A.V. Holden
Freshwater Fisheries Laboratory
Faskally, Pitlochry, Scotland
The normal function of fish toxicants (piscicides) in fisheries management is to destroy unwanted fish. Attempts to destroy one species completely while protecting other species have generally been unsuccessful, although the use of TFM (trifluoromethyl nitrophenol) to control lampreys, Antimycin to control scalefish in rearing ponds for channel catfish, or Squoxin (1,1'-methylenedi-2-napthol) to destroy squawfish are examples of poisons which have proved reasonably successful in this respect.
Fewer investigations have been made into the use of chemicals to sample fish populations with the intention of obtaining information which may otherwise be difficult to acquire. Lennon et al. (1970) reviewed about 40 chemicals which have been used as toxicants for fish or other aquatic life, but only two such toxicants, Rotenone and Antimycin, seem to have been used for fish sampling. The first being probably the oldest and most commonly used of all piscicides, and the second one of the most recently-introduced types. This review attempts to summarize the main characteristics of both the piscicides and the techniques used for sampling.
All toxic chemicals used in fisheries management, even anaesthetics, are capable of causing death of fish if a sufficient concentration is used. No piscicide, however, is limited in its effects to those on fish, and many chemicals are lethal, at concentrations used on fish, to planktonic and benthic invertebrates, reptiles and amphibians, and occasionally toxic (but not necessarily lethal) to birds or mammals. The time of year at which the chemicals are used must be selected carefully. Some amphibians in Europe are in danger of extinction as the result of environmental pressures, while the destruction of fish which are a source of food for fish-eating birds could be serious at times when the young are being reared. Where the water being treated is destined for human consumption, both the potential toxicity and organoleptic properties must be considered.
Hall (1975) reported that sodium cyanide, cresol, copper sulphate and toxaphene have been used in waters of the Tennessee Valley Authority (TVA), U.S.A., in addition to rotenone, the most commonly-used fish toxicant. Antonioni and Baumann (1975) described the use of Antimycin A as a management and sampling tool. Of the others, sodium cyanide is unacceptable in water supply reservoirs in view of its mammalian toxicity, cresol is likely to cause tainting and toxaphene is undesirably persistent. Copper sulphate, at concentrations likely to kill fish, is very toxic to venegation. Meyer et al. (1976) reviewed the system introduced in the United States in 1972 for the registration of chemicals used in fisheries management. Rotenone and Antimycin were approved for use as piscicides, but fish contaminated by them are not acceptable for human consumption, primarily because no suitable methods are vailable for residue analysis.
Rotenone, the active constituent of derris root, is generally accepted as the chemical for fisheries management purposes with the fewest disadvantages in use. It is lethal, at concentrations used against fish, to many species of zooplankton and to some other invertebrates, but the effects on these elements of the fauna are not generally long-lasting. It is not dangerous to birds or mammals except at very high concentrations, but it, or the other chemicals present in commercial formulations, will taint water used for drinking purposes at concentration lethal to fish.
The stability or persistence of rotenone is dependent on pH, temperature, oxygen concentration and the presence of suspended matter, among other factors, but in practice, when used in lakes, the toxic effects usually last only for a period of a few days at summer temperatures. In winter, however, at near-zero temperatures the water of treated lakes can remain toxic for weeks (Holden, unpublished). Rotenone can be destroyed rapidly by treatment of the water with potassium permanganate, an effective technique which adds significantly to operational costs. An appropriate concentration of permanganate is 1 mg/l, but 5 mg/l may itself be toxic. Alternatively, activated carbon can be used in small but unpolluted streams (Engstrom-Heg, 1974). In dense weed beds, distribution of poison is difficult, and rotenone is rapidly destroyed.
As with all chemicals, sensitivity to rotenone varies appreciably between species, and between fry, juveniles and adults of one species, and is affected by temperature and certain chemical factors such as pH. Meadows (1973) found that to ensure a complete kill in a few hours it would be necessary to use 1 mg/l of a 5 percent rotenone formulation for common perch and at least 8 mg/l for crucian carp. The toxicity increased with temperature, but hardness had little influence. The concentration most commonly used of formulations containing 5 percent rotenone appears to be 0.5 or 1.0 mg/l (0.025–0.050 mg/l rotenone), which is capable of killing most fish within an hour or two at temperatures of 15°C or more, but at much higher temperatures death may result in a few minutes. At temperatures below 10°C, death may take several hours. As young fish are usually affected first, larger predatory fish may gorge on the smaller individuals before they themselves are affected.
Antonioni and Baumann (1975) described the merits of Antimycin as a fish poison, one advantage being that, unlike rotenone, fish do not avoid it. This could be an important advantage in some situations, when an escape route was available, or where natural springs or other areas of low concentration of toxin were present in which the fish could survive. The toxicity of Antimycin varies considerably with species, the LC100 value at 12°C being 0.8 microgrammes/1 for fish such as rainbow and brown trout, and yellow perch, but 120 microgrammes/1 for black bullheads. The latter concentration is of the same order as that usually used for rotenone. Marking (1975) found that the toxicity of Antimycin decreased gradually from pH 6.5 to pH 8.5, and abruptly above pH 8.5, which may limit its usefulness in some circumstances. Antonioni and Baumann found that Antimycin was unsuitable for use at sub-lethal concentrations as a sampling tool, as no behavioural abnormalities occurred in cases where ultimate death did not occur. They considered that detoxification with potassium permanganate was essential if apparently non-toxic concentrations were not to cause fish kills downstream to non-target species. It seems to have most promise in sampling rivers in conjunction with permanganate as a detoxifier. Antimycin has received less general approval than rotenone in many countries.
The techniques employed in using rotenone or other piscicides for fish sampling can be sub-divided into those used in lakes or reservoirs, and those in rivers. One basic difference concerns the concentrations employed in the two situations. The time taken to kill fish in a lake, using a low concentration to minimize cost, may be a few hours, but in rivers it may be necessary to maintain a higher concentration for a much shorter period, usually no more than 30 minutes. On the other hand, the quantity of rotenone required for river sampling is usually less than that necessary for lakes, unless the areas sampled in the latter are small. In both types of situation, precautions must be taken to ensure that fish in the area poisoned do not escape from the area, and also that fish from outside the area cannot move into it during the sampling operation.
Hocutt et al. (1970) used a block net in a river to retain all fish drifting down from the treated area above it, rotenone being added 68–91 metres upstream. Potassium permanganate was added immediately below the block net to prevent fish kills downstream. Although a complete sampling of the full width of the river could not be attempted, the technique was judged to be more successful than seine netting, as almost twice as many species of fish were collected. The 2.5 percent emulsified rotenone was added at the rate of 0.26 litres per cubic metre per second of water flow, over a period of 30 minutes, giving an average concentration of 0.15 mg/l of 2.5 percent rotenone over the period. This concentration could not be lethal, but presumably was sufficient to incapacitate the fish, which drifted into the block net.
Boccardy and Cooper (1963) used 2 litres of 5 percent emulsified rotenone per cubic metre per second of water inflow, a concentration over 30 minutes about fifteen times greater than Hocutt et al., who consider that the amount used by Boccardy and Cooper was excessive. it would seem that, in flowing waters, it may not be necessary to attempt to kill the fish in the period of the operation, but only to cause partial asphyxiation.
In lakes, the most effective method of sampling appears to be to apply poison to bays which have been divided from the body of the lake by barrier nets, with a mesh sufficiently small to prevent the loss of fish above the minimum size to be counted. However, it must be recognized that species inhabiting littoral areas or bays are not necessarily representative of open or deep water. Hayne et al. (1968) found that cove samples gave species lists similar to the entire study areas examined, although less common species were not present in all coves. Size classes of abundant species were represented in both cove and lake samples, and standing stock estimates as biomass per unit area in coves were similar to those in the lake. Individual cove samples often gave under or over-estimates of numerical abundance of an individual species. Hall (1975), however, states that the information obtained closely reflects the situation in cove and shoreline areas only, although the littoral zone is considered to be the most productive in the lake, and is also where most fish and fishing activity occurs.
Calculation of the volume of water in the area selected must be accurately made, and the poison applied as evenly as possible. Hall (1975) described the method used in TVA reservoirs, in which the area immediately outside the block net is poisoned to reduce the movement of small fish into the area. The concentration of 5 percent emulsified rotenone used is 0.6–1.0 mg/l, the upper limit being more certain of producing a complete kill. For the usual area treated (about 0.4 ha) with an average depth of 5 m, the amount of 5 percent emulsified rotenone required is about 20 litres. The lakes described by Hall are warm-water lakes, with summer temperatures of 24–29°C, much higher than most European lakes. Davies et al. (1975) described an evaluation of the use of rotenone in sampling fish in a reservoir, in which the chemical was used on 29 occasions over five years in coves blocked off by nets. The fish were collected from the surface as quickly as possible and it was found that the technique gave a reasonable estimate of standing crop, and was useful in demonstrating the presence or absence of a species and the balance between groups of species. It was less useful, however, in determining year-class strength.
Cove sampling with rotenone was also used by Balon and Coche (1974) to provide samples of fish from the large man-made Lake Kariba in Africa. Sampling areas of 0.021–5.273 ha were selected as representative of the various parts of the lake, and fish density was found to be inversely related to the sampling area poisoned. Bazigos provides a detailed statistical analysis of the results obtained in an appendix to the report on Lake Kariba.
The technique of complete poisoning, to provide information on the total populations of fish in lakes or other water bodies which can be regarded as representative of similar lakes in the same area, has been used by several workers. HolĎik (1972) used a chemical (not identified) to determine the total biomass of fish and the species diversity in a number of shallow lakes, marshes, oxbows and water-holes in the alluvial plain of the Danube in Czechoslovakia. The water bodies were small, ranging from 8 m2 up to 0.5 ha, and were treated during periods of low water level. All the fish were collected, and at the time of poisoning the stock was found to consist mainly of young-of-the-year specimens of phytophile species, the adults of which had migrated into the area during floods. Thus the populations determined could only be regarded as representative of a limited period of the year, as the spawning adults were apparently only temporarily resident. Loubens (1960) used a similar technique to study fish populations in floodplain waters of the Yaérés floodplains (Chad), and Kapetsky (see addendum to this chapter) studied fish populations in the flood lakes of the Magadalena River using fish toxicants. Sumari (1975) used rotenone in 32 small Finnish lakes of 0.7–6.4 ha, applying sufficient quantities to kill the entire population in each lake. These lakes were selected as being representative of the much larger number in the area. SCUBA divers collected the fish from 100 m2 areas marked out on the bottom of each lake, the depth in most cases being less than 5 m. A greater relative proportion of small fish and bottom-feeding species were found on the bottom. In the lakes studied, an average of 26 percent of the biomass of ruff, 50 percent of burbot, 81 percent of pike, 74 percent of perch and 66 percent of roach were collected from the surface.
Hocutt et al. (1973) found that, in one river, rotenone sampling averaged 26 species per collection, as compared with 14 species per collection by seine netting over the same area. Boccardy and Cooper (1963) found rotenone up to 35 percent more efficient than electrofishing in small streams, where population estimation is possible.
When lakes are treated with a toxicant, some fish surface within minutes but quickly sink. Decomposition usually occurs within 24 hours at high temperatures, but fish which sink at first may float to the surface later, when they can be collected. In most lakes collection of the fish is made from the surface, but this can result in a substantial error in the population estimate for some species. This error was pointed out by Carlander and Lewis (1948), who treated a 0.12 ha pond with sufficient rotenone to kill all the fish present. The water was turbid, and only fish in water less than 10 cm deep could be seen. A proportion of the population had been fin-clipped two days before poisoning, and estimates of the efficiency of recovery were made, ranging from 14 to 91 percent after five days.
Estimates of the recovery of all fish from a treated area, using previously marked fish, have seldom exceeded 70 percent and are usually about 60 percent in lakes, although returns vary among species and sizes (Hall, 1975). Complete collection may require two or three days, but relatively more of the small fish are lost. The proportion of fish sinking to the bottom may vary between lakes, and may depend on temperature.
The use of marked fish to estimate populations, or to measure the efficiency of collection after rotenone treatment, will involve a risk of error if some of the marked fish die (perhaps as the result of handling or wound-damage) before poisoning is carried out, but to reduce this error the interval between marking and poisoning should be as short as possible. Bonetto et al. (1965) used as mark-recapture method for sampling two lagoons of 19 and 2.3 ha with mean depths of less than 1 m, and treated the lagoons with 1 mg/l of rotenone to confirm the estimates of 4.15 and 0.34 fish per m2. Jensen (1975) used similar procedure to estimate a spawning perch population in a lake. Only about 10 percent of the population was recovered after poisoning, the rest sunk in the deep water. Using the proportion of marked fish in the sample of dead fish obtained after poisoning, good agreement was achieved between the total population calculated from this sample, and the number assessed from the mark-recapture technique.
The difficulty of recovering fish from deeper water was dealt with by Rupp and De Roche (1965) by the use of divers as did Sumari (1975). They used rotenone preparations in three lakes in Maine, 7.7–22.2 ha in area with maximum depths of 9.3–12.0 m. Apart from surface collection, snorkel divers worked in shallow water down to 3 m, and SCUBA divers in the deeper water. Both groups operated in teams of three, sampling the shallow water entirely but the deeper water in transects, over periods of two days after poisoning, although all fish died within 24 hours. For some species, the numbers recovered from the bottom were as large as those collected from the surface, but one species burrowed into the silt and the estimate for this species may have been low.
As mentioned earlier, the use of piscicides will result in the destruction of some species of invertebrates, but if only relatively small areas of streams or lakes are treated it can be assumed that recolonization will take place in due course. However, the balance of species may differ from that originally present. Anderson (1970) found that a zooplankton population which has not reached its reproductive peak is more vulnerable than one which has passed it, while among crustacea the egg stage may be very resistant to poisoning. Morrison and Struthers (1975) found that, in Scottish lochs treated with 0.04–0.06 mg/l rotenone to destroy fish, the majority of invertebrate populations survive or re-establish themselves within a relatively short time.
Fewer studies have been made on the recovery of stream populations after rapidly toxic concentrations of piscicides have been used. Morrison (in preparation) examined Scottish streams treated with concentrations of 0.5 mg/l of rotenone for 30 minutes to kill brown trout quickly, but found no evidence that such treatment caused a significant decrease in the population size of a given invertebrate species one week after treatment. He concluded that the numbers killed represented only a small fraction of the total population, although some effects could be seen on animals 50 m below the point of application. Little (1965) used rotenone concentrations as high as 5 mg/l for one hour and 1 mg/l for a further 5 hours. but although there was some evidence of a reduction in the numbers of insects after eight days, the totals after a year were generally higher than those originally present.
The recovery of fish populations in streams may begin fairly soon after treatment. Phinney (1975) found that repopulation of brook trout was completed in a year, primarily by fry, but some individuals appeared within a month. Not all species of fish returned, however, Olmsted and Cloutmann (1974) also reported the recolonization within a year of a pesticide-polluted stream by juvenile fish of most of the original 29 species from above the poisoned area.
| Lakes: | (a) | Select suitable areas for sampling, primarily shallow water, and preferably bays which can be divided off from the main area by barrier nets. |
| (b) | Measure the area and mean depth of each section to be treated. Over-estimation of the mean depth due to insufficient depth measurements in shallow water at the margins is acceptable. | |
| (c) | Calculate the volume of each section, and determine the quantity of rotenone extract or Antimycin to give a concentration of 0.05–0.2 mg/l of rotenone or 0.0001- 0.1 mg/l of Antimycin, depending whether a kill of all spe- cies or of only the most sensitive species is required. | |
| (d) | Suspend barrier nets (fine cotton or muslin, or small- mesh netting) from the surface to the bottom, using floating and weighted lines, at the water boundaries of the area to be treated. Add marked fish at this stage if desired. | |
| (e) | Apply the poison from a boat or boats to the surface, using outboard motors to mix thoroughly. In water of depth greater than 3 m pump sufficient poison to the bottom, or drag perforated containers of poison sus- pended by rope at the appropriate depth. Treatment should begin as early in the day as possible, to in- crease the success of subsequent collection of fish. | |
| (f) | Where dense submerged vegetation is present, the use of a pump or a perforated container is advisable in all depths. | |
| (g) | Fish collection by hand net should begin as soon as fish are seen to surface, the time varying from a few minutes at high temperatures to over an hour in winter. Some fish sink at first and may float to the surface after a day or so. Thus complete collection may re- quire 2 or 3 days. (Eels may move out of the lake and cross the shore, and can survive for many hours.) | |
| (h) | In weed beds, collection is difficult but a water telescope may be useful to locate fish. Ropes may be drawn through the weed beds to displace fish. Some fish may attach themselves to vege- tation or the bottom by mouth, and can only be dislodged by SCUBA divers. | |
| (i) | SCUBA divers can be used to search the deeper water, which in small areas can be sub-divided by weighted lines if desired. | |
| (j) | All fish should be returned to a central collec- ting point, and sub-divided into species and areas of recovery as required. [See Hall (1975) for further details.] | |
| Rivers: | (a) | Select suitable areas for sampling, as far as possible navigable by boat (for sample collection). |
| (b) | Measure the width and mean depth of cross-section, and determine the surface velocity over a suffi- cient length of stream. Calculate the rate of flow. | |
| (c) | Calculate the volume of the poison required to give a concentration of 0.05–0.1 mg/l rotenone for a minimum period of 30 minutes. | |
| (d) | Install a stop-net (block-net) across the river at the lower end of the section to be treated, the mesh being sufficiently small to retain all fish of the sizes required. | |
| (e) | Calculate the quantity of potassium permanganate required to give a concentration of 1–2 mg/l for 30 minutes. | |
| (f) | Apply the rotenone extract across the river at the head of the section to be treated so that the re- quired concentration can be maintained for 30 min. At the same time apply the permanganate (as crys- tals or in concentrated solution) below the stop- net for 30 minutes, if desired to destroy the rotenone. | |
| (g) | Collect all fish on the stop-net and inspect the area by boat to recover any fish which have been trapped on the bottom or in weed beds. |
Poisoning as a means of sampling fish is limited to certain shallow areas in lakes, to relatively small bodies of standing water and to shallow, slow-moving rivers. The areas sampled can be limited by barrier nets and by a chemical such as potassium permanganate to destroy the toxicant outside the net.
The action of piscicides can be rapid, especially at higher temperatures and in sufficient concentrations it is non-selective between species and sizes of fish. However, larger fish, being affected more slowly by rotenone than juveniles or small species, may gorge themselves on smaller fish precluding the use of stomach content for analysis and producing some bias of estimates of numbers of small fish. With Antimycin, size has little influence of its toxicity.
Being practically non-selective the poisons cannot be used for sampling particular species and/or sizes of fish.
If the sampled areas are representative of the entire water body or of a series of water bodies then information on species composition, size distribution and on standing stock, in terms of numbers and weight per unit area can be obtained. The accuracy of the above estimations depends largely on the application of a sufficient concentration of toxicant, and on the ability to collect all fish. The latter can be difficult, thus, much care should be taken in collecting dead fish but, as loss of small fish is more likely than the large ones, the estimates of standing stock can be fairly accurate.
The cost of the technique can be relatively low in terms of initial outlay and labour required for fish collection; the use of detoxifying chemicals increases the cost but it is not necessary in warm standing waters.
Among the other disadvantages of the technique its destructive action on invertebrates and on eggs of fish and invertebrates (e.g., Antimycin) should be taken into account. Piscicides can be lethal or toxic to reptiles, amphibians and occasionally to birds and mammals. Their use can be prohibited by local or country authorities, if not, they cannot be applied to waters destined for human consumption.
