Natural lakes, which abound on the earth were formed by tectonic, volcanic, glacial, or other phenomena. The Pleistocene glaciations were by far the most important of the lake-producing processes. In more recent times, however, man has created numerous impoundments and reservoirs for a variety of purposes. In the United States alone there are 1,320 reservoirs with a surface area of 3.2 million ha (8.8 million a). A reservoir is defined as an impoundment with a mean annual minimum pool of 202 ha (500 a).
Lakes are exploited for sport and commercial harvest of fishes. In many fresh-water lakes, sport fishing is now the predominant method of fish harvest. Annual nationwide sales of state fishing licenses in the United States during the period from 1933 to 1967 brought in revenues ranging from 6.7 million US dollars in 1933 to $73.3 million in 1967. Data from 1965 indicate that fresh-water fishermen, 24 million strong, spent on the average of $89 per year per person. It is estimated that fresh-water fishing alone generated $2.1 billion of gross business activity in 1965. Estimations indicate that by the year 2000 sport fishing will be 78 percent greater than in 1965.
The tremendous fishing pressure being placed on many lakes necessitates the development of a sound management program to ensure a sustained yield of desirable fishes. What constitutes an undesirable population of fish, which may need to be controlled, depends on the tastes of local fishermen and the management philosophy of the fishery biologist. Often changes in a particular lake may shift a normally desirable species to an undesirable one. For example, in many lakes bluegill overpopulate and become stunted. This highly sought species then becomes undesirable to the sport fisherman because of small size.
Solman (1950) and Rounsefell and Everhart (1953) have listed the major reasons given for the control of undesirable fishes. Major problems may be natural or man-made. In the northern United States the fish in many shallow lakes winterkill due to extended periods of snow cover. In most cases the kill of fish is not complete, with only the less desirable species surviving. To provide a “balanced” fish population, it may be necessary to eliminate the remaining fish population and restock the lake. Some lakes may harbor fish populations so heavily parasitized that they are unacceptable to the fisherman. These populations can be removed and the lake restocked (Webster, 1954).
Man has created some of the problems through mismanagement of aquatic resources. The introduction of exotic or non-native species may cause destruction of native fauna through predation or establishment of fish which are highly competitive and prolific. Both legal and illegal stocking have caused many of the pest fish situations. Introduction of yellow perch into brook trout waters by fishermen destroyed quality trout fishing in a number of lakes. Disposal of pet goldfish also has brought about imbalances in fish populations always in favor of the pest fish. The widespread planting of carp in North America destroyed valuable waterfowl habitat and in numerous cases introduced a vigorous competitor which soon became the dominant species in the lakes. Introduction of certain species of Tilapia into some Rhodesian waters also has caused problems by reducing needed aquatic vegetation (Junor, 1969).
Management of waterfowl habitat may require removal of some or all of the fish in order to prevent predation on young waterfowl by such species as the northern pike (Solman, 1945). In North America much effort also has been placed on carp control to preserve aquatic vegetation for waterfowl (Weier and Starr, 1950).
Intensive, selective removal of predator species such as northern pike by anglers may result in circumstances favorable for a population explosion of forage species. Unless the proper predator-to-prey ratio is re-established, the population will become unbalanced and sport fishing will decline in quality.
Development of lake shore property for residences often has contributed to upsetting the ecology of lakes. Many land developers filled in marsh areas to increase available land. In most cases this destroyed valuable spawning habitat especially for northern pike, a prime predatory species. In addition, home or cottage owners put in sand beaches which provided additional spawning habitat for such species as bluegill which quickly overpopulated these lakes.
Discharge of human wastes from dwellings further adds to the problem by heavily fertilizing the lakes, thereby causing deleterious increases in aquatic vegetation. Dense aquatic weed beds afford protection from predation for the small, overpopulated forage species. These weed beds also reduce the efficiency of fishermen. In many lakes the aquatic vegetation itself becomes a problem, and must be controlled so that boating and fishing can be enjoyed again.
Bennett (1962) and Rounsefell and Everhart (1953) have discussed various methods used to control undesirable fishes, but have judged them to be inadequate. These include netting, weirs, biological controls (stocking of predators), electrical fishing, aquatic vegetation control, water level manipulation, and fishing regulations.
The use of nets and traps has met with indifferent success, because an inadequate fraction of the population is removed. It has been estimated that more than 80 percent of a stunted population must be removed from a lake before any measurable change can be achieved.
Stocking of predatory species should be part of an integrated control program. If enough stunted forage fishes are not removed before the predator species are introduced, corrective stocking usually fails. In many cases the predatory species are vulnerable to fishing, hence stocking must be a continuous program or suitable spawning habitat preserved or created to sustain the predatory species. A long-term stocking program must be integrated with on-going hatchery production. Hatchery production of warm-water predatory species such as northern pike still is somewhat limited in scope. Future demands undoubtedly will require improvement and expansion of warm-water hatchery facilities.
Many small artificial lakes can be drained in order to control the composition of the fish population. Provisions must be made for temporary liveholding of desirable species collected when the pond is drained. The cool days of early spring and late fall are best for draining operations, because fish can be handled at these times with minimum loss. Because most lakes cannot be completely drained, it may be necessary to treat water pockets or channels with toxicants to finish the job.
Planned fluctuations of water level in reservoir lakes can be used to 1) destroy spawn; 2) strand the young of some species, especially centrarchids; and 3) crowd species so that density stresses will increase and situations for increased predation will be maximized. Also, a vegetative crop can be planted on the exposed lake bed to provide food for waterfowl, or to recycle nutrients that are found in bottom soils.
Liberalizing angling regulations to permit the harvest of less desirable species by such methods as spearing, bow and arrow hunting, and SCUBA spearfishing has had little effect on controlling pest species.
Several state agencies have initiated programs to encourage utilization of less desirable species of fish for food and sport. Leaflets describing the preparation and cooking of such species as carp and suckers have been printed, and numerous articles have been written praising their food and game qualities. The impact of these information campaigns has been less than encouraging.
Chemical toxicants provide the only economical and effective way to control or remove undesirable fish populations from undrainable bodies of water. This is particularly true if complete removal of the fish population is desired. Prévost (1960) states it this way “Attempts to remove coarse fish with fishing gear or by other physical methods have generally been unsuccessful; even the drainage of a farm pond, unless there is complete drying-out, generally fails to remove all the fish. Partial removal of coarse fish could sometimes bring improvement, but the results are usually only temporary since most of the coarse fish are very prolific and the lake will resume its maximum capacity with them in a very short time. Thus, unless there is a complete removal of the fish, the coarse fish continue to be troublesome”.
The first recorded attempt to eliminate undesirable fishes from salmonid habitat was that of Titcomb (1914). He applied copper sulfate to Silver Lake in Vermont in 1913 to remove northern pike, chain pickerel, walleye, yellow perch, and bullheads. However, two applications failed to give a complete kill as some northern pike survived. He attributed the failure to the rapid precipitation of copper sulfate. From 1934 to 1938 four head=water lakes in Nova Scotia, Canada, were treated with copper sulfate to remove undesirable species from brook trout habitat (Catt, 1934 and Smith, 1935, 1938, and 1940). In reviewing this work Smith (1950) concludes that although poisoning may eliminate or reduce competition and predation from undesirable species, usually white perch and yellow perch, the extent of improvement in production of desired game species will be conditioned primarily by the productive capacity of the waters.
Eradication of undesirable fishes improved and expanded greatly with the introduction of rotenone as a fish toxicant. The first recorded use of rotenone in the United States was in 1934 in the State of Michigan (Ball, 1948). Two small ponds were treated to remove populations of carp. In that same year, Eschmeyer (1937) treated South Twin Lake, Michigan to remove a population of stunted yellow perch. He poured the toxicant (powdered derris root containing 5 percent of rotenone) on the surface of the water in the wake of an outboard motor. In an attempt to get the toxicant thoroughly mixed into the lake, 100 sticks of 40-percent dynamite were discharged in deep water. This method of toxicant dispersal was unsuccessful, however, as not all perch were killed.
Canadian workers taking a lead from Eschmeyer applied rotenone to obtain a disease=free water source for a new hatchery (M'Gonigle and Smith, 1938). The use of rotenone in several formulations then spread rapidly throughout the United States and Canada (Krumholz, 1948; Solman, 1950; Smith, 1950; Prévost, 1960; and Stroud and Martin, 1968).
In the 1940's, chlorinated hydrocarbon insecticides came into wide usage and some fishery managers feared that they would have dire environmental consequences. Surber (1948) found that toxaphene killed most species tested at 0.02 mg/l (ppm). Lawrence (1950) confirmed the toxicity of toxaphene to fish. The first field trial using toxaphene as a fish toxicant was conducted in Arizona in 1951 by Hemphill (1954). Lyman Reservoir was treated at 0.1 mg/l (ppm) to remove carp. This experiment had only partial success due to 2 days of heavy rain which diluted the toxicant and stopped the kill.
Canadian workers first applied toxaphene in 1956 (Stringer and McMynn, 1960). Eventually it was more widely used in Canada than in the United States (Stroud and Martin, 1968). In most cases toxaphene was chosen over rotenone because of cost. Toxaphene, however, usually rendered the lakes toxic to fish for periods up to 3 years. Canadian workers also experimented with endrin and Thiodan as fish toxicants (Prévost, 1960). Neither of these compounds has been used to any great extent in North America.
Removal of undesirable fish from hatchery water supplies was first reported by M'Gonigle and Smith (1938). Armstrong (1949) reported using calcium hypochlorite (HTH) to destroy the fish population in the water supply of the Dorian Rearing Station, Ontario. The chlorine generated by HTH in the water was neutralized by adding sodium thiosulfate at the diversion dam. Calcium hypochlorite applied in a heavily weeded pond and in a small trout lake in southern Ontario proved ineffective (Smith, 1950). More recently, Silver (1962) worked with chlorine in New Hampshire waters. It offers little or no advantage over rotenone.
Sodium cyanide as a fish toxicant was suggested by Bridges (1958). He also found that the cyanide could be employed to collect live fishes for laboratory or other uses. Lewis and Tarrant (1960) followed up the work of Bridges, and suggested that sodium cyanide is useful in preparing rearing and brood ponds. The compound has been used in fish removal operations in at least three states (Stroud and Martin, 1968).
Derse and Strong (1963) first suggested the antibiotic antimycin as a fish toxicant. Walker, Lennon, and Berger (1964) followed up this suggestion with laboratory and preliminary field studies. Additional field trials of antimycin in lakes and streams were conducted by Gilderhus, Berger, and Lennon (1969). Antimycin, marketed as Fintrol by Ayerst Laboratories, Inc., has been applied successfully in a wide variety of fish habitats, both marine and fresh-water (Finucane, 1969 and Lennon, 1970a).
The use of organic phosphorus pesticides in fish control also has been reported. Hoff and Westman (1965) found that a 3:2 mixture of Dibrom and malathion applied at the rate of 0.1 mg/l (ppm) appears to offer an effective and practical method for controlling bluegill and pumpkinseed populations in many warm-water ponds.
Although most lake reclamations have been accomplished in cold water, complete renovation, partial poisoning, and selective poisoning also have been used in warm-water lakes with varying degrees of success. Hooper and Crance (1960) have shown that in Alabama lakes partial poisoning with rotenone is an effective and economical method of restoring population balance in bass-bluegill-redear populations. Partial poisoning is ineffective, however, if bullheads and/or crappie contribute to the overcrowding problem.
Selective poisoning of gizzard shad, first used in the 1950's (Bowers, 1955), is a technique that has contributed greatly to reservoir management in southern United States (Zeller and Wyatt, 1968). This management tool, if properly used, improves fishing success at a reasonable cost for periods of 3 to 5 years. Another example of selective removal of a target fish is the work of Radonski (1967), who eliminated yellow perch from a soft, acid lake in Wisconsin while leaving the rest of the fish population intact.
Removal of undesirable fish populations from European water has been accomplished with rotenone in Denmark, United Kingdom, Finland, Ireland, and Sweden, with toxaphene in Germany, and with polychlorpinene (PCIP) in Germany and the Union of Soviet Socialist Republics (Larsen, 1961; Tuunainen, 1970; Almquist, 1959; Anwand, 1968a, 1968b; Schäperclaus, 1963; and Burmakin, 1965 and 1967).
Rotenone apparently was introduced in Europe as a fish management tool by Swedish workers in the 1950's. In the late 1950's and early 1960's use of rotenone spread to Denmark, Finland and Ireland. A change in the law of the United Kingdom in 1965 makes it legal to use rotenone as a fish toxicant there.
The unavailability of rotenone preparations led Russian investigators to research on toxaphene, PCIP, and saponins. Toxaphene and PCIP also have been employed in Germany, but German investigators indicate a high degree of reticence about introducing long-lasting chlorinated pesticides into their fish-producing lakes. Again, as in North America and the U.S.S.R., the cost of the chemical becomes the overriding consideration for use in spite of the detrimental effects on the environment.
South American countries also have used rotenone for fish control. Fontenele (1963) records the prodigious effort made in Brazil to control piranhas. In the Northeastern state of Ceara alone, over 965 km (600 mi) of the Acarau River were treated with 4.5 metric tons of timbo powder. The cost of this program was more than compensated for by increased income from fishing licenses generated by the absence of piranhas. In addition, there was reduced danger of physical injury to fishermen as well as an increased yield of fish.
One early example of good planning and execution of a chemical reclamation of a lake was described by Vestal (1942). His report includes the five essential steps in any fish control program: 1) definition of the problem; 2) pre-treatment survey including bioassay; 3) treatment application; 4) post-treatment survey; and 5) management follow-up. The historical development of methods used in reclamations can be found in reports by Siegler and Pillsbury (1946), Krumholz (1948), Hayes and Livingstone (1955), Zilliox and Pfeiffer (1956), Prévost (1960) and Meyer (1966).
A biological survey is necessary to establish the composition of the fish population in any particular body of water. Adequate sampling by netting and/or electrofishing should indicate the relative abundance of desirable and undesirable species. On the basis of this information, a decision can be made as to whether or not the population requires further management. If, for example, 80 to 90 percent of the fish population were undesirable fish and fishing were considered poor, then management action would be necessary to restore quality fishing. A decision to renovate the body of water usually engenders further work such as: 1) contacting all property owners bordering the water to obtain their permission to do the treatment; 2) determining the uses of the water so that all regulatory agencies involved with the body of water can be contacted and clearances approved for chemical treatment; 3) arranging for disposal of the dead fish for health or aesthetic reasons; and 4) preparing a detailed operational plan that includes a) manpower needs, organization, and responsibility; b) equipment needs; c) division of the target area into units to expedite treatment, based on the unit's water volume; d) allocation of the chemical to the various areas, based on water volume and 3) a pre-treatment briefing to explain the operation and to assign duties.
Routine analyses of water quality should be made on the body of water to be treated. In particular the temperature profile and pH should be measured. Diurnal fluctuations in pH also should be recorded so maximum effectiveness of the toxicant can be realized.
Careful estimates of the volume of water in a lake or reservoir are essential to ensure that the proper amount of toxicant is applied. If a survey is necessary, adequate soundings must be taken to estimate the volume. A depth contour map should be plotted for larger lakes to determine the volume of water in each area assigned for treatment. All inlets and outlets to the lake, springs in the lake, and potholes or ponds in the lake drainage should be taken into consideration in the treatment planning. Areas of heavy aquatic vegetation should receive special care to ensure that the toxicant is properly dispersed.
Actual choice of a toxicant is governed by the following considerations: 1) is it registered and cleared for use in aquatic habitats by the governing regulatory agencies; 2) is it commercially available; 3) is the cost justifiable in terms of the return on the investment; and 4) are there proper equipment and trained personnel to handle it. The toxicant selected should be one that will degrade in as short a period of time as possible and leave no toxic residues, especially those which would magnify in the food chain. In some cases, it may be necessary to detoxify the chemical by means of an oxidizing agent such as potassium permanganate.
In reviewing past reclamations it is readily apparent that improvement in techniques in toxicant application was associated with improved formulations of the toxicants (Smith, 1950; Solman, 1950; Prévost, 1960; and Lennon, 1970a). Solman (1950), in discussing the developments in rotenone poisoning, indicated the rapid advance in technology from the powdered derris root, to the wettable powder, to the emulsifiable formulation was accompanied by a shift from towing burlap bags, to gasoline-powered pumps, to airplanes and helicopters for applying the new formulations. Lennon (1970a) described the formulations of antimycin available for fishery uses. These formulations are the most sophisticated on the market to date.
Toxicants in powdered formulations must be mixed with water before they can be applied. Unless the workers are provided with proper safety equipment, such as masks, the powder can present a real hazard in terms of dermal irritation and inhalation exposure to the toxicant.
Antimycin formulated on sand (Fintrol) can be applied by dumping the sand in the propeller wash, by using a seed spreader (hand- or electric-powered), and by helicopter. The usual safety precautions should be followed when applying Fintrol, especially the use of protective goggles.
Liquid formulations of toxicants are poured into the propeller wash of an outboard motor, sprayed out or pumped into deep water using a centrifugal pump, or sprayed from shore, boat, airplane, or helicopter. In the latter cases, care must be taken to use formulations having carriers that will not evaporate before the toxicant reaches the water. Most fish toxicants used today are rather water insoluble so that if the carrier is lost before reaching the water, the toxicant fails to do the job.
Both rotenone and antimycin can be detoxified with potassium permanganate (Lawrence, 1956; Barrows, 1957; Hester, 1959; Berger, Lennon, and Hogan, 1969; Gilderhus, Berger, and Lennon, 1969; and Loeb and Engstrom-Heg, 1970). However, there are no reports of potassium permanganate being used on any large body of water to detoxify either toxicant. Cohen et al. (1960), Cohen, Rourke and Woodward (1961), and Cohen et al. (1961) have shown, however, that rotenone can be used to reclaim public water supplies because it can be removed from raw water by activated carbon.
Attempts at evaluating the completeness of kill have involved 1) visual sightings of fish both from the surface and by use of SCUBA; 2) netting; 3) post-treatment bioassay of lake water; 4) re-treatment with a toxicant; 5) creel census; 6) draining; and 7) electrical fishing; or 8) a combination of the above.
Zilliox and Pfeiffer (1956) gave the following criteria for a complete kill: “Failure of observation, angling and netting for two successive years following reclamation to indicate any species of fish present in a reclaimed pond, except stocked trout, would appear to be a reasonable indication of a complete kill”. In 1960 the same authors indicated that this held true for non-native introduced species but not for competitive native species.
It seems reasonable to assume that a combination of the above mentioned techniques should be used to assess the reclamation effort.
If the toxicant used does not kill fertilized eggs of the target species, then the reclamation must be accomplished after the eggs hatch. All adult target fishes may be eliminated and the reclamation considered an apparent success, but the fertilized eggs could be a source for rapid re-population of the body of water. Because of this, it is necessary to define the life cycle of the target organisms in the pre-treatment survey.
There are five major factors involved with the success or failure of a chemical reclamation 1) water chemistry; 2) toxicant; 3) formulation of toxicant; 4) differential toxicity of toxicant to various fish species; and 5) method and thoroughness of application.
Rotenone and antimycin are more effective in soft, acid or neutral waters than in hard, alkaline waters, other things being equal (Prevost, 1960; Foye, 1964; and Gilderhus, Berger, and Lennon, 1969). Recent work at the Fish Control Laboratories indicates that the pH of water dramatically affects the biological activity of fish toxicants, especially alkaline pH's (Marking, 1970). It goes without saying that temperature is an important factor to be considered.
Prévost (1960) lists these other physical factors to be considered in poisoning a lake: 1) thermocline present or absent; 2) weed beds which affect toxicant distribution; 3) floating islands; 4) springs at the bottom which will dilute toxicant; 5) mud bottom; 6) turbidity; 7) inflowing or outflowing streams; and 8) light.
The toxicity of rotenone to fish is affected by light, temperature, oxygen concentration, alkalinity, and turbidity (Almquist, 1959). The fact that rotenone intoxification is reversible is a disadvantage at times. Fish that move out of a toxic concentration of rotenone into areas such as springs or inflowing streams may recover. Lennon and Parker (1959) observed that many fish affected by the chemical during a reclamation sink to the bottom and appear dead, but they may recover completely if currents or seeps expose them to fresh water.
Rotenone is available as a wettable powder, emulsified, and emulsified and synergized liquid formulation. The liquid formulations repel fish, therefore driving them to any available source of fresh water (Miller, 1950 and Lennon, 1970a). Numerous reports indicate that these formulations do not consistently penetrate the thermocline (Clemens and Martin, 1953; Turner, 1959; and Foye, 1964).
Clemens and Martin (1953) list one of the causes of failure as incomplete dispersal of rotenone through surface waters, especially in weed beds. Foye (1964) found that in 21 of 22 ponds which had marshy shoreline areas and/or tributaries only incomplete kills were affected, while in 23 of 26 ponds without these traits complete kills were accomplished. In general, complete kills with rotenone have been achieved in lakes that are relatively acidic, cold, deep, clear, weedless, unstratified, without either inlet or outlet (Hooper et al., 1964).
Greenbank (1941) was the first biologist to use the thermocline barrier to toxicant penetration for his advantage. He treated the epilimnion of two lakes with powdered derris root (5 percent rotenone) to eliminate the population of warm-water species (rock bass, yellow perch, largemouth bass and forage species) so that the lakes could be managed for trout. Hayes and Livingston (1955) repeated partial poisonings of a lake to improve brook trout fishing. They applied rotenone annually to 5.6 ha (13.8 a) (area within the 3-m contour) of a 21.6-ha (53.5-a) lake for 5 years. The 0.5-mg/l (ppm) concentration applied during summer stratification reduced the undesirable species and increased the catch of trout to about 230 percent of the pre-treatment value. Reduced treatment costs can also be realized if the oxygen is depleted in the hypolimnion after thermal stratification so that only the epilimnion need be treated.
Antimycin is available in three formulations. Fintrol-5 and Fintrol-15 are coated on sand grains in such a way as to release the toxicant evenly in the first 1.5 m (5 ft) and 4.4 m (15 ft), respectively. Fintrol-Concentrate is a liquid for use in streams and shallow waters. None of these formulations is known to repel fish (Lennon, 1970a). Additional formulations are needed to treat deep water lakes effectively.
Antimycin also is affected by temperature and pH. However, if a fish gets a toxic dose of antimycin, it will not survive even if placed in fresh water, because antimycin irreversibly binds in the cells of fish.
Regardless of which toxicant is used, the chemical must be evenly distributed throughout the water in the proper concentration for the length of time necessary for the fish to get a lethal dose.
Most toxicants are not lethal to all species or to all life stages of a species of fish at a given concentration. This fact can be used to good advantage in certain cases where selective removal of fishes is needed as a management technique (Burress and Luhning, 1969a and 1969b). It can, however, work a distinct disadvantage when a complete kill is desired. Ictalurids, the bullheads in particular, are resistant to rotenone and antimycin poisoning. Krumholz (1948) found that concentrations of 1.0 and 1.5 mg/l (ppm) failed to produce a complete kill of bullheads even though the recommended dose was 0.5 mg/l (ppm). Burdick, Dean, and Harris (1955) observed that the brown bullhead is resistant to rotenone poisoning. Walker, Lennon and Berger (1964) also found bullheads to be quite resistant to antimycin poisoning.
There have been various definitions given for a successful reclamation. The one most commonly accepted is that of Zilliox and Pfeiffer (1960) who state that: “The success of a reclamation programme should be measured in terms of quality of fishing produced rather than in terms of complete eradication of all fish life”. Hooper et al. (1964) surveyed 22 states on the degree of acceptance of and value assigned to reclamation procedures, and arrived at the following general conclusions:
1) Rehabilitation is a widely accepted method of management.
2) A relatively high rate of successful projects is indicated (65 to 100 percent). Many states considered a project successful if fishing improved only for 1 year.
3) Public acceptance is, in general, favorable, but nearly all agencies stressed the need for adequate “education” of the public prior to renovation.
4) Partial thinning must be undertaken on a continuing basis. Many states do not favor this approach.
5) In northern states most rehabilitation has been for the purpose of introducing trout. Very few states provide interim fishing while a new population is becoming established.
6) Stream reclamation projects have, in general, had limited success. Re-establishment of undesirable species is rapid.
7) Our knowledge of population dynamics is inadequate, particularly for warm-water lakes.
Hooper et al. (1964) also reviewed the work in the State of Michigan. Their summary on warm-water lakes rehabilitated from 1955 to 1963 indicates that good success was achieved in 40 percent of the treatments, 33 percent were fair and 27 percent were considered poor. In lakes rated good and fair the average duration of success was 4.7 years, while that of lakes rated poor was 0.5 years.
Stroud and Martin (1968), in surveying the successful use of fish toxicants by state fishery personnel, found that there was decided improvement in fishing in a vast majority of lakes where total reclamation was attempted. For cold-water lakes, good fishing prevailed following treatment and restocking for an average of 7 years (range 3 to 10 years). For warm-water lakes, it averaged 5 years with a range of 1 to 15 years.
Partial treatments of lakes with fish toxicants have not been as successful as total treatments. Stroud and Martin (1968) give the following resumé: up to 50 percent improvement (or more) in fishing has occurred in 37 percent or more of the cases; up to 25 percent improvement took place in about 44 percent; and questionable improvement was experienced in the remaining 19 percent. The greater benefits experienced were evident for 3 years or occasionally longer in about 33 percent of the cases. The lesser benefits, applicable to the remainder, lasted for periods of 1 to 2 years. Re-treatment has been frequent -- as many as three times -- in about 20 percent of the cases, twice in another 20 percent, and once in the remaining cases. The vast majority (87 percent) of the water subjected to partial treatments was managed for warm-water fishes.
The most successful reclamations with rotenone have been carried out in trout waters with the targets being non-native, introduced species. Zilliox and Pfeiffer (1960) found that such species as chain pickerel, northern pike, carp, rock bass, smallmouth bass, largemouth bass, black crappie, yellow perch, and walleye could be controlled using emulsifiable rotenone products in trout waters of the Adirondack Mountains in New York. They also state that for most of the waters concerned, the native species survived the reclamation.
Gilderhus, Berger and Lennon (1969) found antimycin to be an effective fish toxicant against about 50 species of fresh-water fishes. Finucane (1969) found 38 species of marine fish were killed by 7 μg/l (ppb) of antimycin in a salt-water impoundment.
The data of Stroud and Martin (1968) and others show that total reclamations have been most successful in trout waters which were usually soft and relatively acidic. The greater percentage of failures were associated with hard, alkaline, warm-water lakes. Spitler (1970) in reviewing reclamations with rotenone in Michigan waters found that in waters with pH's greater than 8.0 the success rate was only 28.5 percent. Other factors influencing success were methyl orange alkalinity and temperature. Success was 44.4 percent when alkalinities fell in the range of 150 to 200 mg/l (ppm) while the best range of temperatures was 15° to 20° C (60 to 69° F). Spitler (1970) also found that if the toxicant concentration was less than 1.6 mg/l (ppm) the chances for success were greatly reduced. Although the formerly recommended concentration of rotenone was 0.5 mg/l (ppm), one finds in talking to fishery managers that 1.5 to 5.0 mg/l (ppm) are most commonly used.
Gilderhus, Berger and Lennon (1969) demonstrated that pH and temperature are important water parameters to be considered when using antimycin as a fish toxicant. For example, less toxicant may be required in soft water than in hard water; less is required in warm water than in cold water; and less is required in water with low pH than high pH. An on-site bioassay should be conducted to establish the proper concentration of any fish toxicant to be used.
Reclamations are carried out at various seasons in the year. The timing of the reclamation is usually influenced by the following factors:
1) sport fishing season
2) accessibility of the body of water to be treated
3) temperature of the water
4) presence or absence of a thermocline
5) presence or absence of waterfowl
6) availability of seasonal help
7) formulation of toxicant to be applied
8) availability of the proper size fish for restocking
9) spawning of target species
Prévost (1960) has listed various combinations of reclamations and restocking according to season which result in continuous sport fishing or with interruptions up to 3 years. To produce continuous sport fishing within the fishing season, lakes were poisoned with rotenone in October when the fishing season was closed. Brook trout 39.4 to 59.1 cm (10 to 15 in) long were planted in November if the toxicity had disappeared, or in the following spring after the ice melted. A reclamation with rotenone in May and restocking with fish 39.4 to 59.1 cm (10 to 15 in) after the toxicant had dissipated usually meant that fishing resumed in August and September.
Interrupted fishing followed 1) poisoning in May with rotenone, restocking with fry in June or fingerlings in October; or 2) poisoning in the fall and restocking with fry the following spring. This meant that the lake was taken out of production for one or two summers.
The use of chlorinated hydrocarbons such as toxaphene may take a body of water out of production for 1 to 4 years (Stringer and McMynn, 1960). In addition, there are residues of the toxicant in the ecosystem for extended periods of time (Johnson, 1966 and Terriere et al., 1966).
Re-infestation of reclaimed waters is a problem commonly encountered. The problem usually can be traced either to an inlet or outlet stream or to re-introduction by fishermen. If there is water flowing into the lake or reservoir to be treated, it must be free of the undesirable species or be reclaimed along with the target body of water. Or, it may be necessary to erect barriers to fish passage. Prévost (1960) and Zilliox and Pfeiffer (1956) suggest barrier dam construction as the most practical solution to upstream migration. Proper legislation and enforcement can reduce or prevent the re-introduction by fishermen.
Regulation of fishing on reclaimed lakes usually is necessary. If fry or yearling fish are stocked, the fishing may be prohibited until the fish reach legal size or have had at least one successful reproductive season. Catch limits that are geared to hatchery production capacity can be used on lakes managed on a put-and-take basis. Often, control over the use of live fish as bait is rigid in reclaimed waters.
Special follow-up studies on ecological consequences of undesirable fish removal with toxicants are not necessary if the proper toxicant is used at the correct concentration. Any long-term effects should be revealed by routine observations that are an inherent part of careful programs of lake management.
The management plan for the body of water will determine the species stocked and the stocking rate. Usually in cold-water situations the lakes are completely cleaned out and restocked with trout. If there is adequate natural reproduction of trout, stocking following renovation will be limited. The fishery is then managed through regulation of harvest and control of re-introduction of the pest fish. If natural reproduction is lacking, then stocking, renovation, and restocking may be scheduled on a 3 to 5 year cycle.
When an attempt at eradication results only in a partial kill, re-treatment may be necessary after a year or two. Also, the re-entry of the undesirable species, either through illegal stocking, inadequacies in a barrier system or by migration from connecting waters, may necessitate re-treatment.
Warm-water lakes that are managed for more than one species of fish usually are reclaimed only when all other forms of management have failed to provide quality fishing. They may be restocked with a diverse fish fauna thought to be representative of the type of water. Both costs and the frequent inability to get a complete kill have discouraged fishery managers from attempting reclamation of large warm-water lakes. In addition, most states do not have enough warm-water fishes of catchable size to restock a large lake to provide continuous fishing.
Periodic re-treatment is necessary where partial poisoning is used to control forage species. A close scrutiny of the population dynamics of forage species is essential in determining when re-treatment is needed. Lack of information on life history, population dynamics, and ecology usually has made partial poisoning less than successful. The reduction of forage species may or may not be followed by stocking of predator species.
