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M. Pursiainen
Evo Inland Fisheries and Aquaculture Research Station
Evo, Finland


K. Westman
Finnish Game and Fisheries Research Institute, Fisheries Division
Helsinki, Finland


The Siikajoki River flows into the Baltic Gulf of Bothnia. Previous to changes in the natural state of the water system it was one of Finland's most productive crayfish waters.

In the late sixties, construction of a 28 km2 reservoir on the upper river and intensive dredging of the main river and its tributaries combined to reduce the quality of the water to the point that the crayfish population was almost completely destroyed. The lost crayfish catch was calculated at 0.5 million individuals per year.

The water quality below the reservoir remained rather bad (low oxygen and pH) for many years. Short-term fluctations in the water level (because the reservoir power plant discharges only during the day) strongly limit suitable areas for crayfish. Since the water quality had improved, however, introductions of crayfish were begun.

The restoration possibilities were tested by stocking mature crayfish in some selected areas. Test fishings in the following years showed that crayfish survived well in areas with low fluctuations in the water level. In shallow areas and areas having a soft bottom, where erosion is strong, the results were poor.

After the test stockings it was concluded that the water quality was good enough for crayfish to survive, but the strong fluctuations in water level due to discharges would reduce the areas suitable for crayfish to 50 percent of the natural situation.


Le Siikajoki se jette dans le golfe de Botnie. Avant que son état physique ne soit modifié, c'était l'une des principales sources d'écrevisses de la Finlande.

A la fin des années soixante, la construction d'un réservoir de 28 km2 sur le cours supérieur du fleuve et le dragage intensif de l'ensemble du réseau ont entraîné une dégradation de la qualité de l'eau, dégradation telle que les populations d'écrevisses ont été presque complètement détruites. Les captures ainsi perdues ont été estlmées à 0,5 million d'individus par an.

La qualité de l'eau en aval du réservoir (oxygénation et pH) est restée assez médiocre pendant plusieurs années. Les fluctuations fréquentes du niveau des eaux (l'eau du réservoir n'est déversée dans le fleuve que durant la journée) font que les zones où pourraient s'lnstaller les écrevisses sont très limitées. Néanmoins, on a commencé à en introduire lorsque la qualité de l'eau s'est améliorée.

Les possibilités de reconstitution des stocks ont été étudiées en repeuplant un certain nombre de zones avec des écrevisses matures. Les essais de pêche effectués les années suivantes ont montré que le taux de survie des écrevisses était bon dans les zones où les fluctuations du niveau des eaux étaient faibles. En revanche, les résultats ont été médiocres dans les eaux peu profondes et sur les fonds friables où l'érosion est forte.

On a conclu de ces expériences de repeuplement que la qualité de l'eau était suffisamment bonne pour la survie des écrevisses mais que les fortes fluctuations de niveau dues à la centrale hydro-électrique réduiraient à 50 pour cent de la superficie originale les zones adaptées aux populations d'écrevisses.


Astacus astacus L. is the only native freshwater crayfish species in Finland. Its original area of distribution has extended to latitude 62°N. In order to improve and extend the crayfish fisheries, crayfish have been introduced to many waters where the species has not earlier occurred. Nowadays there exist self-reproducing crayfish stocks up to latitude 65°N in eastern parts of the country and to 67°N in western parts (Westman, 1973).

One of the water courses in which crayfish were introduced at the beginning of this century is the River Siikajoki, which is one of the large rivers flowing into the Baltic Gulf of Bothnia (see Fig. 1). Crayfish thrived there so well that the river was one of the most important fishing areas for the species in the country before hydraulic construction in the late sixties. The annual crayfish catch in the approximately 96-km long main river was assessed at about 460 000 individuals (Pursiainen et al., 1981).

In connexion with the construction of the 28 km2 Uljua Reservoir in 1969 the whole crayfish stock disappeared almost completely in the main river below the reservoir. As a reason of the destruction, the sudden worsening of the water quality was apparent during the construction period (Westman, 1974). Two smaller reservoirs were built and intensive dredging operations carried out in connexion with the construction of the big reservoir. The water quality in the Uljua Reservoir and also in the river below it remained bad for many years. In particular, oxygen level (min. 2 mg/l) and pH (5.8–6.0) were low and suspended solids high (Alasaarela, 1979), which are the most important factors adversely affecting crayfish survival (Lindroth, 1950; Niemi, 1977; Jarvenpaa et al., 1981). Short-term fluctuations in the water level below the reservoir (the reservoir power plant discharges only during the day) have in addition strongly changed the environment of the crayfish (see also Fig. 1).

About 10 years after the construction operations the water quality was so much improved that it was considered feasible to reintroduce crayfish in the river. This report supplies information concerning the three years period 1978–80, when test stockings were made and the development of the new crayfish stocks monitored.


2.1 Water quality

The Uljua Reservoir remains the most important factor affecting water quality in the River Siikajoki. Although a part of the negative effects on water quality have decreased, the oxygen level during the late winter was very low even at the end of the seventies (2 mg/1). On the lower part of the main river small tributaries and rapids aerate the water and so improve the oxygen situation somewhat (Alasaarela and Salmela, 1980). Other factors (e.g., the pH value and suspended solids) affecting the life of crayfish are slowly improving even though they still continue to be worse than they were before the reservoir was constructed.

2.2 Discharge and water level fluctuations

The water regime of Finland's western river systems comprises high spring floods, a slow decrease in water level during summer, autumn flooding and minimum levels again during the winter. The natural changes in the discharge and water level are slow and do not adversely affect the life of the crayfish.

Since the construction of the Uljua Reservoir the situation has completely changed. The reservoir itself is regulated to an annual amplitude of 8 m by collecting the top of the spring flood in the reservoir and then discharging the water during the low water periods. The discharge from the reservoir power plant must be arranged such that the daily mean current is not less than 1 m3/s in the main river and the maximum current through the power plant does not exceed a daily mean of 50 m3/s (Vesihallitus, 1978).

Neither the daily mean current nor discharge describe the river environment sufficiently clearly, because the power plant discharges water according to the peaks in electrical consumption. This means, of course, that during low water periods (summer and winter) the discharge occurs during the day, while during nights and weekends the discharge is at its minimum. The impact of this kind of regulation is clearly seen in Fig. 2, which presents the fluctuations of the water level 10 km below the reservoir and the daily mean discharge from the power plant at the end of the spring flood and during the low water period in the summer. The daily fluctuations in the water level are not so great further down the river.

2.3 Crayfish habitats

Crayfish prefer hard and relatively sloping bottoms with a lot of refuges and hiding places (Westman and Pursiainen, 1978) and in general this kind of habitat exists in abundance in river environments (Niemi, 1977). This was the situation, too, in the River Siikajoki before the construction. However, the river has been dredged for timber floating and flood protection and this has resulted in great changes in the habitats for crayfish and also increased the effects of short-term regulation. It must, however, be noted that a proportion of the negative effects of the dredging operations diminished slowly through so-called “naturalization”. A great part of previous crayfish habitats have been destroyed ultimately through the loss of all refuges and hiding places. Short-term regulation also causes sliming, which fills all possible refuges and prevents to a large extent the production of zoobenthos and vegetation which are the source of crayfish nutrition.


In order to discover what was the status of crayfish populations in the River Siikajoki before stockings, some trial catches were made in 1978 in areas 1, 2, 3, and 6 in the main river and in areas 4 and 7 in tributaries (see Fig. 1). Test catches have been made at the end of July, when crayfish are most active, with an Evo-trap (Westman et al., 1978) developed specially to research purposes. 25–50 traps were used overnight in each area at 5-m intervals. The catch amounted to only two crayfish on area 1 and one crayfish on area 7, on the Lamujoki tributary. Some crayfish stocking was carried out prior to 1978 by local fishermen and some of them have been successful, as can be seen in area 1. Test catches, however, clearly showed that the crayfish stock in 1978 was very weak in selected stocked areas.


Crayfish for stocking purposes were caught from the nearest available river, the River Oulujoki, about 80 km north of the River Siikajoki, in order to obtain crayfish which were as far as possible adapted to similar conditions and latitude as in the River Siikajoki. In Finland it is thought that only crayfish originating from running waters should be used for stocking rivers, and “lake” crayfish for stocking lakes. All stocked crayfish were marked by cutting the left uropod in 1978 and the right one in 1979. Only crayfish with a hard carapace and in good condition were employed for stocking purposes. Transportation was carried out dry in wooden or plastic boxes.

At the stocking sites crayfish were kept in net cages for one day prior to release since it has been observed that as a result of this method crayfish remain more attached to the stocking site than they do after having been released immediately after transportation. Crayfish were spread over an area of 100–150 m in the littoral zone. In stocking, the density aimed at was the same as observed some natural populations in rivers (2.5 individuals per m2, adults) (Westman and Pursiainen, 1982). In 1978 about 3 500 crayfish were released at six different sites. The mean carapace length was 52.6 ± 0.37 mm. In 1979 stockings were made in five areas with 2 300 crayfish having a mean carapace length of 53.0±0.25 mm. Different stocking areas and the numbers released are shown in Table 1.


To study the success of stocking test trapping was carried out in 1979 and 1980 in selected stocking areas by the same method as in 1978 prior to stocking, described earlier.

5.1 Catches

In 1979 the total catch was 249 crayfish, which corresponds to 0.83 crayfish per trap night. In 1980 the catch was 200 crayfish which is 1.00 crayfish per trap night. Catches from different areas are listed in Table 2.

In connexion with catches from different areas, the following points are of interest:

-   In area 1 no crayfish were caught in 1979 from the 1978 stocking area, which was a shallow rapids, but the re-stocking made in 1979 200 m below the rapids was successful. The reason for this is probably the ice situation in the rapids, which was very adverse in the winter of 1978–79.

-   In area 2 only a few crayfish were caught in 1979 and therefore no fishing was done in 1980. It seems obvious that shallow muddy areas like area 2 are too difficult for crayfish to move along because of the erosion of the bottoms due to regulation.

-   In area 3 the catch was rather good in both years and in 1980 stocked individuals both from 1978 and 1979 were caught, although the 1979 stocking was done 300 m downstream from the previous stocking area which served as a test fishing area in both years. This clearly shows how crayfish can move rather far from their stocking site.

-   On tributary area 4 the catch was good in the first year after stocking, but the reduction in 1980 probably shows how crayfish have become distributed over a larger area.

-   In area 5, where stocking was carried out in 1979, the catch was one of the best, which is a good sign for high survival over the first winter.

-   In area 6 and in tributary and control areas 7 and 8 a certain proportion (7–20 percent of the catch) of crayfish were unmarked, indicating that there existed crayfish either as survivers over a longer period or as the result of previous stockings. It was surprising especially in area 6, which was fished in 1978 without captures being made. It seems obvious that these unmarked individuals have moved into the area during the research period.

5.2 The condition of the crayfish

Crayfish had moulted in both test fishing summers before trapping, which is also clearly apparent from the mean size of specimens captured. For example, the carapace mean length of stocked crayfish in 1978 was 52.6±0.37 mm and that of trapped individuals (excluding crayfish without uropod cutting) varied from 55.4±2.52 to 58.9±0.79 mm in different areas. This means that the carapace length had grown by 3–6 mm, which is the expected normal growth in one year (Abrahamsson, 1972).

All crayfish caught by test trapping were in good condition and almost all females were ready to reproduce, which might be considered a sign of suitable enviornmental conditions. No information about reproductive success could be obtained, however, because electric fishing was impossible in turbid waters and so newly-hatched juveniles could not be caught.


According to the test trapping it seemed obvious that in the River Siikajoki there were only few crayfish left prior to stocking. These crayfish originated from some previous stocking done by local fishermen. However, the population was very weak. Test trapping also showed that on the upper part of the river and in some tributaries a part of the crayfish population may have survived in spite of dredging and suspected crayfish plague.

Stocking of crayfish proved rather successful except in two areas, the rapids area (1) and shallow soft bottom area (2). It seems obvious that this kind of biotope offers relatively poor possibilities for supporting productive crayfish stocks. In the winter, short-term fluctuations in discharge lead to the freezing of the rapids from the bottom during heavy frost, which naturally can destroy a crayfish population during the winter. On areas with soft bottoms crayfish do not seem to thrive, probably because all shelters will be filled with loose mud due to the fluctuations in water level. On such areas, where regulation is not so considerable, i.e., on deep areas with relatively hard bottoms, the crayfish can survive well. It is, of course, clear that the upper part of the river and also the tributaries, where no regulation exists and where the negative effects of dredging have diminished, offer rather good possibilities for crayfish production.

The stocking density used (about five individuals per metre) seemed successful, because the density of the population one-two years after stocking was sufficient for reproduction, though crayfish seemed to spread slowly in larger areas. No information about the success of reproduction could be gathered because juveniles born in the river were not caught. All females in the catch of the test fishings were, however, in reproductive condition, which can be seen from the cement glands under the abdomen.

The results allow the conclusion that crayfish production can be restored at least in part in the main river below the Reservoir Uljua. According to the profile of the river (rapids, other shallow areas, deep areas) and the extent of regulation, it was estimated that crayfish production may reach half the level of that occurring in the natural state before any construction. This would mean an annual production of 230 000 legal-sized (total length over 10 cm) crayfish.

In order to obtain the expected production an effective stocking programme is needed. Stocking areas should have hard bottoms where the fluctuation of the water level due to the short-term regulation are less than 10 cm. Stocking should be concentrated only in the best biotopes and the number of crayfish should be the same as that shown to give results in test stockings, i.e., 400–500 individuals for each area and 4–5 specimens per metre.

If rapid results are required, mature crayfish exceeding 8 cm in length should be used. The stocking of large-sized males should be avoided as they terrorize smaller specimens and occupy the best biotopes. The sex-ration of the stocking material should be 3 females to 1 male. Only specimens with a hard exoskeleton should be released.

The most suitable time for stocking would be the end of the summer when all crayfish have already moulted. They still have enough time to become adapted to the new environment before mating and the onset of winter. To avoid migration from the stocking areas the crayfish should be kept in net-cages, etc., in the new water area for a couple of days before they are released.

The prerequisite for the success of stocking is, however, that there be no new hydraulic constructions and that the water quality is good enough for crayfish to reproduce. The development of the new crayfish population should be monitored for several years along with the water quality and other conditions on the whole river so as to organize the crayfish fisheries in the best way.


Abrahamsson, S., 1972 Fecundity and growth of some populations of Astacus astacus Linne in Sweden. Rep.Inst.Freshwat.Res., Drottningholm, (52):23–37

Alasaarela, E., 1979 Siikajoen yhteistarkkailu: osa II. Siikajoen vesistotarkkailun tulokset v. 1978 ja vesiston veden laadun kehittyminen v. 1963–78. Pohjois-Suom.Vesitutkimustoimisto, 22 p. (mimeo)

Alasaarela, E. and K. Salmela, 1980 Siikajoen yhteistarkkailu. Siikajoen vesistotarkkailun tulokset v. 1979 ja Uljuan altaan vaikutus Siikajoen veden laatuun ja ainetaseisiin v. 1969–1979. Pohjois-Suom.Vesitutkimustoimisto, 33 p. (mimeo)

Jarvenpaa, T. et al., 1981 Effects of hypoxia on the haemolymph of the freshwater crayfish, Astacus astacus L., in neutral and acid water during the intermoult period

Lindroth, 1950 Reactions of crayfish on low oxygen pressure. Rep.Inst.Freshwat.Res., Drottningholm. (31):110–2

Niemi, A., 1977 Population studies on the crayfish Astacus astacus L. in the River Pyhajoki, Finland. In Freshwater crayfish, edited by O.V. Lindquist, Kuopio, Finland, University of Kuopio, vol. 3:81–94

Pursiainen, M. et al., 1981 Ravun elinmahdollisuudet Siikajoessa ja rapukantojen hoitosuunnitelma. 40 p. (MS)

Vesihallitus, 1978 Pohjanmaan pohjoisosien vesien kayton kokonaissuunnitelma. Vesihallituksen asettaman tyoryhman ehdotus. l. osa. Yleiskuva suunnittelualueesta, vesivarat ja vesien nykyinen kaytto. Vesihallitus, tiedotus, 137:1–327

Westman, K., 1973 The population of the crayfish, Astacus astacus L. in Finland and the introduction of the American crayfish Pasifastacus leniusculus Dana. In Freshwater crayfish, edited by S. Abrahamsson. Lund, Studentlitteratur, vol. 1:42–55

Westman, K., 1974 Uljuan tekoaltaan vaikutukset alapuolisen Siikajoen rapukantoihin. Riista-ja Kalatalouden Tutkimuslaitos, Kalantutkimusosasto. Tiedonantoja, 1:37–55

Westman, K. and M. Pursiainen, 1978 Development of the European crayfish Astacus astacus L. and the American crayfish Pacifastacus leniusculus Dana populations in a small Finnish lake. In Freshwater crayfish, edited by P.J. Laurent. Thonon-les-Bains, Institut National de la Recherche Agronomique, pp. 243–50

Westman, K. and M. Pursiainen, 1982 Size and structure of crayfish (Astacus astacus) populations on different habitats in Finland. Hydrobiologia, 86:67–72

Table 1 Crayfish stockings in various areas in the River Siikajoki (Numbers of individuals released)

115706001 170
325946001 194
Total3 5032 3195 822

1 In 1978 stocking was done in the rapids in area 1 and in 1979 200 m below the rapids
2 The stocking sites are about 300 m apart

Table 2 Test trapping and the crayfish catches in the River Siikajoki in 1979 and 1980 Number of individuals, individuals per trap night and mean length of carapace

AreaTotal catchCatch/ trapCarapace lengthTotal catchCatch/ trapCarapace length
10--220.4458.6 ± 0.96
250.1055.4 ± 0.52   
3501.0058.9 ± 0.79350.7059.8 ± 0.83
4561.1257.5 ± 0.62120.4863.3 ± 1.78
5   421.6859.2 ± 0.65
6611.2260.4 ± 0.83461.8462.9 ± 0.89
7771.5456.8 ± 0.62   
8   431.7257.1 ± 0.59

Fig. 1

Fig. 1 The River Siikajoki water course and the test areas

Fig. 2

Fig. 2 Fluctuation of the water level 10 km below the Uljua Reservoir in centimetres (curved line) and the mean daily discharge of the Uljua Power Plant in cubic metres per second (straight line)

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