M. Fürst
Institute of Freshwater Research
Drottningholm, Sweden
ABSTRACT
Crayfish are in great demand by the Swedish people but the crayfish plague Aphanomyces astaci has decreased the yield to 20 percent of its previous level. Nearly all attempts to restore old populations by using the native species Astacus astacus have failed. In many cases the reason for this is probably that the plague is kept alive in the lakes and streams by a few surviving crayfish. These crayfish infect each other and stocked crayfish continuously.
At least some North American crayfish species are resistant to the plague, e.g., Pacifastacus leniusculus. This species was first introduced into Sweden in 1960. Up to 1982 it had spread to 260 lakes and rivers in this country. Most introductions have been very successful, and the catches have in several cases been reported to exceed the former catches of Astacus in the same lakes. The two species are very similar from an ecological and taste point of view.
During moulting Pacifastacus is highly susceptible to the plague spread by an infected sympatric Astacus population. During experimental conditions in aquaria or in farms it was found that Pacifastacus had lost its resistance and died. It is also possible that other diseases, heavy metal ions or biocides cause reduction of resistance. No other harmful disease or parasite has as yet been recorded on Pacifastacus in Sweden.
RESUME
Les écrevisses sont très recherchées par les Suédois mais comme elles sont victimes d'une maladie Aphanomyces astaci les rendements ont baissé de 20 pour cent. Presque toutes les tentatives pour régénérer les anciennes populations avec l'espèce indigène Astacus astacus ont échoué. Dans de nombreux cas, cela tient probablement à ce que la maladie continue à être propagée par les quelques écrevisses survivantes dans les lacs et les cours d'eau. Les écrevisses s'infectent les unes les autres ainsi que les nouveaux sujets.
Tout au moins quelques espèces d'écrevisses d'Amérique du Nord notamment Pacifastacus leniusculus résistent à la maladie. Cette espèce a été introduite pour la première fois en Suède en 1960. Jusqu'en 1982, elle a été propagée dans 260 lacs et cours d'eau du pays. La plupart de ces peuplements ont réussi et dans plusieurs cas on a indiqué que les captures de cette espèce dépassaient celles d'Astacus effectuées dans les mêmes lacs. Les deux espèces ont pratiquement les mêmes exisgences du point de vue écologique et la même saveur.
Pendant la mue, Pacifastacus est fortement exposée à la maladie propagée par une population sympatrique contaminée d'Astacus. Au cours des expériences effectuées en aquarium ou dans les fermes d'élevage on a constaté que Pacifastacus avait perdu sa résistance et mourait. Il est également possible que d'autres maladies, la présence d'ions métalliques louds ou de biocides aient affaibli cette résistance. On n'a encore enregistré aucune maladie ou parasites qui nuisent à Pacifastacus en Suède.
The yearly catch of crayfish in natural waters in Sweden was estimated as being about 1 000 t before the crayfish plague started to spread in 1907. Today the yield is reduced to 100 t in spite of a much greater interest in the utilization of this natural resource. The plague is a fungus, Aphanomyces astaci Shikora, and was probably introduced to Europe from North America in 1860 when it first appeared in Italy. Crayfish were probably introduced or transported in the ballast of ships from the Mississippi River in Louisiana. This is a deduction arrived at from recent research on the biology of the crayfish plague (Unestam, 1969, Unestam and Weiss, 1970, Unestam, 1972) which has shown that several (probably all) of the North American crayfish species are resistant to Aphanomyces but are frequently carriers of it.
Apart from the economic loss to man the ecosystem received a considerable blow because crayfish interacted directly in most trophic levels, e.g., on periphyton, macrophytes, zooplankton, bottom fauna and fish. If the nature of the bottom is good for crayfish the littoral zone has less periphyton and macrophytes and such lakes seem to be less influenced by eutrophication.
The crayfish Orconectes limosus Rafinesque was introduced from North America to Germany in 1890 (Pieplow, 1938) and spread over part of northwestern Europe (Müller, 1954) without being affected by the plague. It was presumed that other American crayfish might also be resistant, since it was probably not just coincidental that this particular species was resistant. In 1960 Svärdson (1965) therefore introduced Pacifastacus leniusculus from California to Sweden for experimental purposes. He found that Pacifastacus might possibly fulfil the demands required of a new species and compensate for the losses of the old one.
In 1981, 21 years after the first introduction, Pacifastacus was stocked in 260 Swedish lakes and rivers. (Each river was stocked in several places but this was counted as only one stocking.)
Permission to stock a new water is granted by the National Board of Fishery and, in principle, those parts of Sweden are excluded where no plague is registered. The aim is to try to preserve areas for Astacus astacus so that populations of this species will have a chance to survive in the future.
Stocking is normally undertaken by using newly-hatched fry after the second moult. This method is found to involve a high mortality probably during the first summer and only 5–10 percent of the stocked material reach sexual maturity (Fürst, 1977). No other material has been available until recently when adult crayfish caught in lakes and rivers are also moved from one water to another.
Reproduction has been registered in most of the stocked waters. As yet probably no population has reached the carrying capacity in the whole lake or river but locally the populations seem to be even higher than for the earlier Astacus populations. A local yield of 55 kg/ha has been registered and a maximum of 90 crayfish in one trap at one emptying. The taste is comparable to Astacus and the appearance is pleasing to the crayfish lover. The price is as high as for Astacus, about 80 Swedish crowns/kg to the fishermen and about 190 over the counter.
Pacifastacus is resistant to crayfish plague under natural conditions and is principally a carrier of the plague. Most of the brown-blackish spots between 1 and 5 mm in diameter which may be registered on all parts of the body contain hyphs of Aphanomyces astaci. The frequency and intensity of visible symptoms vary depending on age, length, sex and locality (Fürst and Boström, 1978). In populations with low fishing intensity both frequency and intensity are higher. Frequencies are found to be between 0 and 35 percent, intensities are from 0 to 4 spots per individual. “Normal” frequency for Pacifastacus caught by traps seems to be about 5 percent and intensity 1–4. In one case there were higher numbers of larger spots and this was suspected to be due to abnormal concentrations of some heavy metals in the sediment.
It has been shown in experiments that Pacifastacus is susceptible during moulting to infestation of zoospores of Aphanomyces arising from dead or dying Astacus (Appelberg and Fürst, 1978). In these cases Pacifastacus die.
Astacus has such a low resistance to the plague that all infested animals die. Purely by chance a few escape infestation and survive. Sometimes they build up new populations which are found very locally in most of the old area of dispersal. In less than 5 percent of the cases, however, do populations reach their former densities and that only for a limited time. There are very few examples of such populations having survived the last four decades. In most cases Astacus survives only in scattered populations which no longer have any economic value.
Aphanomyces is said not to produce any resistant spores (Unestam, 1969a) but it spreads zoospores from infested crayfish some days after they have succumbed. It seems that Aphanomyces is kept alive by continual infestation of only a few specimens of Astacus. This, in turn, prevents the development of permanent and productive Astacus populations. This theory does not exclude the possibility that Aphanomyces could also be reintroduced from other waters.
In several cases the Astacus population finally disappeared when Aphanomyces-carrying Pacifastacus were introduced. In a few cases, however, the stocked fry obviously did not carry the Aphanomyces. No visible symptoms were found and the start of a parallel development of the two species took place.
In ten cases more than ten specimens of Astacus were trapped together with Pacifastacus. In one lake the two populations developed as follows:
| Year | Fishing effort No. traps/night | No. Pacifastacus | No. Astacus |
| 1972 | 150 | 7 | 5 |
| 1973 | 300 | 0 | 2 |
| 1974 | 410 | 9 | 63 |
| 1975 | 300 | 9 | 36 |
| 1976 | 286 | 0 | 0 |
| 1977 | 250 | 0 | 0 |
| 1978 | 200 | 0 | 0 |
| 1979–80 | 200 | 0 | 0 |
In 1976 crayfish plague was recorded in a nearby lake and since then no crayfish have been trapped. One specimen was observed in 1977 indicating that at least some Pacifastacus have survived. This example indicates that the presence of a population of Astacus may imply a risk that, for example, the plague carrying Pacifastacus infests Astacus which, in its turn, dies and spreads zoospores to such an extent that most of the moulting Pacifastacus die. If this happens during the peak of the moulting period the result is devastating. To avoid the risk of such a situation crayfish plague must be spread to the Astacus population before stocking with Pacifastacus.
During experimental conditions in aquaria or in farms it has happened that Pacifastacus has lost its resistance to the plague and died. In aquaria especially the intensity of the plague increases and often legs or chilipeds are lost. Too much handling seems to stress crayfish so that they lose their resistance. It is also possible that other diseases or the influence of heavy metals or biocides causes reduction of the resistance.
The above example is the only recorded case of an introduction of Pacifastacus failing because of a disease or a parasite. However, the development of most of the populations of Pacifastacus is being followed and great interest is being shown by the local people and any unusual situation has, therefore, a great likelihood of being recorded.
There have been fears that some known or unknown disease or parasite native to Europe might affect Pacifastacus very hard because of a lower resistance than that of the native Astacus.
The imported Pacifastacus were reported to carry a great many small or microscopic organisms. Xironogiton instabilis (Branchiobdellidae) was found in great quantities on the chilipeds. Neither this species nor others have been recorded on Swedish Pacifastacus during the past 21 years. This seems very hopeful and if we can learn how to handle Pacifastacus so that it does not lose its resistance against plague there seems to be a real possibility of restoring the old crayfish waters and building up farms producing Pacifastacus.
Appelberg, M. and M. Fürst, 1978 Försök att smitta signalkräfta under skalömsningen med hjälp av pestsmittad flodkräfta. In Frekvens av en skalsvamp (kräftpest) pa signalkräftor, edited by M. Fürst and U. Boström. English summary: Frequency of visible symptoms of crayfish plague in populations of Pacifastacus leniusculus Dana. Inf.Inst.Freshwat.Res., Drottningholm, (1):24 p.
Fürst, M., 1977 Introduction of Pacifastacus leniusculus into Sweden: methods, results and management. In Freshwater crayfish, edited by O.V. Lindquist. Kuopio, Finland, University of Kuopio, pp. 229–47
Fürst, M. and U. Boström, 1978 Frekvens av en skalsvamp (kräftpest) pa signalkräftor. English summary: Frequency of visibile symptoms of crayfish plague in populations of Pacifastacus leniusculus Dana. Inf.Inst.Freshwat.Res., Drottningholm, (1):24 p.
Müller, H., 1954 Die Flusskrebse. In Die neue Brehm-Bucherei, 121. Wittenberg, A. Ziemsen Verlag, 40 p.
Pieplow, U., 1938 Fischereiwissenschaftliche Monographie von Cambarus affinis. Z.Fisch., 36(3)
Svärdson, G., 1965 The American crayfish Pacifastacus leniusculus (Dana) introduced into Sweden. Rep.Inst.Freshwat.Res., Drottningholm, (46):90–4
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., 1969a On the adaptation of Aphanomyces astaci as a parasite. Physiol.Plant., 22:221–35
Unestam, T., 1972 On the host range and origin of the crayfish plague fungus. Rep.Inst.Freshwat.Res., Drottningholm, (52):192–8
Unestam. T. and D.W. Weiss, 1970 The host-parasite relationship between freshwater crayfish and the crayfish disease fungus Aphanomyces astaci: responses to infection by a susceptible and a resistant species. J.Gen.Microbiol., 60:77–90
