K. Westman
Finnish Game and Fisheries Research Institute
Helsinki, Finland
and
M. Pursiainen
Evo Inland Fisheries and Aquaculture Research Station
Evo, Finland
ABSTRACT
Since 1893, numerous unsuccessful attempts have been made in Finland to control the disastrous crayfish plague. No resistant strains of the only native crayfish, Astacus astacus, have developed. In order to restore crayfish production, the plague-resistant American crayfish Pacifastacus leniusculus was introduced to Finland in 1967. In 1967–74 a total of approximately 40 000 juvenile and adult Pacifastacus were stocked in 53 crayfish lakes devastated by the plague. In a few lakes, both Astacus and Pacifastacus occur simultaneously, offering excellent possibilities for comparative studies of both species. Pacifastacus seems to be able to survive and reproduce under Finnish condition at least up to 63°30'N and to tolerate fishing pressure. Yearly removal of legal-size (10 cm) Pacifastacus from some of the experimental lakes seems not to have caused any harm to the populations or their renewal. According to studies run in the same lake, Pacifastacus seems to grow faster and become sexually mature earlier than Astacus. Both species seem to prefer similar biotopes and have similar bionomics and life histories. No signs of hybridization have been observed. No harmful effects of the Pacifastacus population have so far been observed, either to the Astacus population or to the ecosystem of the lake. However, the niches of both species seem to overlap to such a great degree that competition for living space will probably occur.
RESUME
Dès 1893, la Finlande a commencé à combattre la maladie des écrevisses, aux conséquences catastrophiques. Ses efforts sont restés vains. Aucun type résistant d'Astacus astacus, seule écrevisse indigène. n'est apparu. Pour relancer la production, la Finlande a décidé d'introduire en 1967 l'écrevisse américaine Pacifastacus leniusculus qui, elle, résiste à la maladie. Entre 1967 et 1974, on a utilisé environ 40 000 juvéniles et adultes de cette espèce pour repeupler 53 lacs dévastés. Astacus et Pacifastacus coexistent dans certains lacs, ce qui permet des études comparatives. Le second cité paraît à même de survivre et de se reproduire en Finlande, du moins jusqu'à 63°30'N, et de supporter la pêche. Le prélèvement annuel de Pacifastacus de taille légale (10 cm) dans certains des lacs expérimentaux ne semble pas avoir nui aux populations ni à leur renouvellement. Par ailleurs, Pacifastacus semble avoir une croissance plus rapide qu'Astacus et atteindre plus vite la maturité sexuelle. Les deux espèces ont apparemment une préférence pour les mêmes biotopes et ont un cycle biologique et une bionomie identiques. Aucun signe d'hybridation n'a été observé. Pour l'instant, la population de Pacifastacus ne paraît pas avoir eu d'effets néfastes sur la population d'Astacus ni sur l'écosystème. Toutefois, les niches de ces deux espèces semblent se recouvrir à tel point qu'il y aura vraisemblablement concurrence entre Astacus et Pacifastacus.
There are some 60 000 lakes in Finland, with a total area of 31 613 km2, and owing to their irregular shores, their total shoreline is very long, measuring about 130 000 km. The total length of the numerous Finnish rivers exceeds 20 000 km. Consequently, the extent of the littoral zone, which is the habitat of the crayfish, is remarkably great, and Finland has particularly well-suited natural conditions for large-scale crayfish production.
The greatest obstacle for the utilization of these possibilities has been the disastrous crayfish plague, Aphanomyces astaci, which has caused great losses to the populations of Astacus astacus, the only endemic crayfish species occurring in Finland. The crayfish plague reached Finland in 1893 and of the 74 major watercourses, excluding the seven flowing north in which crayfish have never lived, only 20 remain uninfected. New cases have appeared yearly, partly in waters which have previously escaped the disease, and partly in waters which have new crayfish populations derived from stockings (Westman, 1973; Westman and Nylund, 1979). Along with the plague, the increasing manipulation of our watercourses, dam construction, draining, dredging etc., have contributed to the disappearance of the crayfish (see Pursiainen and Westman, 1983).
In Finland, many unsuccessful attempts have been made to control the crayfish plague. No resistant strains of Astacus astacus have developed (Unestam, 1969) and the crayfish seems to be incapable of re-establishing itself in chronically infected water courses. In order to restore crayfish production and to improve the earlier important crayfish fisheries, it was decided to start research and stocking experiments with the plague-resistant signal crayfish Pacifastacus leniusculus.
The first specimens of signal crayfish were imported as adults from California, U.S.A. in 1968–69. Due to the danger of inadvertently introducing fish or crayfish diseases or parasites, we abandoned the direct transfer of adult signal crayfish from North America and continued stocking experiments with newly-hatched juveniles, produced from broodstock from controlled cultivation systems in Sweden.
In 1967–74, a total of about 40 000 juvenile and adult signal crayfish were stocked in 53 lakes. The greater part, about 35 500, were newly-hatched juveniles from Sweden, the rest were larger juveniles cultivated in Finland, or adults from the U.S.A. Since 1974, no new introductions have been made, as we wished to obtain the results of previous stockings before continuing.
Preferred sites for stocking signal crayfish were small lakes with no inlets or outlets. Such sites ensured that stocked signal crayfish would not spread, and it also made it easier to follow the development of the populations. Moreover, all the selected lakes had previously supported native A. astacus populations which had been devastated by the crayfish plague. However, it was later revealed that there were also Astacus in some lakes in which Pacifastacus juveniles had been stocked. It would appear that some Astacus escaped the attacks of the plague fungus and gave rise to new Astacus populations.
Lakes in which both species occur simultaneously offer excellent possibilities for making comparative studies of the native and the exotic crayfish species. Since 1977, the Finnish Game and Fisheries Research Institute has conducted extensive research of this type, especially in one of the small lakes with self-reproductive Astacus and Pacifastacus populations. The aim of the research is to investigate and compare the populations of Astacus and Pacifastacus: their structure and densities in different biotopes, the biology of both species, e.g., growth, activity, reproduction, etc. Special attention has been paid to the relationships between both species, especially to possible competition on biotopes, for example (Westman and Puralainen, 1979).
Signal crayfish greatly resemble the native crayfish in appearance. The body weight in relation to length is considerably higher in Pacifastacus than in Astacus. The difference is connected with the greater development of the chelipeds in Pacifastacus, which is an advantage for utilization. The taste of Pacifastacus is as good as that of Astacus, when traditionally prepared.
Signal crayfish seem to be able to survive and reproduce under Finnish conditions at least up to 63°30'N. According to studies made on the same small lake, signal crayfish seem to grow faster and become sexually mature earlier than the native crayfish. According to Abrahamsson (1973), the egg production of Pacifastacus is approximately 90 percent higher than that of Astacus. Although egg production is higher and maturity occurs earlier in the signal crayfish than in the native crayfish, the development of Pacifastacus populations has been very slow in all of the experimental lakes. No signs of uncontrolled, vigorous reproduction and spread of the signal crayfish have been observed (Westman, 1973; Westman and Pursiainen, 1979).
Signal crayfish seem to tolerate fishing pressure. Yearly removal of legal-size (10 cm) Pacifastacus from some of the experimental lakes seems not to have caused any harm to the populations or to their renewal. This seems to indicate that in Finnish conditions signal crayfish would be able to develop populations that are economically viable, i.e., populations which are able to support fishing and give profitable catches.
Signal crayfish seem to be more active than the native crayfish, and the species is easy to catch with traditional methods and traps. Both crayfish species seem to prefer habitats of the same type, as the catches of both have been greatest in the same areas. The preferred biotopes are those with hard bottoms of rock, stones or gravel; and those with bottoms suitable for digging, covered with sunken tree trunks, twigs, litter and vegetation which provide shelter for the crayfish.
No signs of hybridization between Astacus and Pacifastacus have been observed in the specimens examined. There is, however, the possibility that Astacus males copulate with Pacifastacus females and Pacifastacus males with Astacus females, which might cause some disturbances in the “normal” copulation, thus slowing the increase of the populations in the same water bodies.
No harmful effects of the signal crayfish populations have so far been observed, either to the Astacus populations or to the ecosystem of the experimental lakes. However, the niches of both species seem to overlap to such a great degree that competition for living space will probably occur (Westman and Pursiainen, 1979).
Signal crayfish show high, but not complete, resistance against the crayfish plague fungus (Unestam, 1973) and infected specimens, if stocked or migrating to new water areas, may spread the fungus, thus causing harm to the highly susceptible native Astacus astacus. The cultivated juveniles may be free of the plague fungus as indicated by the simultaneous presence of Astacus and Pacifastacus in some lakes for several years.
Survival and reproduction of signal crayfish in some selected lakes does not necessarily prove that it would be a good substitute for the native crayfish in all the chronically infected crayfish waters. Introductions of new animal species often have a number of consequences that are not sufficiently well known in advance (see Westman and Tuunainen, 1983). Accordingly, caution must be exercised in expanding the introduction of Pacifastacus leniusculus, and special care must be taken to ensure that stocking does not out-distance research on the subject, as happens very easily with a new and interesting species.
The results are very promising and further stocking will be made in the near future. The importation of large numbers of living crayfish is expensive, and there is also the constant danger of introducing communicable fish or crayfish diseases or parasites not earlier found in Finland. For example, two Branchiobdellidae previously unknown in Finland, Xironogiton instabilius Moore and Cambarincola sp. were found on the exoskeletons of Pacifastacus imported from California. Accordingly, it seems to be safest and most advantageous in the long run to produce the crayfish needed for restoration of the devastated crayfish waters in Finland itself. Hatching and rearing investigations with Pacifastacus have been made in Finland since 1967 (Westman, 1973a). The main purpose is to develop cultivation techniques for producing crayfish suitable for stocking on a large scale.
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Pursiainen, M. and K. Westman, 1984 The restoration of the crayfish (Astacus astacus) stock in River Siikajoki, Finland. EIFAC Tech.Pap., (42)Vol.2:412–21
Unestam, T., 1969 Resistance to the crayfish plague in some American, Japanese and European crayfishes. Rep.Inst.Freshwat.Res., Drottningholm, (49):202–9
Unestam, T., 1973 Significance of diseases on freshwater crayfish. In Freshwater crayfish, edited by S. Abrahamsson. Lund, Studentlitterature, vol.1:136–50
Westman, K., 1973 The population of the crayfish, Astacus astacus L. in Finland and the introduction of the American crayfish Pacifastacus leniusculus Dana. In Freshwater crayfish, edited by S. Abrahamsson. Lund, Studentlitteratur, vol.1:41–55
Westman, K., 1973a Cultivation of the American crayfish Pacifastacus leniusculus. In Freshwater crayfish, edited by S. Abrahamsson. Lund, Studentlitteratur, vol.1:211–20
Westman, K. and M. Pursiainen, 1979 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, vol.4:243–50
Westman, K. and P. Tuunainen, 1984 A review of fish and crayfish introductions made in Finland. EIFAC Tech.Pap., (42)Vol.2:436–47
