THE PREVALENCE OF CRAYFISH PLAGUE (APHANOMYCES ASTACI) IN TWO SIGNAL CRAYFISH (PACIFASTACUS LENIUSCULUS) POPULATIONS IN FINLAND

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1 JOURNAL OF CRUSTACEAN BIOLOGY, 20(4): , 2000 THE PREVALENCE OF CRAYFISH PLAGUE (APHANOMYCES ASTACI) IN TWO SIGNAL CRAYFISH (PACIFASTACUS LENIUSCULUS) POPULATIONS IN FINLAND Viljo Nylund and Kai Westman (VN) Finnish Game and Fisheries Research Institute, P.O. Box 6, FIN Helsinki, Finland. Present address: Långvik, FIN Kirkkonummi; (KW, corresponding) Finnish Game and Fisheries Research Institute, P.O. Box 6, FIN Helsinki, Finland ( ABSTRACT The occurrence of dark brown melanized spots indicative of the presence of crayfish plague fungus (Aphanomyces astaci Schikora) was investigated in three Finnish lakes inhabited by the introduced American signal crayfish (Pacifastacus leniusculus Dana) in 1979, 1983, , 1990, 1993, and The crayfish were caught with crayfish traps during August September, 7 8 weeks after the moult. In one lake in which P. leniusculus population had developed from juveniles imported from Sweden in 1971, the crayfish showed no signs of infection. The lake also contained a population of the native noble crayfish (Astacus astacus L.). The coexistence of the two species for nearly 30 years in the same habitats in this small lake indicates the absence of crayfish plague. In two lakes P. leniusculus populations originated from adults imported from Lake Tahoe, U.S.A., in In 1979 the prevalence of infected P. leniusculus in the lakes (n = 1,841) was high, 52% in Lake Karisjärvi and 47% in Lake Iso-Majajärvi. By 1988 the prevalence had decreased to only 11% and 12%, respectively. During the 1990s, the number of infected specimens began to increase, reaching 24% in Lake Karisjärvi in 1993, and 18% in Lake Iso-Majajärvi in The black spots most commonly occurred on the walking legs (40% and 39%, respectively) and the chelae (28% in both lakes). The mean number of spots per crayfish was 2.0 and 2.4; many (43% and 36%, respectively) were > 3 mm in diameter. Crayfish plague does not appear to have affected the P. leniusculus populations in the study lakes in any way, nor have any mortalities been reported in either lake. Since 1967, the Finnish Game and Fisheries Research Institute (FGFRI) has studied the suitability of introducing American signal crayfish (Pacifastacus leniusculus Dana) resistant to the crayfish plague fungus (Aphanomyces astaci Schikora) into waters in which plague has repeatedly destroyed the native noble crayfish (Astacus astacus L.) (Kirjavainen and Westman, 1999; Westman et al., 1999). To date, P. leniusculus has been introduced into some 300 water bodies in Finland, mainly lakes. Populations are expanding, and commercial fishing has begun in some lakes (Mannonen and Westman, 1998). When stocking began, it was known that P. leniusculus could carry latent infections and thus infect endemic European crayfish species (Svärdson, 1965; Unestam, 1969). Fungal hyphae live inside the carapace of P. leniusculus, but their growth is inhibited by the natural resistance of the host animal. Symptoms of infection are apparent as melanized, dark brown or black spots on the carapace. All North American crayfish species show natural resistance to plague, in contrast to freshwater species from other continents (Unestam, 1969, 1972), suggesting that crayfish plague originated in North America (e.g., Alderman and Polglase, 1988). If resistance of P. leniusculus is for some reason diminished, e.g., due to stress (Persson and Söderhäll, 1983; Persson et al., 1987; Svärdson et al., 1991; Svärdson, 1992), a latent infection may become acute, resulting in rapid death of the individual. In Finland, appreciable numbers of P. leniusculus have died in a couple of lakes due to the interaction of plague and some other stress factor; yet the populations have revived fairly quickly (Tulonen et al., 1998). Despite the importance of this issue, very little research on the occurrence of crayfish plague has been conducted on wild stocks of North American species, either in their original localities or in their new habitats in Europe (cf., Fürst and Boström, 1978). Pacifastacus leniusculus has thus far been introduced into at least 21 European coun- 777

2 778 JOURNAL OF CRUSTACEAN BIOLOGY, VOL. 20, NO. 4, 2000 Table 1. Number of Pacifastacus leniusculus examined and mean total length (TL) and length range of samples from Lake Karisjärvi in 1979, 1983, , 1990, and 1993; Lake Iso-Majajärvi in 1979, 1983, , 1990, 1993, and 1994; and Lake Slickolampi in (SD = standard deviation). Lake Karisjärvi Lake Iso-Majajärvi Lake Slickolampi n Mean TL mm SD n Mean TL mm SD n Mean TL mm Females Males Total , Range tries, and the stocking programmes are still expanding (Holdich, 1988; Huner, 1988; Westman and Westman, 1992). Knowledge of plague occurrences and fluctuations is vital for the planning of P. leniusculus stockings and for efforts to combat crayfish plague and protect native species. Kobayashi and Söderhäll (1990) and Söderhäll (1990) postulated that all P. leniusculus carry plague, but the coexistence of signal crayfish and native species suggests that plague-free populations of P. leniusculus may also occur (e.g., Fürst and Boström, 1978; Westman and Pursiainen, 1979; Lowery and Holdich, 1988). With reference to the increase in transfer stockings it is important to record the occurrence of plaguefree P. leniusculus populations in natural waters (Nylund and Westman, 1992, 1995). The purpose of the present study was to investigate the occurrence of crayfish plague on the basis of melanized spots in two P. leniusculus populations originating from adults imported from North America. By examining a population developed from cultivated juveniles, we also attempted to determine whether plague-free P. leniusculus populations occur in Finland. MATERIALS AND METHODS Pacifastacus leniusculus Studied The three lakes studied were Karisjärvi (Lake KJ) (described in Kirjavainen and Westman, 1999), Iso-Majajärvi (Lake I-M) (described in Westman et al., 1999), and Slickolampi (Lake SL) (described in Westman et al., 1995). All three are natural, unpolluted forest lakes covered by ice for an average of six months (November April) a year. The average water temperature in winter is C, and for only 3 4 months a year, during May September, does it reach C. The P. leniusculus population of Lakes KJ and I-M originated from adults imported from Lake Tahoe, California, U.S.A. in (Westman, 1973). In Lake SL, the population had developed from cultivated secondstage juveniles imported from Sweden (Simontorp, Blentarp) in 1971, but their parents had also been brought from Lake Tahoe in An A. astacus population has most probably occurred in Lake SL since at least the early 1960s (Westman et al., 1995). In all three lakes, crayfish were trapped using baited cylinder traps (mesh size 8 mm) for research purposes almost every year from 1971 to 1994 without any significant changes being made to catch effort or crayfish handling (measuring, marking, etc.). Individuals longer than 70 mm were harvested from Lake KJ in , and individuals longer than 100 mm from Lake I-M in and from Lake SL in The P. leniusculus examined in Lakes KJ and I-M were caught during an eight-year period, in August September 1979, 1983, , 1990, and 1993, and in Lake I-M also in 1994, in general 7 8 weeks after moulting. Catching and handling of the catch have been described elsewhere (Kirjavainen and Westman, 1999; Westman et al., 1999). The total number of P. leniusculus examined from the Finnish lakes and their lengths (all lengths are given as total length = TL) are presented in Table 1. In addition, in 1982 a sample of 100 P. leniusculus (mean 104 mm, 53 females, 47 males) was examined from a commercial consignment imported from Oregon, U.S.A. (Metro-Auto Oy), and in 1994 a sample also from Oregon of 29 P. leniusculus (mean 127 mm, 3 females, 26 males) (Arctic Isle). There were no labels on the packages, and no information was available on the exact catching time, locality, catcher, etc. Examination of P. leniusculus Samples and Infection Experiments The P. leniusculus from Lakes KJ and I-M were checked for the occurrence of the plague fungus by taking random samples comprising 2 or 3 infected individuals from annual catches during the study years. The carapace tissue at the melanized spots (35 specimens, 42 spots) was examined (author V. Nylund) as a fresh mount with a light microscope (100 ). Two spots of two individuals were studied from the Oregon samples. The presence of plague fungal hyphae was verified by keeping 3 or 4 infected P. leniusculus (total 53 individuals, mm) from Lakes KJ and I-M in separate aquaria together with 6 8 healthy A. astacus (total 110 intermoult individuals, mm). The controls (6 8 A. astacus from the same batch) were kept under the same conditions in another aquarium but without P. leniusculus. Astacus astacus in Lake SL most probably originated from individuals that avoided infection during the 1960 plague epidemic (Westman et al., 1995). To ascertain whether the long coexistence of A. astacus and P. leniusculus was a consequence of the absence of plague infection among P. leniusculus or of unusually strong resistance by A. astacus to the plague, random samples comprising 10 A. astacus from Lake SL were kept in an aquarium together with one P. leniusculus from Lake KJ

3 NYLUND AND WESTMAN: CRAYFISH PLAGUE IN FINLAND 779 Table 2. Number of Pacifastacus leniusculus examined and prevalence of plague infection in lakes Karisjärvi and Iso-Majajärvi during study period Lake Karisjärvi Lake Iso-Majajärvi Year n Females % Males % Total % n Females % Males % Total % Total , with plague spots. The health of A. astacus were established by keeping 8 and 12 specimens from the same batch from Lake SL as a control group in another aquarium under the same conditions, but without P. leniusculus. These experiments continued until all of the A. astacus had died. The absence of A. astaci among P. leniusculus in Lake SL was ascertained by keeping 10 of these specimens together with 6 and 8 healthy A. astacus in the same aquarium, respectively. Eight A. astacus from the same batch were kept as controls in another aquarium. The experiments were continued until it was certain that the P. leniusculus were not carrying plague. The experiments lasted 3 7 months, i.e., 6 14 times the normal period from plague infection to death. The aquaria (100 l, cm) used for the experiments were supplied with a through current (8 l/h) of well-aerated Helsinki tap water (O 2 concentration 90%, ph , temperature C) filtered through active carbon. The illumination was arranged with natural and artificial light in the current daylight rhythm. The bottom of each aquarium was covered with gravel, and sections of plastic piping were provided as shelter for the crayfish. The crayfish were fed on carrot and decaying alder leaves. Waste water was drained to the sewer system. RESULTS Melanized Spots as an Indication of Plague Fungus Each year, we observed hyphae and biflagellate zoospores characteristic of A. astaci (Alderman and Polglase, 1986, 1988; Cerenius et al., 1988) in the melanized spots (n = 50) on P. leniusculus (n = 39) from lakes KJ and I-M and in the Oregon sample. On no occasion were any other types of fungus found. In all eight infection experiments, behavioural disturbances characteristic of crayfish plague were seen within 7 10 days in A. astacus kept in the same aquaria as infected P. leniusculus. All of the A. astacus (n = 110) died within days, and fungal hyphae were found in the carapaces of the dead individuals. In contrast, the A. astacus control groups in aquaria without P. leniusculus were in good condition after the 3 7-month experiments in each of the study years. Because the melanized spots of P. leniusculus used for the infection experiments and microscopic examinations were of the same size and appearance as those of the P. leniusculus caught in the lakes in our other studies, and because no other fungus was ever detected on the spots, all of the melanized spots were considered evidence of fungal infection. Each study year, we trapped 3 12 (48 in total) P. leniusculus that had moulted 1 5 days previously and which had black spots as clearly visible as in hard-shelled intermoult specimens. Thus, not all plague spots disappear during moulting or they appear again very soon afterwards. Prevalence of Infection The overall prevalence of plague was 22.7% for P. leniusculus from Lake KJ and 23.3% from Lake I-M. The prevalence of infected specimens in Lake KJ was 51.7% and in Lake I-M was 47.0% in the first study year, i.e., 10 years after stocking (Table 2). Despite great annual fluctuations, the prevalence gradually declined until 1988 in both lakes (Fig. 1). During the 1990s the prevalence increased, amounting in the last study year to 23.6% in Lake KJ and to 24.1% in Lake I-M. In 1992, 43% of P. leniusculus from Oregon were infected and in 1994, 72 percent were infected (Fig. 1). The follow trends existed in the data. The prevalence of infection in Lake KJ was slightly lower in females than in males except in 1986 and 1988, but in Lake I-M the proportion of infected females was slightly

4 780 JOURNAL OF CRUSTACEAN BIOLOGY, VOL. 20, NO. 4, 2000 Fig. 1. Prevalence of crayfish plague (Aphanomyces astaci)-infected Pacifastacus leniusculus in a commercial batch originating from Oregon, U.S.A., and in lakes Karisjärvi and Iso-Majajärvi. higher (except in 1993) (Table 3), as it also was in the Oregon samples. The mean total length was considerably greater in infected than in healthy P. leniusculus in both lakes (except in Lake I-M in 1985) (Table 3). Moreover, the larger the animals, the greater the proportion of infected individuals (Fig. 2); the smallest individuals observed with plague spots were 66 mm in Lake KJ and 64 mm in Lake I-M. Occurrence and Size of Crayfish-Plague Spots The average number of plague spots per infected P. leniusculus from Lake KJ (n = 167) was 2.0 (range 1 9), from Lake I-M (n = 255) 2.4 (range 1 16), and from the Oregon samples (1982 n = 43 and 1994 n = 21) 2.1 (range Fig. 2. Proportion of plague (Aphanomyces astaci)-infected Pacifastacus leniusculus in different size groups in lakes Karisjärvi (infected 169) and Iso-Majajärvi (infected 255) during the study period, and 1 5). A single spot was observed in nearly half of individuals in the Finnish lakes but in about 1% and 33% in the Oregon crayfish. More than 3 spots were found in 13.2% (Lake KJ) and 17.7% (Lake I-M) but in 27.8% and 4.8% in the Oregon samples (Fig. 3). No significant differences were noted in the locations of spots between sexes. In Lake KJ a total of 68.8%, in Lake I-M 67.5%, and in the Oregon samples 66.7% and 60.0% of spots occurred on the walking legs and chelae (Fig. 4). The spots were situated especially at the ends of broken appendages, i.e., in 25.2% in Lake KJ, in 28.6% in Lake I-M, and in 41.7% and 42.2% in the Oregon samples. It is noteworthy that in the Oregon samples 15.6% and 8.9% of the spots occurred on the Table 3. Number of Pacifastacus leniusculus examined and mean total length (TL) of plague-free and plague-infected specimens in Lakes Karisjärvi and Iso-Majajärvi during study period (SD = standard deviation). Lake Karisjärvi Lake Iso-Majajärvi Healthy Infected Healthy Infected Year n Mean TL mm SD Mean TL mm SD n Mean TL mm SD Mean TL mm SD Total ,

5 NYLUND AND WESTMAN: CRAYFISH PLAGUE IN FINLAND 781 Fig. 3. Number and frequency of plague (Aphanomyces astaci) spots on infected Pacifastacus leniusculus in Oregon samples in 1982 (n = 43) and 1994 (n = 21) and in lakes Karisjärvi (n = 167) and Iso-Majajärvi (n = 255) during the study period, uropods, which was rare in Finnish samples (Fig. 4). The diameters of the plague spots ranged from 1 to 8 mm but were seldom greater than 5 mm. The size distribution varied slightly from year to year. Spots > 3 mm in diameter were most common among P. leniusculus from the Finnish lakes (42.5% and 35.7%) (Fig. 5). There were no significant differences between the sizes of spots between sexes. We did not find any dead or weak individuals in the course of the study. The Plague-free P. leniusculus Population Astacus astacus and P. leniusculus have coexisted in the Lake SL for 29 years without observations of infections or deaths in A. astacus due to plague. No melanized spots were found on the 933 P. leniusculus examined during this study, nor were any observed in conjunction with annual samplings conducted in other studies, when thousands of both species were examined (Westman et al., 1995). No signs of crayfish plague were observed in the test groups in which healthy A. astacus were kept in the same aquarium with P. leniusculus from Lake SL for 3 7 months. These observations reinforce the assertion that the P. leniusculus population is plague free. Instead, when A. astacus from Lake SL were kept together with plague-infected P. leniusculus, all the A. astacus died of plague within 3 or 4 weeks. The control groups (Lake SL A. astacus without P. leniusculus) remained alive after 3 months, after which the Fig. 4. Location of plague (Aphanomyces astaci) spots on Pacifastacus leniusculus in Oregon samples in 1982 (n = 43) and 1994 (n = 21) and in lakes Karisjärvi (n = 167) and Iso-Majajärvi (n = 255) during the study period, test was terminated. Consequently, A. astacus from Lake SL have no exceptional resistance to the plague. DISCUSSION Pacifastacus leniusculus from Lake SL were not infected nor acted as reservoirs for A. astaci. Plague spots have never been detected among these signal crayfish (Westman and Pursiainen, 1979; Westman et al., 1995). Fig 5. Size distribution of plague (Aphanomyces astaci) spots on Pacifastacus leniusculus in Oregon samples in 1982 (n = 43) and 1994 (n = 21) and in lakes Karisjärvi (n = 167) and Iso-Majajärvi (n = 255) during the study period,

6 782 JOURNAL OF CRUSTACEAN BIOLOGY, VOL. 20, NO. 4, 2000 Moreover, P. leniusculus have shared the habitats of this small lake with A. astacus population for nearly 30 years, a period spanning several generations. Because A. astaci is transmitted via zoospores and no resting stages are known (Cerenius and Söderhäll, 1992), the plague could not have been confined as a latent infection to P. leniusculus in Lake SL. Furthermore, in infection experiments, A. astacus from Lake SL were no more resistant to plague than were any other A. astacus, and in the aquarium experiments P. leniusculus from Lake SL did not infect A. astacus. Consequently, our observations do not support the hypothesis of Kobayashi and Söderhäll (1990), and Söderhäll (1990) who postulated that all P. leniusculus are infected with the plague, even when they show no external symptoms. Plague-free P. leniusculus populations have been reported from France (Laurent and Vey, 1986), Great Britain (Lowery and Holdich, 1988), Lithuania (Cukerzis, 1988) and Sweden (Svärdson et al., 1991). Fürst and Boström (1978) documented two Swedish lakes supporting plague-free populations originating from stockings of cultivated juveniles as in Lake SL, and two other similar lakes with P. leniusculus populations marked by plague spots. The FGFRI has developed a method for producing plague-free juvenile P. leniusculus by stripping the eggs from brood crayfish carrying the plague and incubating them in isolation from the mother (e.g., Pursiainen et al., 1989). Plague-free juvenile production is important for the endeavour to prevent the disease from spreading with P. leniusculus. Fürst and Boström (1978) postulated that it may be harmful to attempt to maintain P. leniusculus populations entirely A. astaci free, because this may gradually weaken the high natural resistance of the species. Relevant data are, however, lacking. The problem is more-or-less academic in Finland, as it is impossible to keep chronically infected large waterways plague free. Because we invariably observed A. astaci hyphae on the melanized spots of P. leniusculus and noted that these individuals also infected A. astacus, we can infer that the occurrence of the same type of spot in P. leniusculus is evidence of plague infection, as was also postulated by Söderhäll et al. (1981). Consequently, P. leniusculus with spots should not be introduced into plague-free waters. Smith and Söderhäll (1986) and Söderhäll et al. (1988) suggested that, for investigations of the prevalence of crayfish plague, P. leniusculus must be caught before moulting, because plague spots may not appear until 3 or 4 months after moulting. Fürst and Boström (1978) were likewise of the opinion that plague spots may disappear in association with moulting and that spots are generally absent in juveniles of less than 60 mm. It is interesting that in each study year we found several P. leniusculus with spots but also with carapaces that were still soft, i.e., only a few days had elapsed since moulting. The smallest P. leniusculus examined were 50 mm, but no spots were observed in specimens less than 64 mm. The absence of spots in juveniles is presumably due to their high moulting frequency and, especially in small individuals, cannot be considered as evidence of the absence of infection. Correspondingly, the occurrence of spots was higher in large P. leniusculus, presumably because the lower moulting frequency gave infections time to develop into visible spots. Thus, the lack of melanized spots does not conclusively prove the absence of infection. To ensure that P. leniusculus are free of A. astaci, the animals must be kept for several weeks in corfs, aquaria, etc. with known healthy A. astacus at water temperatures greater than 10 C. Methods for isolation and in vitro culture have been presented by Alderman and Polglase (1986) and Cerenius et al. (1988). An appreciable number of P. leniusculus were presumably already infected with plague when imported into Finland in the late 1960s. The high plague frequency found in the first tests carried out in the present study was of the same order of magnitude as that found in the first Oregon sample and in two North American P. leniusculus populations (Fürst and Boström, 1978). During the late 1980s, the proportions of infected individuals in the present study were only slightly higher than those in seven lakes studied in Sweden (prevalence 3 11%) by Fürst and Boström (1978). In contrast, in two other studies conducted in Sweden (Söderhäll et al., 1981) the proportions of A. astaci-infected individuals were 30% and 40%. According to Fürst and Boström (1978), in one Swedish lake the prevalence of plague was as high as 61%, but

7 NYLUND AND WESTMAN: CRAYFISH PLAGUE IN FINLAND 783 this was presumably caused by heavy metal discharges disturbing the defence reaction to the A. astaci. The marked decrease in the number of A. astaci-infected individuals in both of our study lakes may be attributed to the fact that harvesting culled large and old individuals bearing many plague spots. The increase in the proportion of infected individuals during the 1990s may in turn be due to the decline in harvesting after The implication is that it might be possible to reduce the proportion of specimens with plague spots by intensive harvesting. In contrast, in a Swedish lake, there was no difference in the prevalence of plague between repeatedly and rarely trapped populations (Fürst and Boström, 1978). It is also possible that the reduction in our study lakes was partially due to low population density (Kirjavainen and Westman, 1999; Westman et al., 1999) compared with North American populations and also to the cold water resulting from the long winters. Aphanomyces astaci spreads slowly in cold water, and in our study lakes the water temperature was higher than 10 C for only 3 or 4 months a year. No changes have been noted in the conditions of lakes KJ and I-M to explain the reduction in the number of infected crayfish. More research needs to be done, however, into the differences in the abundance of A. astaci-infested P. leniusculus and changes in the prevelance of plague in different environments. Plague spots on the P. leniusculus were common at the tips of appendages. This indicates that A. astaci infections are concentrated on easily damaged parts. We do not know, however, whether all the spots were a consequence of damage or whether the fungus also becomes attached to undamaged parts of the carapace. Some researchers (e.g., Smith and Söderhäll, 1986; Cerenius et al., 1988) have suggested that P. leniusculus might not be as plague resistant as thought by Unestam (1969, 1972) and at moulting are actually not much more so than A. astacus. Persson and Söderhäll (1983) considered that the resistance of P. leniusculus might even have declined. If that were so, the plague would pose a major threat to P. leniusculus stockings. Similar views have been expressed in Finland (e.g., Rantamäki, 1990). However, long-term observations in the present study do not support these arguments. Crayfish plague does not appear to have affected the P. leniusculus populations in these lakes and no dead or moribund specimens have been observed in either lake in the 30-year study period (Kirjavainen and Westman, 1999; Westman et al., 1999). Infected specimens were repeatedly caught with baited traps. On the other hand, we do not know if infected specimens are more susceptible to predators and cannibalism than healthy crayfish. Only in a couple of Finnish lakes have P. leniusculus deaths been reported in exceptional circumstances (Tulonen et al., 1998), but even then the populations have revived rapidly. Pacifastacus leniusculus populations in Finland appear, therefore, resistant to plague and thus suitable for managing plague waters, which was the main reason for importing the species into Finland in the first place (e.g., Westman, 1995). Transmission experiments made in aquaria have also demonstrated the good plague resistance of Finland s P. leniusculus populations (Järvenpää et al., 1986). Similarly, neither Fürst and Boström (1978) nor Svärdson et al. (1991) observed any P. leniusculus deaths due to plague in the lakes they studied. The effects of the plague in the wild must be insignificant, because the heavily infected populations in the U.S.A. (e.g., Sacramento River) give high yields (Fürst and Boström, 1978; Svärdson et al., 1991). The resistance of individual P. leniusculus may nevertheless be impaired under some circumstances, causing latent infection to develop into an acute and destructive form within a few hours (Persson and Söderhäll, 1983; Persson et al., 1987). In Sweden, cases of large-scale P. leniusculus mortality observed in natural waters (Smith and Söderhäll, 1986) may have been caused by exceptional weather conditions during the winter or sewage water containing copper; however, the populations affected have rapidly attained their former status (Fürst and Boström, 1978; Svärdson et al., 1991). This pattern differs markedly from that in A. astacus, in which the plague destroys all infected individuals within 1 3 weeks depending on the water temperature (Järvenpää et al., 1986) and eventually the entire population (e.g., Westman and Nylund, 1979; Nylund et al., 1993). From the perspective of crayfish stock management, it is important that the plague

8 784 JOURNAL OF CRUSTACEAN BIOLOGY, VOL. 20, NO. 4, 2000 should not be spread by stocking P. leniusculus carrying melanized plague spots, because they spread infection persistently in contrast to A. astacus, which produce plague zoospores for only a short period after death. ACKNOWLEDGEMENTS We are deeply grateful to the following persons who participated in the field work: Pekka Ahlfors, Esa Erkamo, the late Aimo Järvinen, Jarmo Louhimo, Jorma Kirjavainen, Eero Kuittinen, Matti Mielonen, Markku Pursiainen, Riitta Savolainen, Klaus Sundbäck, Jukka Sutela, Jouni Tulonen, Irmeli Wallin, and Pia Westman. The owner of Iso-Majajärvi, Metsä-Serla Oy (formerly Serlachius Oy) and especially Leo Häggman and Jorma Laitinen of that company, and the owners of Lake Karisjärvi, in particular Kalle Hakala, helped and supported the field work in many ways. Our sincere thanks are also due to Professor H. Lehtonen, Professor Pekka Tuunainen, and Dr. Magnus Fürst for their constructive criticism. We also thank Gillian Häkli for checking the English language, Taina Kytöaho for drawing the figures, and Kati Manninen for technical assistance. LITERATURE CITED Alderman, D. J., and J. L. Polglase Aphanomyces astaci: isolation and culture. Journal of Fish Diseases 9: , and Pathogens, parasites and commensals. Pp in D. M. Holdich and R. S. Lowery, eds. Freshwater crayfish. Biology, management and exploitation. Croom Helm, London. Cerenius, L., and K. Söderhäll Crayfish diseases and crayfish as vectors for important diseases. EIFAC Workshop on Crayfish Management and Stocking. Kuopio, Finland August, Finnish Fisheries Research 14: ,, M. Persson, and R. Ajaxon The crayfish plague fungus Aphanomyces astaci diagnosis, isolation, and pathobiology. Freshwater Crayfish 7: Cukerzis, J. M Astacus astacus in Europe. Pp in D. M. Holdich and R. S. Lowery, eds. Freshwater crayfish. Biology, management and exploitation. Croom Helm, London. Fürst, M., and U. Boström Frekvens av en skalsvamp (kräftpest) på signalkräftor. (Summary: Frequency of visible symptoms of crayfish plague in populations of Pacifastacus leniusculus (Dana)). Information från Sötvattenslaboratoriet, Drottningholm 1: Holdich, D. M The dangers of introducing alien animals with particular reference to crayfish. Freshwater Crayfish 7: XV XXX. Huner, J. V Status of crayfish transplantations. Freshwater Crayfish 7: Järvenpää, T., V. Nylund, E. Railo, and K. Westman The effects of the crayfish plague fungus Aphanomyces astaci on the haemolymph of Astacus astacus and Pacifastacus leniusculus. Freshwater Crayfish 6: Kirjavainen, J., and K. Westman Natural history and development of the introduced signal crayfish, Pacifastacus leniusculus, in a small, isolated Finnish lake, from 1968 to Aquatic Living Resources 12: Kobayashi, M., and K. Söderhäll Comparison of concanavalin A reactive determinants on isolated haemocytes of parasite-infected and non-infected freshwater crayfish. Diseases of Aquatic Organisms 9: Laurent, P. J., and A. Vey The acclimation of Pacifastacus leniusculus in Lake Divonne. Freshwater Crayfish 6: Lowery, R. S., and D. M. Holdich Pacifastacus leniusculus in North America and Europe, with details of the distribution of introduced and native crayfish species in Europe. Pp in D. M. Holdich and R. S. Lowery, eds. Freshwater crayfish. Biology, management and exploitation. Croom Helm, London. Mannonen, A., and K. Westman Crayfish situation in Finland. Pp in T. Taugbøl, ed. Nordic-Baltic workshop on freshwater crayfish research and management, May 22 26, 1998, Sagadi Training Centre, Estonia. Østlandsforsking, ØF. Rapport nr. 26/1998. Nylund, V., J. Kirjavainen, J. Tulonen, and K. Westman The spread of crayfish plague (Aphanomyces astaci) and its effects on the noble crayfish (Astacus astacus) population in the Lake Ormajärvi waterway in Finland in Freshwater Crayfish 9: , and K. Westman Crayfish diseases and their control in Finland. EIFAC Workshop on Crayfish Management and Stocking. Kuopio, Finland August, Finnish Fisheries Research 14: , and The Crayfish Mortality Register as an aid in control of crayfish diseases in Finland. Freshwater Crayfish 10: Persson, M., L. Cerenius, and K. Söderhäll The influence of haemocyte number on the resistance of the freshwater crayfish, Pacifastacus leniusculus Dana, to the parasitic fungus Aphanomyces astaci. Journal of Fish Diseases 10: , and K. Söderhäll Pacifastacus leniusculus Dana and its resistance to the parasitic fungus Aphanomyces astaci Schikora. Freshwater Crayfish 5: Pursiainen, M., T. Järvenpää, J. Tulonen, and K. Westman Crayfish culture in Finland. Pp in J. Skurdal, K. Westman, and P. I. Bergan, eds. Crayfish culture in Europe. Report from the Workshop on Crayfish Culture, Nov. 1987, Trondheim, Norway. Rantamäki, J Rapurutto tappaa täpläravunkin. Suomen Kalastuslehti 97: [In Finnish.] Smith, V. J., and K. Söderhäll Crayfish pathology: an overview. Freshwater Crayfish 6: Söderhäll, K Rapurutto vakava uhka täpläravulle. Kräftpest är ett allvarligt hot mot signalkräftan. Suomen Kalankasvattaja-Fiskodlaren (5): [In Finnish and Swedish.], R. Ajaxon, and M. Persson Amerikanska kräftor och kräftpest. Information från Fiskeristyrelsen. 6 pp. [In Swedish.], M. Persson, and R. Ajaxon Parasiter och sjukdomar hos kräftor. Vattenbruk 2: [In Swedish.] Svärdson, G The American crayfish Pacifastacus leniusculus (Dana) introduced into Sweden. Report of the Institute of Freshwater Research, Drottningholm 46: Ecological co-evolution of the parasitic fungus Aphanomyces astaci and its crayfish host. Finnish Fisheries Research 14:

9 NYLUND AND WESTMAN: CRAYFISH PLAGUE IN FINLAND 785, M. Fürst, and A. Fjälling Population resilience of Pacifastacus leniusculus in Sweden. Finnish Fisheries Research 12: Tulonen, J., E. Erkamo, T. Järvenpää, K. Westman, R. Savolainen, and A. Mannonen Rapuvedet tuottaviksi. Riistan ja kalantutkimus. 152 s. Helsinki. [In Finnish.] Unestam, T Resistance to the crayfish plague in some American, Japanese and European crayfishes. Report of the Institute of Freshwater Research, Drottningholm 49: On the host range and origin of the crayfish plague fungus. Report of the Institute of Freshwater Research, Drottningholm 52: Westman, K The population of the crayfish Astacus astacus in Finland and introduction of the American crayfish Pacifastacus leniusculus Dana. Freshwater Crayfish 1: The 6th Sture Abrahamsson Memorial Lecture. Introduction of alien crayfish in the development of crayfish fisheries; experience with signal crayfish (Pacifastacus leniusculus (Dana)) in Finland and the impact on the native noble crayfish (Astacus astacus (L.)). Freshwater Crayfish 10: 1 17., and V. Nylund Crayfish plague, Aphanomyces astaci, observed in the European crayfish, Astacus astacus, in Pihlajavesi waterway in Finland. A case study on the spread of the plague fungus. Freshwater Crayfish 4: , and M. Pursiainen Development of the European crayfish Astacus astacus L. and the American crayfish Pacifastacus leniusculus Dana populations in a small Finnish lake. Freshwater Crayfish 4: , R. Savolainen, and M. Pursiainen Development of European noble crayfish Astacus astacus (L.) and American signal crayfish Pacifastacus leniusculus (Dana) populations in a small Finnish lake a twenty-years study. Freshwater Crayfish 8: ,, and Development of the introduced North American signal crayfish, Pacifastacus leniusculus (Dana), population in a small Finnish forest lake in Boreal Environmental Research 4: , and P. Westman Present status of crayfish management in Europe. EIFAC Workshop on Crayfish Management and Stocking. Kuopio, Finland August, Finnish Fisheries Research 14: RECEIVED: 20 September ACCEPTED: 7 June 2000.