***reviewed by : Muhammad Fahri 2009
It is well known to avid fishermen that fish have different appetites. Some are strictly plant eaters feeding on algae. These are the herbivores. The gizzard and threadfin shad are of this type and are rarely, if ever, taken on hook and line. Some are strictly meat eaters. These are known as carnivores. The bluegill bream, eating worms, insects and larval forms, are carnivorous. Some meat-eating fish have a preference for other fish and must have them in their diet to reach their maximum size. These fish are the piscivores and include the largemouth bass, striped bass, and crappie. It might be noted here that a bream may occasionally eat a smaller fish or minnow, but he does not have to have these in his diet to achieve his maximum growth. Some fish prefer a diet of meat and vegetables. The sucker which seems to prefer algae, will and does also eat a good number worms, and therefore can be taken on hook and line by the fisherman with red wigglers. He is omnivorous.
Within a fish population, as a result of the different appetites of the fish, they perform different functions. The smaller fish in the population such as young bluegill must by necessity be eaten by the larger ones such as largemouth bass. Otherwise, these smaller ones will become too numerous, will not have enough food to go around and will stunt or stop growing. These small bluegill are therefore food for the larger fish and are designated as prey.
These fish, which fall prey to the larger fish and are small enough to be handled by the average size piscivore are termed forage fish. A one-pound bluegill is not considered forage, because he is too large to be consumed by an average size (two or three pound) largemouth bass. The bass himself plays the role of predator, a role, which is lacking will lead to small undesirable bluegill populations, and nobody enjoys catching small fish.
Also in the fish population we have fish, which might be thought of as constituting a clean-up detail. These are those (and are not essential in the population), which feed on scraps and bits of dead organic matter such as dead fish. The fish performing this function are known as scavengers and are represented by the catfish family. If they are not present to clean up, bacteria and fungi will take care of the organic matter. The scavenger then, although a part of most wild fish populations, is not an essential component of the fish population.
The Aquatic Ecosystem
The aquatic environment of the fish is an excellent example of what is known scientifically as an ecosystem. An ecosystem is defined as a unit of living and non-living components in which the exchange of materials between living and non-living follows a circular path. The living portion of the ecosystem is part of the biosphere and includes all living organisms in the system. The non-living portion of the eco-system is part of the geosphere and includes the water and soil and all of the components dissolved or contained in them. The ecosystem is a self-sustaining unit.
Within the system there are three categories of living organisms according to the function, which they perform. These are producers, consumers, and decomposers.
The producers are green plants, the most important of which in aquatic habitats are small free-floating green algae call phytoplankton. These give a bright green color to very productive waters. Phytoplankton contain chlorophyll and are capable of absorbing the energy of the sun through the process of photosynthesis. It is this absorbed energy, which makes the body of water productive. The more green plants, the more energy absorbed and available to the pond and the more pounds of life, which can be supported.
The consumers include the animals from the very small protozoans (one-celled animals) to small many-celled animals, insect larvae, small fish and large fish. The number of consumers present depends upon how much is produced for them to consume. The producers again determine this.
The decomposers act upon the dead organisms within the body o9f water such as the dead phytoplankton, insects, and fish. They decay the organisms, break them down, release the elements and compounds from them, and return them as dissolved materials to the water. These materials can again be used by the producers to produce. If it were not for the decomposers, all of the essential nutrient materials would be tied up in the bodies of living organisms and no longer available to the producers.
It should now become apparent what is meant when we say that in an ecosystem the exchange of materials between living and non-lining follows a circular path. The nutrient materials dissolved in the water (a part of the geosphere) are used by the producers, become a part of their living substance and thus become a part of the biosphere. These are eaten by small animals, which in turn are eaten by larger animals and so on up the line. The nutrient materials are passed from one consumer to another and remain a part of the biosphere. When these plants and animals die, they are acted upon by decomposers and the materials are returned to the geosphere. They can then again be utilized by the producers and become a part of the biosphere. Thus, a circular path is followed in the exchange of materials between living and non-living.
The role of nutrient materials in the aquatic ecosystem should now be apparent. The number of little green algae present is dependent on the concentration of nutrient materials. The more nutrients, the more phytoplankton. The more phytoplankton, the more energy absorbed. Ultimately, more living organisms, including fish.
Each body of water has a maximum number of pounds of organisms, which it can support at any given time. This poundage constitutes the carrying capacity of this body of water. This should not be confused with what is actually present and is known as the standing crop. This may be less than the carrying capacity.
The basic determining factor of carrying capacity is energy, which comes from the sun. The amount of energy, which strikes a given body of water, can have little effect on carrying capacity unless this energy is absorbed and retained by some component. Again, the role of phytoplankton becomes apparent. It absorbs the energy of sunlight and makes it available to the other organisms, which are present.
You might think of all of the light striking a pond or lake as total energy, which is available for absorption. This is significantly more than is actually absorbed. The amount, which is absorbed, might be thought of as phytoplankton energy. This is now available to other life forms. The organisms, which feed directly on the phytoplankton, are known as primary consumers. The energy of the phytoplankton is available to them. The other consumers, which feed on up the line, are known as secondary consumers and the energy is passed to them. Since each category of consumer uses some of the energy which he obtains for his own consumer lever. This can be well depicted as a pyramid showing total energy, absorbed energy, and energy available at each consumer level up to the last group of consumers which might be, in a fish population, the largemouth bass, designated as top carnivore.
Carrying capacity can be manipulated. We do it each time we fertilize a fishpond. We put nutrient materials into the water and by so doing, increase the number of phytoplankton organisms. This increases the energy absorbed and the amount of energy available to each consumer level. The pyramid, which is discussed above, would have a much larger foundation, and larger layers. More of each type of organisms can be supported and we have manipulated (increased) the carrying capacity of the habitat.
If a fishpond with a specific carrying capacity is fertilized, the carrying capacity is increased and the population present in the pond will expand to the increased carrying capacity.
Ponds and lakes, much like people, have different characteristics and personalities. It is difficult to find two aquatic habitats, which are exactly alike. They do, however, have characteristics in common.
Very definite zones from edge to middle can be found in the deeper lakes and ponds. The first zone encountered, as one moves from the edge outward is the littoral zone. This is the shallow water zone with light penetration to the bottom. Rooted plants, such as water lilies, sedges, rushes, etc., typically occupy this zone unless there is phytoplankton bloom sufficient to keep sunlight from reaching the bottom. There are numerous insect larvae and burrowing type worms in this zone. The small bluegill bream feed in this zone and as a result, largemouth bass move into this area, cruise the edges and feed on them.
As you move on out toward the middle, the water becomes deeper and the light rays no longer strike the bottom. The open water zone, to the depth of effective light penetration (effective enough for photosynthesis), is the limnetic zone. Organisms in this zone include free-floating types and free-swimming types. Large and small bream and bass may be found in this region. In large reservoirs, shad are usually always found in this zone and as a result, predaceous species such as largemouth and white bass are also found here. It might be pointed out here that shad are detrimental to a bluegill bream population. They school in the open water region away from the littoral zone and are a favorite prey of predators such as the largemouth. Since they travel in schools and are easy to catch, the largemouth moves away from the shallow water regions to the schooling shad. The bass then neglect the small bluegills and they become overpopulated and stop growing. This is the case in many reservoirs with shad populations.
The deeper region below the depth of effective light penetration is known as the profundal zone. In very deep lakes there is no oxygen here during the summer months so the only organisms occupying this area are those, which are very special and require no oxygen. Some larval forms of life fall into this special category. If the lake is not so deep, oxygen-requiring forms such as catfish may be found in this area.
Distinct regions or zones can also be encountered by moving from the surface to the bottom of a deep lake. This is particularly true of deep lakes during the summer months.
During the summer months, the upper waters become warmer than the deep waters. As the temperature continues to rise on the surface, the two regions, the upper and deeper waters, become definitely separated with a zone. This zone is known as the thermocline. This area serves as a barrier to the mixing of warm surface waters and cold bottom waters, just as oil and water won’t mix. The upper warmer water is known as the epilimnion. The water in this area circulates freely, contains a sufficient supply of oxygen, and will therefore support a considerable number of living forms. The temperature of the water in the epilimnion is relatively constant.
The deeper colder water below the thermocline is known as the hypolimnion, and is usually below the level of effective light penetration. Oxygen is depleted rather rapidly in this area through processes of decay, etc., and since the green plant and surface source of oxygen is cut from this region, it remains devoid of oxygen during the summer months. The temperature remains relatively constant in this area with only a slight drop in temperature from top to bottom of the hypolimnion.
While the temperature is relatively constant in the epilimnion and hypolimnion, it is not in the thermocline. By dropping a thermometer down from the surface through all of these layers of water, you will find that the temperature is relatively constant in the epilimnion. You can readily tell when you have reached the thermocline. There is a rapid temperature drop of one degree centigrade per each meter of water. When the thermometer reaches the hypolimnion, the temperature drop is again very gradual and remains relatively constant as it goes deeper into it.
A lake in this condition is said to be thermally stratified. This condition is due to water temperature. The oxygen is restricted to the upper warmer waters; therefore, this is where you should fish. Since there is no oxygen below the thermocline, there are no fish present. The fact that there is a difference in oxygen concentration in these regions can also be demonstrated by the use of a water sampling bottle lowered into them and water collected from each.
It should have become apparent by this time that there are factors which limit the success of fish in a body of water. One of these has already been mentioned several times in this discussion. It is oxygen. Fish have different levels of tolerance for oxygen concentration. Some can live at oxygen levels, which are far too low for other fish. Mudfish, for example, survive at low levels of oxygen, which will be fatal to most other fish.
Oxygen depletion is one of the major causes of fish kills. The rapidity with which oxygen is depleted has a decided effect on the extent of the fish kill. If the oxygen concentration drops slowly, the fish can adjust and may survive in concentrations as low as two parts oxygen per million parts of water. If, however, the dissolved oxygen in the water drops rapidly and there is no time for adjustment, most fish will begin to die at a concentration of three parts oxygen per million parts of water.
Pollution is a major cause of oxygen depletion. As organic wasted and other pollutants are emptied into streams, they must be decomposed. The decomposition process carried on by aerobic bacteria and fungi requires oxygen.
An overabundance of organic matter causes the oxygen to be used up rapidly and almost completely. As a result, other forms of life suffer.
While carbon dioxide is also a limiting factor, very little is known about its role as such. High concentrations are definitely limiting to fish, especially since high carbon dioxide concentrations are associated with low oxygen concentrations. Fish react vigorously to high concentrations and will be killed if the water is too heavily charged with it.
Degree of acidity or alkalinity (p.H.) is also a limiting factor. Again, some fish can stand greater acidity or alkalinity than others, but there are limits of tolerance. Gradual changes are not as severe as rapid changes. This is a factor, which must be considered when changing fish from one habitat to another. Even though survival of the fish may be satisfactory when subjected to such a change, reproduction may be decreased or cease completely.
Salinity or concentration of salt is one of the more obvious limiting factors. Some fish can survive in either salt or fresh water and do not realize this as a barrier. Others, however, are strictly either fresh or salt water fish and only a slight change can lead to fatality. Increased salt concentration is often the cause of fish kills in ponds along coastal areas following extremely high tides which causes a spilling over of salt water into the ponds.
Turbidity is a very definite limiting factor. Turbid water is muddy due to soil particles not heavy enough to settle out. It has an effect on fish in several different ways. It cuts down on productivity by decreasing the amount of light available to phytoplankton. This makes it difficult for predaceous species to catch prey, so both predator and the pre suffer. If it becomes severe enough, it can be fatal to fish by covering the gills and preventing oxygen and carbon dioxide exchange.
The running water habitat—river, stream, creek, or branch—is obviously quite different from the pond or lake habitat and has one primary characteristic which does not exist in the type habitat which we have discussed previously. This factor is current.
Current in the running water habitat limits phytoplankton, green algae, which are the chief producers in ponds and lakes. Attached algae and various types of bacteria are the primary producers here. Food organisms for fish, such as larval forms, must have special adaptations. Some of the adaptations of organisms living in running water are: permanent attachment to a firm substrate, hooks and suckers, sticky undersurfaces. Streamlined bodies, flattened bodies and positive rheotaxis (orientation upstream).
In small streams such as are found in much of Georgia, there are two major zones. One of these is the rapids zone where shallow water with velocity of current great enough to keep the bottom clear of silt and other loose materials provides a firm bottom to which organisms may attach. The other is the pool zone, deeper water where velocity of current is reduced, and silt and other loose materials tend to settle to the bottom, thus providing a soft bottom unfavorable for attachment, but suitable for burrowing.
Fisheries managers collect fish to determine: population numbers; species composition of the population; length frequency (the distribution of lengths of a species); relative condition (measure of the well being of a fish); age composition of the population; fish growth; and to mark fish for later recapture (to determine total, fishing, and natural mortality rates, growth, movements, population estimates). Fish are also collected in the wild to be used a brood fish in hatcheries. The following are the most commonly used techniques to collect fish. It is important to understand that each collection method has its own inherent bias. In other words, seines are not an effective method to collect open water species (threadfin and gizzard shad, white bass) in reservoirs. Electrofishing does not collect species deeper than 15-20 feet. Therefore, fisheries managers use several techniques to gain an overall picture of the fish population.
Gill nets: Gill nets are vertical walls of netting set out in a straight line. Capture is based on fortuitous encounter with the net. Fish are captured when they become wedged, gilled (mesh slips behind the operculum), or tangled (fins, teeth, and/or spines) in the net. Most often, fish are gilled. Gill nets can be set in many different ways depending on the species desired and the habitats involved. The most common set is the shoreline bottom set where the net is anchored on shore and on the opposite end, perpendicular to shore. They are usually set in late afternoon and retrieved (or fished) early the following morning. Species selectivity (mobile fish the reside in the area of the net are more susceptible), size selectivity (fish of some optimum size are held most securely in a certain mesh size while smaller or larger fish are less likely to be caught), mesh material (monofilament is most efficient) can all influence gill net efficiency. Fisheries managers use experimental gill nets with panels of several mesh sizes to reduce the effects of size selectivity. Most fish captured by gill nets are dead by the time the net is fished (retrieved).
Seines: Seines are mostly used to collect young-of-year and juvenile fish. They are sometimes used to collect adults, but are not there are more effective methods for that. Seines are effective in shallow water, along the shoreline. Seines can be used in streams, but flow and an uneven or gravel bottom can make this difficult. The 15 foot minnow seine is used by Fisheries managers to collect young-of-year fish to develop management recommendations for pond owners. As with gill nets, mesh size determines the size of fish captured by seines.
Electrofishing: Electrofishing, is the use of electricity to capture fish. Many factors, such as fish size (larger fish are more vulnerable), water depth (deeper fish are less vulnerable), water conductivity (affects the size of the electrical field), water temperature, water clarity (affects ability to see stunned fish), substrate (mud and silt reduce current density), and habitat (species which inhabit shoreline habitat, such as sunfish, bass, and minnows, are generally more vulnerable than open water or bottom species like threadfin and gizzard shad, hybrid bass, and catfish) affect electrofishing efficiency. Electrofishing is not an effective method to collect fish from saltwater. Electrofishing units can be boat, barge (used in streams too shallow for a boat and too large for a backpack), and backpack mounted. Electrofishing causes temporary paralysis and unconsciousness in fish that enter the electrical field, causing them to float to the surface where they can be dip-netted. Stunned fish usually recover within a few minutes. Electrofishing is safe for the vast majority of fish collected.
Creel census: Fisheries managers interview anglers to learn: fish species targeted by anglers, number and sizes of fish caught and released or harvested, the length of time, how often and where someone fishes, fishing methods used, likes and dislikes about their fishing experience, opinions on issues, how far they travel to fish, and how much they spend on fishing. This information is used to calculate the effort (man-days) expended on a particular body of water, angler success and harvest, and which species are targeted by anglers. It also provides insight into what anglers like, dislike, desire, and spend.
Fish toxicants: Fish toxicants are used to collect all the fish within a given area. Obviously, fish are sacrificed by this method. Rotenone is the most commonly used fish toxicant. It is an extract from rotenone-bearing plants. Rotenone is used to remove undesirable fish or for a complete kill. Fisheries managers often kill a small area and then expand that information to represent the entire body of water. Rotenone kills fish by blocking oxygen uptake, and the fish suffocates. In recommended concentrations, it is reasonably specific to fishes. Fisheries managers will measure the intended sample site to determine the volume of water to be treated, inject the rotenone into the water column, and collect fish as they struggle to the surface or float following death. Fish are collected for several days following rotenone application. Rotenone looses its toxicity in several days under natural conditions, but potassium permanganate can be used to detoxify rotenone quickly. Rotenone is also used to perform a partial kill to remove undesirable fishes.
Managing and sampling stream fish populations pose unique problems for fisheries managers. Streams are open systems where fish can migrate upstream and downstream. Streams cross state and national boundaries making management especially challenging. Current creates conditions where fish collection methods are more difficult, if not impossible. There are over 70,000 miles of warmwater and coldwater streams in Georgia which makes managing all of them a daunting task. In order to truly manage a stream, you must manage the entire watershed.
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