Yield potential is affected not only by the coregonid species stocked, but by both abiotic and biotic environmental conditions and by factors intrinsic to the population in question (see Chapter 3). These should be considered first before the decision to stock coregonids is made. A three step procedure to manage the yield potential is recommended.
Once the manager is satisfied that the general conditions are favourable he needs to consider the relationship between the yield potential, life stage and stocking rate in determining the target catch from the fishery. It is stressed that the selected target catch should be realistic. In the attempt to increase catches it has been a very common error to use too high an initial stocking rate. The first step is to examine the data from Table 1 which summarizes all known results for coregonids in terms of the requirements to produce 1 kg/ha of catch. For example if in an eutrophic lake the target is to increase the whitefish catch by 1 kg/ha then the target could be achieved by stocking 323 ( = 1 000/3.1) larvae or 38 (= 1 000/26.3) summer juveniles or 15 (= 1 000/66.7) autumn juveniles.
The range of the results shown in Table 1 is very broad. The yield from stocking is affected not only by environmental conditions and stocking rate, but also by fishing. It is self-evident that if there is no fishing, the yield from stocking is nil. From a fisheries management point of view fishing effort should be considered in determining the stocking rate. This can be done in the second step of the recommended process.
Table 1 Yield (kg/l 000 fish stocked) from coregonid stockings by life stages in oligotrophic (Finnish) and in eutrophic (Polish) lakes. Range is presented in parentheses except for y-s (yolk-sac) larvae, where only range is presented. ? shows that results are unknown.
| LIFE STAGE | VENDACE | PELED | WHITEFISH | |||
| OLIGO | EU. | OLIGO | EU. | OLIGO | EU. | |
| Y-s larvae | 0–0.04 | 0.17–5 | 0–14.9 | 0–26.3 | 0–7.3 | ? |
| Larvae | ? | ? | ? | 16.9 (4.7–83.3) | ? | 3.1 (0.9–38.5) |
| Juveniles | ||||||
| - Summer | ? | ? | ? | 8 (0–333) | ? | 26.3 (8.5–83.3) |
| - Autumn | 15.9 | ? | 62.5 (3.0–142.9) | 26.3 (1–200) | 55.6 (2–250) | 66.7 (2–250) |
Where sufficient comparative data exist the second step is to make a statistical analysis of the variables known to be significant in management in a given geographical area. This method allows for the consideration of environmental factors and fishing, and accordingly the adjustment of the stocking rate. This approach has been used in practical fisheries management at least in Poland (vendace and whitefish) and in Finland (whitefish). The Polish experience can be obtained from the Inland Fisheries Institute, Olsztyn. An example of the Finnish application for juvenile whitefish stocking is shown in Appendix 2. This method can give starting points for fisheries regulation and stocking which can later be adjusted according to the monitoring results.
The size of the fish released is an important factor affecting yield potential. It is seen from Table 1 that the yield from stocking increases as a function of the developmental stage. However, the increase is not linear (Figure 2a). Summer juveniles can give nearly as good results as autumn juveniles. This is also supported also by Finnish observations that the size of autumn juveniles has very little effect on the yield from stocking. One prominent feature is that uncertainty of the yield potential decreases at higher developmental stages (Figure 2c), which is an indication of the effect of size on the yield.
The third step in the process should be adaptive management. Management options will need to be changed from time to time, but this should be done in a stepwise manner, with sufficient time lags to monitor the response of the lake fishery to the steps. A variety of methods are available to test the stocking efficiency; for example, lake to lake comparisons, time series, stocking in alternate years, analysis of yield or year class strength. The contribution of any naturally occurring population should also be considered via such mechanisms as egg dredging, comparison of stocked and naturally occurring larvae or marking and tagging.
There can be both short-term and long-term effects on the existing population of the species stocked. A prerequisite is that there should be a self-sustaining population of the species in the lake and hence most of the eutrophic lakes are excluded.
Long-term effects are poorly known, although it is known that hybridization and most probably introgression have taken place (see Chapter 3). Further there can be effects on the distribution and occurrence of parasites and other fish pathogens. The effects of these genetic and pathological factors on fish yield are so far unknown.
Much more is known about short-term effects. Stocking with fertilized eggs and yolk-sac larvae of all coregonids has had very few adverse effects in lakes with endemic stocks (no or minimal catch increase). Juvenile stocking has increased the catch level, but overstocking has in many cases resulted in stunted populations and smaller and less valuable catches. Due to the fact that juvenile stocking in most cases succeeds, there is a greater probability of genetic effects (increase of “hatchery genes”) and at least theoretically it is probable that the stocked population outcompetes the endemic one.
Due to the complex interactions in freshwater ecosystems it is difficult to forecast the effects of stocking on the ecosystem or the overall fish yield. However, if stocking is successful it will affect energy flow in the whole community. Very little is so far known about the detailed community effects of stocking. Some of the known examples are presented in Chapter 3.3 (Ecological Considerations).
Stocking can cause conflicts between different groups of fishermen and other water users. Vendace and whitefish have for the most part been target species for subsistence and commercial fishermen. Whitefish have an increasing significance for angling.
Stocking can change allocation of the catch between commercial, subsistence and recreational fishing. At present there is commercial fishing in lakes in which there was none prior to stocking. Commercial catches are taken with fykes and to a certain extent with trawls, which reduces the catch of gillnet fishing. The mean size of fyke and trawl catches is lower than that of gillnet catches. This may cause conflicts between groups of fishermen. In some cases over stocking has caused stunting of the stocked whitefish. This has reduced the catch of subsistence fishing and the condition of fish has been poor.
In mixed stock fisheries it is necessary to consider the growth rates of the potential introduction and the proposed harvesting method. For example, the introduction of peled to Lake Starnberg was a failure because they became vulnerable to the gill net fishery designed for the slower growing C. lavaretus before they reproduced.
