Self sustaining coregonid populations require oligotrophic and mesotrophic, cool and well-oxygenated waters. This is the reason why their natural distribution area in Europe extends from northern central Europe to northern Scandinavia and from the British Isles in the west to the eastern edge of Europe in Russia. In central Europe they abound in the sub alpine lakes of France, Germany, Italy, Switzerland, Austria and the reservoirs of the Czech Republic and Slovakia.
In the southern edge of its natural distribution area the lakes preferred are deep, but in northern Europe coregonids live in shallow lakes as well. Temperature and oxygen are the most important water quality parameters together with suitable spawning substrates for self-sustaining coregonid stocks. The upper lethal temperature for whitefish is about 22°C, the optimum being < 15°C, and for peled these levels are a little higher. The oxygen level should not fall below 4 mg/l and oxygen saturation on the spawning grounds should be higher than 70%. The degree of sensitivity to adverse water quality depends on the life stage, physiological condition of the fish, acclimatization time, chemical composition of the water and other factors.
Water quality is not the only abiotic factor affecting the year-class strength in natural environments. Weather conditions (temperature, wind, sunshine, etc.) are some of the most prominent factors suggested to affect recruitment of coregonids. The sensitivity of fish to environmental stress usually decreases as a function of fish size (developmental stage). It is possible to stock coregonids in lakes and reservoirs in which they cannot naturally maintain a self-sustaining population, as in eutrophic lakes.
Competition within and between cohorts is significant and cannibalism has occasionally been observed.
A spawner-recruit relationship has seldom been observed in vendace and variability in recruitment is high. Competition within a cohort at the larval stages (density-dependent growth and survival) is said to be one of the most important mechanisms affecting year-class strength in vendace in oligotrophic waters. It is suggested that an asymmetrical competition for food between adult and young vendace is also significant. In eutrophic lakes environmental (density independent) factors are more important.
Recruitment in natural whitefish stocks has seldom been shown to be dependent on population size. Therefore it is suggested that in most cases recruitment of whitefish is determined through density independent environmental abiotic and biotic mechanisms. This explains the success of Finnish juvenile stocking in lakes containing indigenous whitefish populations. Whitefish are known to prey extensively on their own eggs and larvae, which might have some role in population regulation.
It should be borne in mind that when yolk-sac larvae are stocked, the same environmental factors operate as on naturally hatched larvae. This explains at least partly why stocking with yolk-sac larvae so often fails in the presence of naturally reproducing stocks in the lake. In contrast with juvenile stocking it is unrealistic to expect any markedly positive results from stocking with yolk-sac larvae in lakes in which naturally reproducing populations already exist. This is valid for all coregonids. The situation is different in eutrophic lakes in which fishing is efficient and feeding conditions good.
It is logical to assume that at some point compensatory mechanisms should operate when juvenile fish are stocked. In natural systems the number of recruits is generally determined before the juvenile period and therefore stocking with juveniles increases population density and intraspecific competition for resources as well. In such situations the number of recruits is determined through density-dependent (compensatory) mechanisms. The growth of juveniles depends particularly on the cohort size. Growth retardation is supposed to increase vulnerability to predation, which affects the number of recruits to the fishery.
Figure 1: A scheme for planning coregonid stocking. Review boxes on the left illustrate different levels of data collection and processing and decision boxes on the right the respective decision levels with some relevant questions. Stocking should be rejected if answers to the questions are negative.
Therefore it is suggested that in less trophic lakes possessing self sustaining whitefish stocks, stocking with juvenile fish tends to be more effective than stocking with larvae. The same is probably true of peled, although this is not borne out by the observed results reported in Table 1. The wide range of the reported figures should be noted, and it would be useful for further investigations to be published. In more eutrophic lakes, with no natural populations of coregonids, the size of fish stocked is less important.
Interactions between fish species are very complex and can change in different life stages. The interactions can be in the form of competition for resources or predation.
The vendace is a pronounced plankton feeder and dominates over both peled and whitefish in oligotrophic lakes. It has been shown in Scandinavia that stocking with any life stage of vendace into whitefish lakes is likely to succeed, while the contrary is seldom true, if yolk sac larvae of whitefish are stocked. However, when larger stages of whitefish are used they are more frequently successful.
The peled seems to be sensitive to competition for food in oligotrophic lakes in which both vendace and whitefish are present. The yield from peled stocking in these cases has never been good despite the size or age of stocked peled. The general outcome is that the species with greater gillraker numbers tends to occupy the pelagic zone in which it feeds, while the species with fewer gillrakers remain in the littoral zone feeding on benthos. This problem has not been encountered in eutrophic lakes.
Whitefish introduction and stocking has been harmful to char populations in many Scandinavian lakes. The continued persistence of char and whitefish is, however, possible in large oligotrophic lakes. Perch prey on whitefish larvae and in many small lakes whitefish have been reduced to extinction by perch. However it is possible to control perch in small lakes by the introduction of larger stages of whitefish. However, large, colder lakes provide an advantage for whitefish over perch. The overall pattern is that whitefish are dominant over perch in large cold lakes with an extensive pelagic zone, but perch tend to dominate over whitefish in smaller, warmer lakes. Dominance mechanisms also work here during early life stages.
Predator species (pike, burbot, brown trout, etc.) prey on coregonids. Predation is size-dependent and hence natural mortality on smaller sizes is highest. Susceptibility to predation decreases as a function of fish size. The more stocking the better feeding conditions are created for predators. Predation on eggs and larvae may be the means of determining the level of coregonid recruitment in many lakes.
Some beneficial effects have been noted in slightly eutrophic Polish lakes in which stocked whitefish tended to replace small cyprinids in the fish community, thereby improving environmental quality as well as value of the fish catch. Although mass removal of undesirable fish species is not a stocking activity, its impacts can be similar. There are indications that in lakes in which vendace or whitefish stocks consist of large sized specimens and the population abundance has for years remained at a very low level, mass removal of unwanted fish can result in a dramatic increase of vendace or whitefish populations.
Where hybrids of peled and whitefish have been stocked from hatcheries into eutrophic lakes, or have been produced by natural hybridisation, the result has been a slow growing, but rapidly reproducing population which is of low fishery value.
It is very important to consider ecological factors in determining the type of stocking material (species, developmental stage, size, etc.) and season of stocking operations. Sensitivity to handling depends on the physiological condition and size of fish to be stocked and the ambient temperature. There is evidence that the more fatty the stocked fish, the less resistant they are and the higher the water temperature, the more sensitive they are to handling. The size of stocked fish should be determined on the basis of the environmental conditions (physiological water quality requirements, species composition, predation, etc.).
The physical environment and predation rate vary according to the seasons, which can be utilized in determining the proper time for stocking operations. On the other hand the time of stocking operations may be determined on the basis of the production cycle and technically suitable times of stocking. Vendace transfer should be carried out only in winter when Y-O-Y vendace are easy to collect and transportation can be arranged with minimal loss of fish.
Coregonids can carry many of the known bacterial and viral diseases and parasites of salmonids. The probability for the occurrence of these normally increases when age and size increase. The most important observed parasites have been Diplostomum, Proteocephalus and Triaenophorus in pond and hatchery rearing. Dense populations can increase susceptibility to certain parasites and this will lower the value of the catch. Triaenophorus is especially a problem where pike are numerous and the market value of fish with this parasite is practically nil. It is therefore very important as far as possible to use stocking material with minimal parasite loads.
When hatchery fish are released, they may have harmful genetic effects due to inbreeding, hybridization and introgression in natural coregonid populations. These can result in loss of genetic variation (reduced heterozygosity) and increase of nonadaptive “hatchery genes” and hence reduced fitness and overall vigour of coregonid stocks, but very little information is available on this.
Genetic hazards are not important in eutrophic lakes with no naturally reproducing coregonid stocks, but can be very high in oligotrophic lakes with natural stocks. The local material must always be used if this is at all possible. Transfer of coregonids to a new environment or to different geographical areas in which indigenous coregonid populations exist is risky if it is aimed at producing a self-sustaining population, because the spawning time in relation to environmental conditions seems to be at least to some extent genetically determined and the hybrids are likely to be less well adapted than the native stock.
Genetic and health hazards involved in stocking can be reduced with the following general guidelines:
In order to maintain a consistent basis for stocking materials lakes with a local form used for the collection of spawn for hatchery rearing should not be stocked with any material except that local form.
Whitefish from the sea areas should not be stocked in inland waters and sea whitefish roe should be disinfected carefully when incubation takes place in hatcheries located in the interior.
Transportation of fish from one water system to another should be restricted to an absolute minimum.
The origin of the fish used for stocking, their condition and possible health hazards should be studied before stocking.
In order to avoid unintentional selective breeding it is important to use the full range of the spawning population. For example, collection of too much material early in the spawning period will favour the earlier maturing and hence smaller fish.
Intensive vendace and peled stocking might be expected to increase algal blooms and biomass because of heavy predation on zooplankton, and for this reason stocking is not always considered good for drinking water reservoirs. However, there is evidence that intensive stocking of vendace can improve the quality of the environment and slow down the eutrophication process. This is due to the fact that yields from vendace stocking may be very high, and thus substantial amounts of phosphorus are withdrawn from the ecosystem with the fish catch. Therefore, vendace stocking has an indirect effect on the fish stock in the lakes as well, this effect being beneficial. There are lakes in which the ecological balance is preserved mostly due to intensive vendace stocking and heavy fishing (for instance Lake Maroz). The obvious contradiction of the Polish experience to the biomanipulation results can be explained by the fact that fishing in Polish lakes is very efficient multispecies fishing (in fact a large scale biomanipulation exercise).
