THE EFFECTIVENESS OF EEL STOCKING IN POLISH LAKES

M. Leopold and M. Bninska
Inland Fisheries Institute
Olsztyn-Kortowo, Poland

ABSTRACT

Polish lakes have been stocked with elvers for many years but since 1950 the practice has intensified and is now a regular measure. This paper examines eel stocking and catches against a background of increasing eutrophication. General data on stocking and catch from all Polish lakes is presented for the 30-year period from 1951 to 1980. More detailed information from 559 lakes is analysed for the past 24 years. An assessment of the effectiveness of stocking is made from both the commercial and recreational catches. The influence of selected factors on stocking effectiveness is pointed out and some suggestions are made as to the types of lake most suitable for stocking.

RESUME

Cela fait quelques dizaines d'années que l'on repeuple les lacs polonais avec des civelles. Depuis 1950, cette pratique s'est intensifiée et est devenue régulière. Les auteurs présentent les captures d'anguilles et les repeuplements effectués dans les lacs polonais sur la base de l'évolution des stocks sous l'effet de l'eutrophisation. L'efficacité du repeuplement est analysée statistiquement dans le temps et dans l'espace.

Dans le premier cas, les auteurs se fondent sur des données générales concernant le repeuplement et les captures d'anguilles dans tous les lacs polonais pendant 30 ans (1951–80); dans le second, sur des données générales concernant le repeuplement et les captures d'anguilles dans tous les lacs polonais pendant 30 ans (1951–80); dans le second, sur des données détaillées concernant un certain nombre de lacs (559) pendant plus de 24 ans. C'est la première fois en Pologne que l'on procède à une évaluation définitive de l'efficacité du repeuplement en anguilles en se fondant à la fois sur les captures de la pêche commerciale et sur celles de la pêche récréative. En outre, les auteurs indiquent certains des facteurs qui ont un effet sur cette efficacité et présentent des suggestions préliminaires touchant le type de lac le mieux adapté au repeuplement.

1. HISTORY OF STOCKING

Polish lakes have been systematically stocked with eels since 1948. Time series analysis shows the trend in stocking to be an approximately straight line (Fig. 1) which fits the equation:

z = 0.027 + 1.135 x

where:

z = level of stocking in millions of elvers

x = successive years of the analysed period (1,2,3.......31)

The mean level of stocking was 18.182 million individuals, with a S.D. of 11.86 and a coefficient of variation V% = 65.2.

The average annual increment relative to the mean level of stocking was 6.24 percent for the period 1948 to 1978. This represents a yearly increment of 4 elvers/ha for the 280 000 ha of exploited lakes, with a mean level of stocking of 65 elvers/ha/yr. In recent years the mean level of stocking has risen to over 100 elvers/ha/yr.

2. THE COMMERCIAL FISHERY

Catches of eel by the commercial fisheries of Polish lakes show a continuous increase both in total weight and as a percentage of the catch. The average catch during the 1951 to 1981 period was 432.25 t and the avarage annual increase was 4.15 percent i.e., 18 000 kg annually. Variations in catch are less pronounced than those of stocking as are shown by the S.D. of 167.57 and the coefficient of variation V% = 38,77.

The trend in eel catch is approximately linear represented by the equation:

y = 145.51 + 17.92 x

where y = eel catch in metric tons and x = successive years of the analysed period (1,2,3....31). However the trend in catch conforms better to a parabola (Fig. 2) represented by the equation:

y = 219.026 + 4.554 x + 0.418 x²

The parabolic function has a coefficient of convergence of 0.055, as compared to 0.085 for the straight line. It can therefore be concluded that the increments in eel catch increase with successive years. In the first five years of the period increments amounted to 1 to 2 percent, whereas in the last five years they were as high as 6 to 7 percent, equivalent to 30 000 kg/yr.

3. SIGNIFICANCE OF STOCKING TO THE FISHERY

A comparison between the trends in stocking and catch suggests that the effectiveness of eel stocking into Polish lakes has increased in recent years. The commercial catch of other species from lakes over the past 20 years (1961 to 1980) shows an especially strong tendency for cyprinid landings also to increase (Table 1). This, together with a decline in the catch of percid and littoral fish species (Table 2) clearly points to the progressive eutrophication of Polish lakes. These trends in catch agree with the schemes of eutrophication presented by Colby et al. (1972), Hartman (1978) and Leach et al. (1977). In the context of these changes in the environment the increase in catch of stocked fish species, and especially of eel, indicates that the effectiveness of eel management in Poland is high.

Statistical analysis of the general data on eel stockings and catches in Polish lakes during the 31-year periods - 1948 to 1978 and 1951 to 1981 showed that the commercial catch of eel was totally dependent on artificial stocking. Linear correlations were calculated between eel stockings in a given year (year y) and the catch of eel in some subsequent year (year y + ..). The correlation was very high between stocking in year y and catch in years between y + 3 and y + 20. This demonstrates the close dependence of catch on stocking but at the same time indicates the difficulty of Interpreting the results in any detail because eel catches in any given year are determined by stockings in several of the preceding years. Furthermore the degree to which catches are influenced by the various preceding years is determined by a variety of physical, chemical and biological factors such as different thermal regimes, variations in water level, differential growth rates, etc. This is confirmed by the slight influence on the results of the removal of years where eel stocking deviated widely from the general trend from the data set. Fig. 3 compares the real catches with those calculated from regression equations for the relationship between stocking and catch 10 to 16 years later. As the figure shows there is very good agreement between real and calculated catches in both cases.

4. SUITABILITY OF LAKES FOR STOCKING

Data from 454 individual lakes were analysed to find out what types of lakes are most suitable for eel management. The data included morphometric and environmental information as well as average values of certain parameters of eel management for the 24-year period 1950 to 1973. From this analysis it was apparent that eel catch was dependent on the level of eel stocking, an index of intensity of exploitation (expressed as average number of months of fishing in the year), the average depth of the lakes, the shoreline development (expressed as length of shore/unit of surface area). This relationship is expressed by the equation:

y = - 0.70 + 0.0077 z + 0.381 x + 0.0119 u -0.082 v

where: y = eel catch in kg/ha

z = intensity of eel stocking in number of elvers/ha (z = 82.05)

x = average number of months fishing (x = 3.52)

u = shoreline development in m/ha (u = 65.58)

v = average depth in m (v = 6.17)

The coefficient of multiple correlation was R_y.zxuv = 0.66. The equation shows clearly that relatively shallow lakes with well developed shorelines are the most suitable for management for eel fisheries. This conclusion is supported by an analysis of the effectiveness of eel stocking into lakes as classified acording to the Polish typology. For instance in vendace lakes, which are usually deep and large with poorly developed shorelines some 64.4 elvers are needed to obtain 1 kg of commercially caught eel. By contrast in pikeperch lakes, which are shallower, smaller and with well developed shorelines, only 45.5 elvers are needed for the same catch. In these two exteme types of lake the intensity of exploitation of eel is about the same: 3.98 months/yr in the vendace lakes and 3.67 months in the pikeperch lakes. The average yield of eel amounts to 1.09 kg/ha and 2.01 kg/ha respectively. In addition to the features of the lakes described above pikeperch lakes tend to be more eutrophicated and hence productive than vendace lakes.

5. INFLUENCE OF RECREATIONAL FISHERY

The above results are based solely on the commercial catch, but in order to define the true effectiveness of eel management in Poland it is necessary to also take into account the catch from the recreational fishery. Angling is extremely popular in Poland with some 800 000 legally registered anglers in 1980. In 1979 studies showed that the eel catch from this fishery was 1 786 t, or 2.6 times higher than the commercial catch. When this additional catch is taken into account it would appear that the real survival of eels from stocking to capture is at least 15 percent, an estimate that includes natural recruitment to the stock. In these circumstances only 11.7 elvers are needed to obtain 1 kg of eel catch and the real yield works out at 4.6 kg/ha.

6. CONCLUSIONS

The continuously increasing catches of eel in Poland and the greater effectiveness of eel stocking are essentially determined by four factors:

1. An increase in the stocking rate

2. Increasing fertility of the lakes due to eutrophication

3. More intensive and effective fisheries

4. Improvements in the selection of lakes for management.

The last of these particularly can significantly increase production of eel in inland waters because the results of this survey show that eel management is most effective in relatively shallow, fertile lakes with well developed shorelines. Lakes of this type are usually small and have not been utilised properly so far.

7. REFERENCES

Colby, P.J. et al., 1972 Effects of eutrophication on salmonid communities in oligotrophic lakes. J.Fish.Res.Board Can., 29:975–83

Hartmann, J., 1978 Fishereiliche Veranderungen in Kulturbedingt eutrophierended. Seen.Schweiz.Z.Hydrol., 39:243–54

Leach, J.H. et al., 1977 Response of percid fishes and their habitats to eutrophication. J.Fish.Res.Board Can., 34:1964–1971

Table 1 Trend of commercial lake fish catches in Poland in the past 20 years

Fig. 1 Trend of eel stocking in Poland

Fig. 2 Trend of eel catches in Poland (commercial catch only)

Fig. 3 Relationship between eel catches and eel stockings