy balance also determines the Tiers (or levels) of the Food Web of the lake (know what's on the food web, and you have a much better shot at catching the fish that live within it!!). 
The energy balance follows three general rules: 1) The closer the item is to getting its food from the sun, the more food energy it has when it is eaten (there is a cost for going between Tiers on the food web (look at the triangle in the illustration), 2) The lower the tier number (Tier 1 is the lowest, 6 is the highest) the more total biomass (sum of all the masses of the living animals/plants in the tier) the tier has, and 3) every body of water has an optimal ratio of biomasses between the tiers for the fish-if it is upset the fish suffer.  The third rule is enforced by the oxygen balance in the case of Tier 1.   Too little sun can kill the plants, starving the zooplankton, which starves the small minnows and shrimps, which starves the bigger minnows and the larger fishes.  Too much fertilizer makes too many plants, which strip out the oxygen in the lake (an example of the oxygen balance enforcing the energy balance), killing the large fish, allowing the little animals to overpopulate.  Now you understand a little about the energy balance, take a peek at the various food webs (expansions of the upside-down triangle in the illustration) for various lakes and ponds.


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(SIMILAR TO  THE GREAT LAKES)(Click here to jump to illustration)

In the large, deep, coldwater lakes of the northern U.S, Canada, Europe, and Asia, neither temperature (as in too warm) or water depth and volume are a problem.  These ecosystems are very diverse and when not polluted, can hold a vast variety of fishes.  In the Great Lakes of North America, the natural fish diversity (after overharvesting in earlier part of the 20th century) has been enhanced by the addition of Pacific Salmon species (Coho, Chinook), and Steelhead Rainbow trout.  Unfortunately, due to international shipping, heavy industry,  and the creation of the St. Lawrence Seaway, the ecosystem has also suffered from both  pollution and the introduction of unplanned species (the sea lamprey, zebra mussel).  While pollution has ben partially curtailed in recent years, the unwanted additions have impacted the ecosystem (though it will be a while before the effects of some species like the zebra mussel are understood).
The first three tiers (0,1,2) are the same as for nearly any aquatic ecosystem, tier 3 hosts a variety of organisms including crayfish and other crustaceans, larger aquatic insects and larvae, the juveniles of larger fishes, and some smaller species of fishes (silversides, small alewives, small smelts, shiners).  Tier 4 is the first Tier that hosts sportfish (yellow perch, pan-sized sunfishes, smaller trout), as well as some other medium sized fishes (burbot, suckers), and larger minnow and herring species (shad, larger alewives, larger smelts, Carp, large chubs, large shiners). Tier 5 hosts the bulk of the gamefish species present, while tier 6 remains for the leviathans like large chinook salmon, large lake trout, muskellunge and northern pike, and sturgeon.

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(SIMILAR TO A ROCKY MOUNTAIN RESERVOIR )(Click here to jump to the illustration)
While the temperature in a mountain reservoir may not be a problem to oxygen levels, it can inhibit plant and fish growth if it remains too close to freezing or is iced over too much of the year (hurting plant growth).  This problem usually is not major for those lakes large enough to allow animals and fish to find comfort zones, and have tributary streams to enhance the food supply, but the ability of plants to grow will determine the size and number of larger fish such as trout and salmon.  The ecology in these high lakes is heavily salmonid (the family name for trout & salmon) based, due to the cold water, and nearly all of the resident fish will use the tributary streams to host their spawning activities and very young hatchlings.  Therefore, the presence of these tributary streams (and how many good spawning beds they hold-i.e. gravel bars) will also determine fish health and size.  The first three tiers are similar to the cold lake scenario above, except that the aquatic insect population plays a larger role in tier 3 due to the 'boom' nature of plant life in the lake (long winter&short summer=short plant growing season). Tier  4 holds the suckers and minnows, the smaller trouts (ex. brook trout) and a small landlocked variety of the sockeye salmon called the Kokanee.  Higher up the food chain on Tier 5 are the larger trouts such as brown, rainbow, cutthroat, dolly varden, and mid-sized lake trout.  At the top of the chain are the largest Lake Trout, and any large salmon stocked in the lake (ex. chinook).
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(SIMILAR TO SMITH MOUNTAIN LAKE IN VIRGINIA) (Click here to jump to the illustration)
Due to the wonders of geography, civil engineering, and geology, one can find an ecosystem that hold both cold water and warm water species in the southeastern U.S. thanks to the Appalachian Mountains.  Smith Mountain Lake in Virginia is an example of this highly diverse ecosystem.  This deep lake has a cold water river as a tributary, and has been creatively stocked by biologists, and as a result holds a wide variety of fishable species.  The depth of the lake, its latitude, and it tributary allow for constant water levels, a long growing season, and enough water for fish to find a variety of spawning habitats and comfort zones.  In the warm summer the cool water species head deep, while in the moderate winter it is the warm water species that use the depth to seek comfort.  The long growing season and abundance of nutrients provided by the river allow an abundance of phytoplankton and zooplankton (Tiers 1&2).  The variety of habitats also allows a wide variety of  Tier 3 crustaceans and small minnows including copepods, crayfish, freshwater shrimp, and gizzard shad, along with aquatic insects.  The variety is reflect up the chain to Tier 4 which holds both white and black crappie, several species of pan-sized sunfishes (bluegill, green, redear, rock bass, orangespotted, pumkinseed), carps and suckers, bullhead catfish,  yellow perch, and larger chubs and shiners.  The variety on Tier 5 owes itself to both the natural variety and stocking: Hybrid and smaller Landlocked striped bass, white bass; smallmouth and largemouth black basses; channel and smaller flathead and blue catfish; smaller muskellunge, and walleye.  Tier 6 is also well populated with larger muskellunge and striped bass, and large flathead and blue catfish.  This is one of those types of lakes that can offer a different fish for each season, and a variety of possible trip scenarios.
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(Similar to Prairie Lake, St. Charles MO, USA)(Click here to jump to the illustration)
The simpler ecosystem for the midwestern midsized lake or pond is familiar to many anglers, and is effected heavily by seasonal and yearly fluctuations of temperate and water level.  It is also prone to boom/bust cycles of predator and prey, and without careful management, can become stunted (stunted=overpopulated with one species to the point that most of the members of the species are severely limited in size due to overcomsumption of available food).  The system is 5 Tiered (excluding the sun), with plants (also subject to seasonal overgrowth) and zooplankton at tiers 1&2. To help control the plants on Tier 1 many lakes are stocked with sterile Grass Carp which can eat up to five time their weight in plants each day.   Tier 3 holds the basic food base for almost the entire fish population, with crustaceans, mollusks, and juvenile or smaller fishes (including mosquito fish).  The bulk of the fish are on tier 4: pan-sized sunfishes like bluegill and crappie, smaller catfishes such as bullheads and madtoms, and midsized channel catfish and largemouth bass (8"-15").  The fish food chain tops out in Tier 5 with the 'pondmasters', i.e. the largest 5% of the largemouth bass and catfish.

(Several near Miami, FL, USA) (Click here to jump to  illustration)
If you every wanted to know what happens when you mix tropical 'aquarium' fish and everyday gamefish, take a peek at the canals, lakes, ponds, and swamps of south Florida.  These waters are essentially a mix of the midwestern pond scenario above, with a sprinkling of tropical Ciclids from Africa and South America.  While the butterfly (and/or peacock) bass were added with forethought, the Oscars and Tilapia (Mozambique and Blue) were accidental.  The Oscar, as a small but aggressive predator, has not forced out many of the native sunfishes (bluegill, Crappie, redear), while the other accidental addition-the Tilapia-have had severe consequences.  As a true omnivore ('all eating' literally-they eat plants, other animals, and scavenge) and mouth breeder (they defend their eggs by hiding them in their mouths) they have impacted the central 3 Tiers of the ecosystem.   They compete with zooplankton for phytoplankton and plants, they compete with juvenile bass and sunfish for the zooplankton, and the largest tilapia compete with the midsized basses, larger minnows, gar, and full sized sunfishes for small minnows (mosquitofish and mollies), crustaceans (freshwater shrimp and crayfish), and mollusks.  While the bulk of the tilapia diet is vegetable, they like to occupy some of the same spawning beds as bass and sunfish.  On the plus side, the tilapia taste good to both people and to larger predatory fishes like the the Florida strain of the largemouth bass (truly beastly bass- up to 25lbs+) and large Florida gars.  Another interesting side effect is that the high speed growth of Tilapia allow for their commercial harvest in some lakes, and plant control in other lakes (which partially restores the natural balance).  This potpourri also has a benefit for the Eastern US angler- an exotic fishing experience without a needing a passport!

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