From Aquaculture into the Wild

Developing alternative methods for managing predatory fish in lake fisheries

Predatory fish play a very important role in aquatic ecosystems. By regulating the abundance of populations of unprofitable fish, mainly Cyprinidae, these predators maintain the biological equilibrium of the ichthyofauna. They also eliminate diseased and weak fish, thus, directly impacting the health and condition of fish populations inhabiting natural basins. Pike (Esox lucius) and pikeperch (Sander lucioperca) are the most important predators in Polish waters from the perspectives of ecology and fisheries management. These are also two of the most valuable commercial fish species, and most fisheries enterprises engage in stocking both pike and pikeperch, generally stocking early developmental stages of these fish, namely pike hatchlings and pikeperch summer fry. This requires using large quantities of stocking material; additionally, the effects of stocking depend largely on the prevailing environmental conditions (mainly thermal and food base) in a given year. Generally, there is a lack of documented information and data on the impact of stocking natural waters with predatory fish. Notably, there are a variety of significant limitations with traditional pond methods for producing pike and pikeperch stocking material, including the following: outcomes are dependent on atmospheric conditions; stocking material harvest is seasonal and, consequently, so is stocking natural waters; harvesting fish from ponds and transporting them is difficult; the biological quality of material from ponds is frequently poor; environmental conditions are often adverse when stocking material is released, which is frequently done in late fall.

Progress has been made over the past few years not only in strict hatchery work associated with spawn incubation for predatory species, among others, but technologies have also been developed for the artificial reproduction and rearing of pike and pikeperch stocking material in recirculating aquaculture systems (RAS). Significantly, these techniques permit producing material regardless of prevailing atmospheric conditions and at different times of the year. RAS also allows intensifying methods for obtaining stocking material. It is currently possible to produce fish of any size from hatchlings to spawners. Pike and pikeperch stocking material reared in RAS can be use in two ways: (1) for further rearing in this type of system to commercial-sized fish, and (2) as stocking material for release into open waters. Initial pilot studies conducted in ponds indicate that stocking pike and pikeperch material (fingerlings with body weights (b.w.) of 1-10 g) reared with this method is highly effective. One of the drawbacks to utilizing this technology is the lack of fully documented information regarding the stocking effectiveness of material reared under controlled conditions in RAS once they are released into open water environments.

This monograph presents the results of work conducted from 2011 to 2015 within the pilot program entitled “Developing alternative methods for lake predatory fish species management based on stocking material obtained through intensive rearing in recirculating systems”. The primary aim of the project was to maintain and/or increase resources of predatory pike and pikeperch populations in natural aquatic basins. The following work was performed to achieve this aim:

  • testing innovative methods for tagging juvenile stages of fish, and determining the suitability of them for tagging pike and pikeperch material reared in RAS and released into earthen ponds or open waters;
  • developing procedures for and determining the effectiveness of releasing predatory fish (pike and pikeperch) stocking material obtained through intense rearing in RAS into earthen ponds and lakes - specifically, the optimal size and age of the stocking material used and the timing of their release were determined;
  • determining the principles of and methods for handing pike and pikeperch spawners during and following artificial reproduction that would permit the greatest effectiveness when returning them to natural waters.

The project included determining for the first time in Poland the application and effectiveness of coded-wire tags (CWT) for pike and pikeperch. This method was identified as being nearly ideal for pikeperch fingerlings reared in RAS (b.w. > 1 g). This quick, highly effective tagging method (CWT retention < 90%) can be used with large numbers of fish. The cost of this method is relatively low, and it has little or nearly no effect on the tagged pikeperch. It also allows simple, in vivo identification of the tagged fish. The effects of CWT tagging in juvenile pike are not as unambiguous. Short-term tag retention (approximately one month) in pike fingerlings with b.w. < 10 g was not as satisfactory at 55-65%. Not until the fish reached b.w. > 15 g did this indicator increase to > 90%, which is a retention rate that is recommended and considered acceptable for mass tagging methods applied on a commercial scale. This is why, in the nearest future, further work is required that will focus on increasing the effectiveness of tagging smaller size classes of pike stocking material with CWT. Visible implant elastomer (VIE) tags were found to be very effective, and VIE tag retention (determined during rearing in RAS and earthen ponds for periods of 28 to 155 days) in both pikeperch and pike fingerlings regardless of size was 100%. This method can be recommended for short-term pike and pikeperch tagging, for example, during spring-summer-fall rearing in ponds. The CWT and VIE tagging methods could potentially be used in similar work on other commercially and ecologically valuable fish species inhabiting Polish waters. The experience gained and some of the observations made during work on tagging predatory fish were of a universal character and can significantly simplify and streamline similar work with other fish species in the future.

During the project, the use of an innovative method for tagging pike and pikeperch permitted verifying the suitability of material reared in RAS for stocking ponds and open waters. To date, the results obtained and observations made confirm that pike and pikeperch stocking material from RAS can be used to stock both earthen ponds and lakes. The degree of adaptation of fingerlings reared in RAS to new environmental conditions (in ponds or lakes) can depend on the size of the stocking material. This was especially apparent among pike reared in earthen ponds, as the best results (fastest growth rates and highest survival) were achieved with the smallest size group of fish analyzed (b.w. 1-2 g). The results of stocking lakes with different pike size groups are not as unambiguous. In one shallow, eutrophic, pond-like lake, it was more advantageous to use larger sized stocking material (b.w. 15-30 g). No significant impact was noted with regard to the size of the pikeperch stocking material used in either ponds or lakes. In terms of economics, it is recommended to stock smaller size groups of pikeperch since they require a shorter rearing period in RAS, and are therefore less expensive to produce. The effectiveness of stocking with various pike size groups is determined largely by environmental conditions, especially the available food base that is appropriate for given fingerling size groups. In lakes with considerably degraded littoral zones, which are the natural spawning and nursery areas for developmental stages of fish species that smaller pike fry prey upon, it could be more effective to release stocking material of a larger size. This confirms a theory that has long been recognized, but is applied in practice only infrequently: decisions regarding stocking pertaining to species, material type, quantity, and release timing should be made based on fairly detailed knowledge of the environmental conditions in given lakes/rivers. Knowledge not only of thermal and oxygen conditions, but also of water transparency and the availability and structure of the food base are all crucial.

The effects of stocking earthen ponds and the results to date of stocking lakes indicate that currently, in consideration of the possibility of using material from RAS for these aims, efforts should be focused on improving the effectiveness of stocking material production in RAS (including economic aspects) of pikeperch with b.w. 1-2 g and pike with b.w. 15-30 g. The possibility of testing the effects of stocking fish with b.w. < 1 g should also be considered; this would probably entail using another tagging method for such small fish. The results of tag returns obtained from commercial fisheries catches are preliminary. During the study, only some fish from the first stocking year release (2011) had attained catchable size; this means that a representative sample of pike and pikeperch from RAS that were tagged with CWT will not be available to the catches for another two to three years when the year classes stocked even in the past two years (2013-2014) are recruited to the catchable populations of these species. Data collected to date indicate that in work aimed at determining stocking effectiveness in open waters it is very important to devise a system to recover tags from fish caught in natural water bodies that are included in given stocking programs. This is essential as it is the most important condition for conducting reliable scientific analyses of the effectiveness of stocking and also for estimating the abundance of ichthyofauna resources and fish mortality based on tagging. A key condition for conducting this type of study, formulating credible conclusions, and applying recommendations is to collect appropriately robust material, which, in this case, is tag returns. This is impossible to achieve without real cooperation among commercial fishers, recreational fishers, scientists, and representatives of various tiers of the state administration. This is why all fish stocking programs or projects must be accompanied by wide-ranging informational campaigns and training for commercial and recreational fishers to educate them about the benefits of these programs for local communities and, of course, those who exploit the fisheries of these waters.

The pikeperch biotelemetry surveys conducted as part of the project were the first of their kind in the world to track the movements of this species using acoustic transmitters with accelerometers. This permitted determining precisely pikeperch movements and the depths these fish chose to occupy in lakes. The impact of environmental conditions on pikeperch movements and the lake zones they chose to occupy at different times of the year were also tracked. Radio telemetry methods were applied for the first time in Poland to determine pike survival and ecology. Each of these telemetry methods permits, for example, precisely determining causes of mortality among predatory fish in lakes that are exploited by commercial and recreational fisheries. The most significant factors contributing to pike mortality were poaching and recreational fisheries, while for pikeperch they were commercial fisheries and poaching. Biotelemetry was used in the current study of predatory fish on a rather small scale, but the results obtained and the observations made indicate that it could be an excellent tool that could be used to discover new facts about the lives of fish. In the near future, this method could be useful for identifying fish habitats, including spawning grounds or migration routes in river-lake systems. Undoubtedly, this is not only of use in research, but it also has practical applications. Knowledge of this type can be extremely useful when implementing measures to protect and manage natural ichthyofauna assemblages, including, of course, those of predatory fish. For the first time in the world, this project has made a preliminary quantification of the survival of pike spawners used for artificial reproduction and re-released into the lakes in which they were caught. Biotelemetry was one of the methods used to accomplish this. However, with the aim of drawing concrete conclusions and formulating recommendations, it is necessary to expand this type of work to include a variety of environments, populations, and stocking material sizes.

Like all branches of electronics, biotelemetry is developing dynamically, and this means that transmitters are being made in increasingly smaller sizes. Currently, the smallest of them can be used on fish with body weights of about 10 g. These transmitters can already be used in stocking material released into open waters, including in fall fingerlings from ponds or in pike and pikeperch reared in RAS. Additionally, the intense development of fish positioning systems permits determining the location of larger numbers of individuals with transmitters. Automatic systems permit identifying the locations of fish with transmitters twenty-four hours a day. Such systems are adapted to determine the impact of environmental conditions and anthropogenic stress on fish biology and behavior. It is also possible to compare the behavior of fish obtained through artificial reproduction and reared in hatcheries to that of fish from natural recruitment. Biotelemetry also creates opportunities to compare the behavior of predatory fish obtained from artificial reproduction and reared in hatcheries to that of wild individuals from natural populations.

Combining biotelemetry with mass fish tagging methods, such as CWT, provides a nearly complete picture of fish behavior and can be used to assess the effectiveness of stocking juvenile fish stages. The work performed with predatory fish reared in RAS indicated fairly unambiguously that CWT tagging alone is a unique tool that provides important information regarding the effectiveness of stocking, which can then also be very easily used to evaluate the economic and ecological effectiveness of this type of management measure. Including biotelemetry in this type of work would doubtless supplement our knowledge of how stocking material adapts to environmental conditions in natural streams. While the work of gathering data and information on the effects of stocking earthen ponds and lakes with pike and pikeperch stocking material reared in RAS is still in the collection and analysis phase, the knowledge already garnered indicates that it is very helpful, even essential, for formulating management principles for predatory fish, and in the not too distant future, for doing so with other species of fish that are introduced into open waters.

Tekst zaczerpnięto z monografii opracowanej w ramach projektu:

Zakęś Z., Szczepkowski M., Kapusta A., Rożyński M., Stawecki K., Pyka J., Szczepkowska B., Wunderlich K., Kozłowski M., Kowalska A., Hopko M. 2015 – Z akwakultury do natury. Opracowanie alternatywnych metod zarządzania rybołówstwem drapieżnych ryb jeziorowych (Red.) Z. Zakęś, M. Szczepkowski. Wyd. IRS, Olsztyn, 224 s.

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