The reclamation of fish production, farm, and ranch ponds with toxicants has become an established, successful, and economical practice. Many intensively managed production ponds in Asia, Europe, and North America are treated annually with toxicants to eradicate competing or predaceous species, or any fish remaining from the previous crop. Thousands of farm and ranch ponds in the United States and Canada are treated periodically with general or selective toxicants, either totally to remove all fish or partially to reduce numbers of undesirable fishes. Restocking with preferred species often restores fine recreational fishing.
The problems of securing adequate distribution of toxicants in fish production, farm, and ranch ponds, assessing kills of fish, and evaluating the success of reclamation and subsequent management are relatively minor because the ponds are small. Many such ponds have water control structures which permit manipulation of water levels to enhance the reclamation process.
Total and partial reclamations of lakes and reservoirs, ranging up to hundreds of hectares in surface area, have become common and effective management practices within the past two decades. Streams, too, are often treated with toxicants in conjunction with lake reclamations, and more recently the reclamation of river systems has been practiced on a wider scale. A developing technology in reclamation will spur greater numbers and more ambitious projects. Some of the problems frequently encountered and affecting the success of reclamations of lakes, reservoirs, and streams are similar and include:
1) The need for reclamation often is not demonstrated adequately. Therefore, the target
fishes may not be well defined, or no proof is offered that they indeed are the cause of
a fishery problem. Thus, their elimination may not result in an improvement of the fishery.
2) The biology of the target species in the water to be treated seldom is investigated
or reported. Consequently, alternatives to poisoning cannot be given appropriate consideration.
3) A crew assigned to reclaim a body of water often is composed of novices in control
methodology; and some crews are too small for pre-treatment, treatment, and post-treatment
operations.
4) Pre-treatment surveys on the biology and chemistry of receiving waters frequently are
lacking or inadequate to detect and evaluate factors that may influence the performance of
a toxicant and compromise the success of the reclamation. For example, low temperature
or high turbidity may reduce the effectiveness of rotenone, or high pH may cause rapid
degradation of antimycin. Few fishery managers, with the exception of sea lamprey control
specialists, acknowledge the importance of on-site bioassays of a candidate toxicant
against target fishes in the particular receiving water to delineate the dose (concentration
plus exposure) needed to produce the desired kill. Also, the fact that a target fish
must be exposed to a given concentration of toxicant for a certain length of time to get
a kill very often is overlooked.
5) Post-treatment surveys of kills are often sketchy, and objective evaluations of subsequent
management are conspicuously absent in much of the reclamation literature. If
evaluations are mentioned at all, they are usually subjective or reported as being in
progress.
6) The toxicants selected may be wrong in nature or formulation for the reclamation.
Residues of toxaphene, for example, were very persistent in some waters, contaminating
invertebrates, fish, and wildlife for months or years thereafter. Moreover, some insecticides
used as fish toxicants are very damaging to aquatic invertebrates that are important
as food for fish. Toxicants applied in agricultural formulations may fail to
penetrate thermal barriers, thus permitting target fish in deep water to escape poisoning.
Much remains to be done in perfecting toxicant formulations suited for aquatic application.
7) Many reclamation failures are attributable to deficiencies in application methods.
Improvised apparatus for distributing toxicants in water too often is primitive and inefficient.
8) Economic considerations rather than biological and chemical facts often govern the
selection of a toxicant or formulation and the application of it. Recognition seldom is
given to the fact that the cost of a toxicant is only a small part of the total cost of a
reclamation project, and that true economy is achieved only by using the best toxicant for
the job.
9) The worth of barriers and other measures to prevent the re-infestation of reclaimed
waters by undesirable fish has been proven, but the benefits of many reclamations are
short-lived because steps are not taken to prohibit the rapid return of unwanted fishes.
10) We have the impression that many fishery managers view fish toxicants as a panacea,
that a single application will correct problems and result in bountiful fishing that
will last for a long time. Few indicate recognition of the fact that some intensively
fished and managed waters may have to be re-treated regularly with toxicants to keep them
in optimum production. On such waters, the costs and efforts invested in thorough biological
surveys are spread over a number of reclamations, realizing efficiency and economy.
On the other hand, the many successful reclamations affirm the value of reclamation as a management tool. Some authors have concluded that reclamation by means of fish toxicants is the best tool available to fish managers. In general, the better studied and more carefully executed projects have the greater number of successes. Continuing research on general and selective toxicants, on formulations for aquatic use, on means for distributing toxicants in water, on controls to be integrated with toxicants, and on survey and assessment equipment and techniques will help to improve the success ratio for reclamations. Provision for training and manuals to be used by control personnel also will help to upgrade reclamation projects.
Our review of literature and responses to a world-wide questionnaire indicate that at least 27 countries, in addition to the United States and Canada, have used or are using fish toxicants in fishery management. The number of reclamations and the number of countries practicing reclamation are certain to increase as information on methods on safe and effective toxicants, and on management benefits is made widely available. Some nations now wish to conduct reclamations, but have difficulty in obtaining supplies of a suitable toxicant. Others plan to use fish toxicants when needs arise.
