Crayfish Plague Research Articles

Temporal dynamics of spore release of the crayfish plague pathogen from its natural host, American spiny-cheek crayfish (Orconectes limosus), evaluated by transmission experiments

The crayfish plague pathogen, Aphanomyces astaci, is one of the most serious threats to indigenous European crayfish species. The North American invasive spiny-cheek crayfish, Orconectes limosus, is an important source of this pathogen in central and western Europe. We evaluated potential changes in A. astaci spore release rate from infected individuals of this species by experiments investigating the pathogen transmission to susceptible noble crayfish, Astacus astacus. We filtered defined volumes of water regularly to quantify spore concentration, and sampled crayfish tissues at the end of the experiment. The filters and tissues were then tested for the presence of A. astaci DNA by species-specific quantitative PCR. Additionally, we tested the efficiency of horizontal transmission to apparently uninfected O. limosus. The experiments confirmed that A. astaci can be transmitted to susceptible crayfish during intermoult periods, and that the pathogen was more frequently detected in noble crayfish recipients than in American ones. The pathogen spore concentrations substantially varied in time, and significantly increased during moulting of infected hosts. Our study strengthens the evidence that although the likelihood of crayfish plague transmission by water transfer from localities with infected American crayfish might increase when these are moulting or dying, no time-periods can be proclaimed safe.

Absence of the crayfish plague pathogen (Aphanomyces astaci) facilitates coexistence of European and American crayfish in central Europe

Summary1. Most European crayfish species are strongly threatened, mainly as a result of the introduced pathogen, Aphanomyces astaci, transmitted by invasive North American crayfish. Long‐term coexistence of American and European crayfish species is therefore regarded as almost impossible, even though some coexisting populations have been observed.2. In this study, crayfish were collected from presently coexisting populations of the introduced spiny‐cheek crayfish (Orconectes limosus) and the native noble crayfish (Astacus astacus) from nine standing waters in central Europe. Our aim was to resolve whether the coexistence resulted from reduced virulence in local strains of A. astaci, increased immunity in the native crayfish or an absence of the pathogen in these populations. We used highly sensitive A. astaci‐specific real‐time PCR to evaluate the crayfish latent carrier status, combined with transmission experiments to further validate the molecular results.3. From the total of 523 crayfish tested (490 spiny‐cheek crayfish, 33 noble crayfish), none positive for A. astaci was detected. Transmission experiments confirmed these results: No abnormal mortality or behavioural changes were seen in noble crayfish kept together with American crayfish from the coexisting populations. If we assume a very low prevalence of A. astaci of 10% in a carrier population, there is a 98% probability of disease being absent in five of the nine coexisting populations tested. Hence, a consistent absence, or an extremely low prevalence, of A. astaci seems to allow the coexistence of European and American crayfish in these central European populations.4. The results are important for native crayfish conservation and management and demonstrate that disease transmission risk may vary substantially between the different populations of spiny‐cheek crayfish in central Europe.

Aphanomyces astaciPsI-genotype isolates from different Finnish signal crayfish stocks show variation in their virulence but still kill fast

We detected significant virulence differences among five tested (PsI-Puujarvi, PsI-Pyhajarvi, PsI-Kukkia, PsI-Saimaa I and PsI-Saimaa II) PsI-genotype crayfish plague (Aphanomyces astaci ) isolates against lake Mikitanjarvi noble crayfish population. The crayfish were inoculated with a dose of 300 m·L-1 A. astaci spores in ambient water under experimental conditions. Mortalities started from four to seven days after inoculation, depending on the PsI-genotype isolate. In all the experimentally infected groups it took no more than three days for all the crayfish to die after the first mortality. The PsI-Puujarvi isolate proved to be the most virulent strain, while PsI-Kukkia was the least virulent. The average day of death for these experimental groups was fifth and ninth day, respectively. We did not discover correlation between the average day of death and gender or level of additional Psorospermium haeckeli infestation. Our results show that there are, from the practical point of view, minor virulence differences among PsI-genotype A. astaci isolates, and that all the tested five isolates are highly virulent. The present results emphasize the necessity to prevent all further spread of highly virulent strains of A. astaci to aid and shelter successful conservations attempts of the native European crayfishes.

Invasive zoopathogenic mastigomycotina in Republika Srpska, Serbia and neighboring countries with special reference to Aphanomyces astaci

Non-native alien zoopathogens have had a profound impact on the health of aquatic zoobiota in Republika Srpska and the Republic of Serbia as well as around the region and the world as a whole. The movement of disease-causing microfungi around the world has taken its toll on many different organisms, and continues to intensify with an increase in global transport of cargo, people, and animals. Those who study fungi have little information about the characteristics of invasive, zopathogenic fungi because of the fact that a particular fungus can inhabit many different organisms, and may be pathogenic to some of these hosts but not to the others. Crayfish plague (Aphanomyces astaci) broke out through the Danube River to the territory of Backa and Banat (today A.P. Vojvodina, the Republic of Serbia) in 1879. Until 1880, the disease spread through the Sava River to the northern Bosnia (today Republika Srpska). From 1955 until 1970, almost all territories of Republika Srpska and Republic of Serbia were afflicted with the crayfish plague disease. Despite the fact that the problems of the spread of crayfish plague in the territory of the Republika Srpska and the Republic of Serbia have been of great significance, there is little data available for analyzing this issue. According to IUCN criteria, and mostly due to the Aphanomyces astaci invasion, the degree of endangerment of noble crayfish in Serbia was evaluated as ?Endangered?, which was a higher degree than the international level determined for this species, i.e., ?Vulnerable?.

Biological invasions: Prospects for slowing a major global change

One of the greatest ongoing ecological changes of the Anthropocene is the rearrangement of the earth’s biogeography, as people deliberately or inadvertently move species around. We have been doing this for a long time – the Lapita people from southeast Asia brought various plants and animals (including the Pacific rat) to many islands in the Pacific beginning about 3,000 years ago. However, the number of species introduced to new regions has massively increased in the last few decades as the forces that transport them have grown. From 1970 to 2010, the amount of cargo shipped by sea tripled, and it is on course to double once again by 2030. Amount of air cargo is growing even more rapidly, and passenger air travel is increasing at almost as high a rate. We could as well call the Anthropocene the Homogeocene. The array of impacts of introduced species is staggering. Most obvious are those of introduced predators on native prey (e.g., rats on many island seabird populations [Pascal, 2011]) and introduced herbivores on native island vegetation (e.g., Cronk, 1989). The spread of the Burmese python (Python molurus bivittatus) in Florida and the subsequent drastic decline of populations of prey species ranging in size from rabbits to bobcats (Dorcas et al., 2012) is a noteworthy recent example. Introduced species can compete with native species and also transmit new pathogens to them. The North American gray squirrel (Sciurus carolinensis) does both to the European red squirrel (S. vulgaris), outcompeting it for nuts and spreading the parapox virus, to which the gray squirrel is resistant and the red squirrel is highly susceptible (Rushton et al., 2006). The red squirrel population has declined drastically in Great Britain, and the gray squirrel is now spreading on the continent from an initial escape from captivity in Italy (Wauters, Tosi and Gurnell, 2002). Competition can be between unrelated species – in New Zealand, the introduced wasp Vespula vulgaris outcompetes threatened native birds for “honeydew” produced by scale insects (Beggs and Wardle, 2006). Introduced parasites and pathogens devastate native species. Examples include avian malaria, a major threat to many native Hawaiian birds (van Riper et al., 1986) after its introduction with Asian caged songbirds; crayfish plague, introduced to Europe with resistant North American crayfish and currently ravaging native crayfish (Lodge et al., 2012); and white-nose disease, a European fungus threatening several North American bat species (Turner, Reeder and Coleman, 2011). Some native species undergo a kind of “genetic extinction” from hybridization with related invaders. For instance, North American mallards (Anas platyrhynchos) introduced for hunting have extensively hybridized with native Hawaiian ducks (A. wyvilliana) and New Zealand grey ducks (A. superciliosa superciliosa)(Rhymer and Simberloff, 1996). In addition to these frequently occurring types of impacts, particular invasions can have idiosyncratic but nonetheless devastating effects. For example, the North American cane toad (Rhinella marina), brought to Australia in an ill-advised attempt at biological control of introduced beetles on cane, kills threatened native predators such as quolls when they attack the highly toxic toad (Smith and Phillips, 2006). Invasion biologists, especially recently, have explored ways in which particular invasions affect entire ecosystems (Vitousek and Walker, 1989; Ehrenfeld, 2011; Simberloff, 2011), for instance by changing nutrient cycles, fire or hydrological regimes, or physical structure. Such changes affect many species simultaneously. In Florida, invasion by Australian paperbark (Melaleuca quinquenervia) leads to more frequent, hotter lightninginduced fires, to which native plants such as sawgrass (Cladium jamaicense) and muhly grass (Muhlenbergia capillaris) are maladapted (Serbesoff-King, 2003). This invasion is thus transforming parts of the “river of grass” into patches of paperbark-dominated forest, with consequent changes in the animal community. Introduced plant pathogens that devastate the dominant native vegetation can dramatically modify an entire ecosystem. The Asian chestnut blight fungus (Cryphonectria parasitica) spread through eastern North America in the Domain Editor-in-Chief Donald R. Zak, University of Michigan

Differing virulence of Aphanomyces astaci isolates and elevated resistance of noble crayfish Astacus astacus against crayfish plague

Crayfish plague epidemics (caused by Aphanomyces astaci) have been causing population collapses among native European crayfish stocks since the late 1800s. Recent indirect and direct evidence has shown that its virulence has been variable, with native European crayfish even acting as carriers. We tested the differences in A. astaci virulence under experimental conditions using both PsI- and As-genotypes with 3 Finnish noble crayfish Astacus astacus populations. We infected crayfish with adjusted quantities of A. astaci zoospores and monitored the symptoms and mortality of the crayfish. The PsI-genotype isolate caused rapid and total mortality among the tested populations, while the As-genotype isolates expressed more variable virulence. In some cases, mortality among the As-genotype-infected crayfish did not exceed the mortality level of the control group. All of the tested noble crayfish stocks showed lower mortality towards the As-genotype of A. astaci isolated from the River Kemijoki epidemic. We conclude that there are clear differences in virulence between different A. astaci genotypes and also differences in virulence within As-genotypes. Furthermore, we observed clear signs of increased resistance in different populations of noble crayfish towards some of the tested strains belonging to the As-genotype of A. astaci.

Current limitations of the stone crayfish distribution in Romania: Implications for its conservation status

The stone crayfish Austropotamobius torrentium is one of three indigenous crayfish species in Romania. Extensive research to assess the conservation status of this highly protected species is overdue in this country. Presently, the populations are concentrated in two distinct regions, the southwest and northwest of the country. Most populations were found living in habitats with little anthropic impact. Using logistic and negative binomial regression models, we determined the ecological preferences of the species. The most favourable conditions suggest an equilibrium between the availability of dissolved oxygen, moderate river velocity, and low concentration of organic pollutants. Due to these ecological requirements, the species is generally restricted to sub-montane areas (around 500 m a.s.l.) leading to a high degree of population fragmentation. The spectrum of limitations is completed by invasive species and the crayfish plague agent Aphanomyces astaci. Contact with non-indigenous Orconectes limosus and the crayfish plague is a real threat, especially to the populations inhabiting the order I tributaries of the Danube, in southwestern Romania. We recommend that the Romanian stone crayfish A. torrentium be taken into consideration as an “endangered species”.

Crayfish plague pathogen detected in the Danube Delta – a potential threat to freshwater biodiversity in southeastern Europe

The crayfish plague, caused by the oomycete Aphanomyces astaci, is probably the most significant reason for declines in European freshwater crayfish species. One of its hosts, the North American spiny-cheek crayfish Orconectes limosus, extends its range in the river Danube and recently reached the territory of Romania. We used highly sensitive A. astaci-specific real-time PCR to test if the native narrowclawed crayfish Astacus leptodactylus in the highly protected Danube Delta about 970 km downstream of the current invasion front of American crayfish is a carrier of the crayfish plague. Thirteen out of 40 analysed native A. leptodactylus tested positive for the crayfish plague pathogen, infected individuals were found at both sampled localities within the Danube Delta. Therefore A. astaci has a much wider range in this river than assumed. The pathogen seems to persist in local populations, as neither crayfish mass mortalities nor alien crayfish species have been reported from the region. Aphanomyces astaci may have reached the Delta by long-range passive dispersal of infected hosts or pathogen spores, or by gradually infecting populations of native crayfish in upstream regions of the Danube in a stepping-stone manner. Alternatively, the crayfish plague may have persisted in the Danube Delta as chronic infection from an old plague wave in the 19th century. In any case, the presence of this pathogen in the lower Danube may become a threat to conservation of European crayfish and to freshwater biodiversity in many regions of southeastern Europe, at present considered “crayfish plague-free”.

Timing and quantifying Aphanomyces astaci sporulation from the noble crayfish suffering from the crayfish plague

Aphanomyces astaci sporulation is crucial for the spreading potential of this disease agent. For the first time, we are reporting timing and quantity of A. astaci spores released from noble crayfish (Astacus astacus) suffering from crayfish plague under practical aquatic conditions. We infected nine noble crayfish with A. astaci PsI-genotype and maintained them in individual 8L tanks. Spores (zoospores and cysts) were quantified from water samples (3×1mL) taken every 12h over 10 d using A. astaci specific qPCR. A clear sporulation trend was found, together with a high individual spore estimate variation. The median spore counts from two days before death to 12h post mortem were from ∼500 to ∼2000sporesL−1. A significant sporulation increase occurred after 24h post mortem (∼12,000sporesL−1) and reached a peak after two days (∼65,000sporesL−1) before declining to or below pre mortem levels from the fourth day. The single most sporulating crayfish released from ∼75,000 to ∼400,000sporesL−1 during the mass sporulating period, yielding a maximum estimate of ∼3,200,000 spores released from a single crayfish if we assume homogeneous spore distribution. The results confirm a mass A. astaci spore release from moribund and recently dead infected noble crayfish, with a sporulation peak one to three days post mortem. The acute crayfish mortality only three days after zoospore exposure confirm the lethal potential of the PsI-genotype. The powerful sporulation potential observed here may be one of the key virulence factors of this genotype.

Invaders for sale: trade and determinants of introduction of ornamental freshwater crayfish

The trade of live ornamental freshwater crayfish has grown rapidly in the last decade and has become the major pathway for new non-indigenous crayfish species (NICS) introductions into Europe. Here, I report on the German ornamental crayfish trade, the main importer of non-indigenous crayfish into Europe. In total, 120 NICS have been available as ornamental aquarium species. One hundred and five species originate from North or Central America and are, therefore, suspected to be crayfish plague vectors. The import rate since 2005 was estimated to be seven new species per year. Despite many species being imported, only eleven species were found to be very common in the trade. In 2009, 16 online shops offered at least 37 NICS. The availability, price, and size of the offered species were used to predict their introduction status. Multiple binary logistic regression analysis showed that species’ availability and size were the principal predictors of the likelihood of being recorded as introduced from aquaria. NICS introduced from aquaria were found to be more available and larger than those present only in aquaria, and their potential invasiveness was also higher. The findings are consistent with the propagule pressure hypothesis in that a greater availability is likely related to more release events, and large species may be released more frequently as a result of overpopulating or outgrowing their aquaria. Efforts to mitigate the risk of further harmful crayfish introductions from aquaria should aim to drastically reduce the availability of high-risk species.

The diversity of the pathogenic Oomycete (Aphanomyces astaci) chitinase genes within the genotypes indicate adaptation to its hosts

The aim of this work was to evaluate the genetic diversity of the crayfish plague pathogen Aphanomyces astaci (Oomycete) among different isolates and genotypes. Partial chitinase genes were cloned and sequenced from 28 A. astaci isolates including four of the five previously identified RAPD (random amplification of polymorphic DNA)-genotypes. The cloned chitinase sequences (n=176) formed three main groups, CHI1, CHI2 and CHI3, with the CHI2 group then further divided into three subgroups, CHI2A, CHI2B and CHI2C. Some of these chitinases were specific for certain genotypes of A. astaci, as CHI2B and CHI2C were only found from the As-genotype and CHI3 from the Ps-genotypes of A. astaci. Highest diversity rate was observed in the CHI2 group, while the CHI3 group specific for Ps-genotypes was highly homologous. Based on our chitinase data, As- and Pc-genotypes seem to be related, while the two Ps-genotypes were identical to each other, but considerably different from the genotypes As and Pc. These are the first genotype specific differences that are located in the coding region of the chitinase gene of A. astaci and the differences observed here also enable the genotyping of A. astaci. The diversity observed here can also reflect differences in the epidemiological properties of the different genotypes.

Monitoring the spore dynamics of Aphanomyces astaci in the ambient water of latent carrier crayfish

The specialized crayfish parasite Aphanomyces astaci causes the devastating crayfish plague in European crayfish. Even though A. astaci sporulation has been thoroughly studied under pure culture conditions, little is known about the sporulation dynamic from its live host. Our purpose was to investigate the A. astaci spore dynamic in its native parasite–host relationship by monitoring the sporulation from carrier crayfish into the ambient water using agent specific qPCR. American signal crayfish (Pacifastacus leniusculus) with known positive carrier status were housed individually and communally in two experimental set-ups using multiple replicates and different temperatures. Water samples were collected weekly, and spore numbers were quantified. We demonstrate here that live latent carrier crayfish continuously released a moderate number of A. astaci spores (∼2700 spores per crayfish/week) in the absence of death and moulting events. In contrast, a pronounced sporulation increase was seen already one week prior to death in moribund crayfish, suggesting a crayfish plague-like condition developing in weakened or stressed individuals. Significantly more spores were produced at 18°C compared to 4°C, while a negative correlation was detected between spore numbers and temperatures rising from 17 to 23°C. This study is the first attempt to quantify the spore release from carrier crayfish on the basis of qPCR applied on water samples, and demonstrate that the approach successfully unravel A. astaci sporulation patterns. The results emphasize that carrier crayfish pose a constant infection risk to highly susceptible crayfish species regardless of crayfish life cycle state.

Associations between stream habitat characteristics and native and alien crayfish occurrence

Human mediated introductions of non-indigenous crayfish species (NICS) are responsible for their rapid colonisation of European freshwaters. The introduction of North American crayfish is furthermore linked to the spread of crayfish plague and the decline of indigenous crayfish species (ICS). As the management of ICS and NICS have become necessary, a detailed knowledge on their distribution and ecological requirements is needed. We studied the current range of native noble crayfish Astacus astacus and stone crayfish Austropotamobius torrentium, as well as alien signal crayfish Pacifastacus leniusculus in Carinthia (Austria) and evaluated environmental and physical habitat features in streams with and without crayfish. Meanwhile, the loss of many ICS populations was recorded and alien P. leniusculus was found to be widespread in this region. Most of the habitat features of streams having crayfish differed significantly from sites lacking crayfish for at least one investigated native or alien species. Furthermore, multivariate and regression analyses showed specific differences in the habitat use of the investigated crayfish. Our results showed that the presence of alien P. leniusculus was associated with larger and smoother sloped lowland rivers, while the occurrence of the two native species was confined to smaller streams either at higher altitudes and with distinct physical habitat conditions (A. torrentium) or with moderate water temperatures (A. astacus). This study helps to identify potential refuge areas for the endangered native species and to predict the further spread of the most common non-native crayfish species in European streams.

PCR detection of the crayfish plague pathogen in narrow-clawed crayfish inhabiting Lake Eğirdir in Turkey

Many populations of the narrow-clawed crayfish Astacus leptodactylus in Turkey, including those inhabiting Lake Eğirdir, declined drastically in the mid-1980s due to introduction of crayfish plague Aphanomyces astaci. However, unlike many other localities, there has been some recovery in the A. leptodactylus population inhabiting this lake even though crayfish plague has been suspected to have persisted since then. In support of this, DNA from 5 of 34 healthy-looking crayfish sampled recently from the lake tested positive by both conventional and real-time PCR using species-specific primers targeting the rDNA internal transcribed spacer region, and product sequence analysis confirmed the identification of A. astaci. This complies with other recent reports of coexistence of native European crayfish with this pathogen, and further research is now needed to identify the key mechanisms allowing it.

Harvest, export and economic status of freshwater crayfish (Astacus leptodactylus Esch.1823) in Turkey

This study was carried out on the export and economic status of freshwater crayfish (Astacus leptodactylusEsch. 1823) in Turkey. Due to the fact that there is a very low domestic consumption of freshwater crayfish in the country, Turkey had been the largest provider of A. leptodactylus to Western Europe from 1970 until 1986. The peak production of the country was attained in the early 1980s, with over 5.000 tons being exported in 1984. After the occurrence of crayfish plague in Turkish waters, crayfish stocks reduced dramatically and harvest declined from approximately 5.000 (in 1984) to 200 (in 1991) tons annually. The harvest of crayfish in Turkey between 2005 and 2009 was 809, 797, 816, 783 and 734 tons, respectively and the commercial values of crayfish, based on these years were 3.033.371, 3.209.396, 3.381.094, 3.366.426 and 2.713.494 USA dollars. The harvest of crayfish in total was 13.648 tons from 2000 to 2009 in Turkey, and the total commercial value was approximately 40.343.870 USA dollars. Majority of crayfish harvest has been exported to European countries and especially Sweden. Key words: Astacus leptodactylus, freshwater crayfish, harvest, exports, economy, Turkey.

Tolerance of signal crayfish (Pacifastacus leniusculus) to Persteril 36 supports use of peracetic acid in astaciculture

Peracetic acid (PAA) is useful in treating a wide variety of serious fish diseases in aquaculture. As no information on PAA toxicity to crayfish is available, we conducted acute toxicity tests of Persteril 36, a commercial PAA-containing product, on three age/size classes of signal crayfish, Pacifastacus leniusculus. Results confirmed high tolerance of the target species to this product, expressed as a 96h LC50 of 77.3±1.0mgL−1 PAA in adults and low post-treatment mortality in 9month old juveniles. As PAA concentrations successfully applied in aquaculture are generally 2.0mgL−1 or lower, PAA can be recommended for use in astaciculture. After necessary evaluations, uses might include limiting spread of crayfish plague, treating saprolegniosis on artificially incubated crayfish eggs, and avoiding translocations of further crayfish-related non-native species when introducing crayfish or stocking them in closed culture systems.

Invasive crayfish and crayfish plague on the move: first detection of the plague agent Aphanomyces astaci in the Romanian Danube

Native European crayfish, such as Astacus leptodactylus, are threatened, among other factors, by the crayfish plague agent Aphanomyces astaci, dispersed by invasive North American crayfish. Two of these invaders, Pacifastacus leniusculus and Orconectes limosus, have extended their distribution in the River Danube catchment; the latter was detected for the first time in Romania in 2008. We monitored, at monthly intervals for over 2 yr, occurrence of native A. leptodactylus and invasive O. limosus at 6 sites on the Romanian Danube and checked for the invasive species in 4 of its tributaries. Between January 2009 and March 2011, the relative abundances of O. limosus steadily increased with time, while the native A. leptodactylus dramatically decreased in abundance. O. limosus expanded downstream at a rate of ca. 15 km yr-1; in August 2011, it was already present in the upper 105 km of the Romanian Danube. An agent-specific real-time PCR analyses demonstrated the presence of A. astaci DNA in at least 32% of the analysed invasive (n = 71) and 41% of the native (n = 49) crayfish coexisting in the Danube. Furthermore, A. astaci was also detected in A. leptodactylus captured about 70 km downstream of the O. limosus invasion front (at the time of sampling). Assuming a steady rate of expansion, O. limosus may invade the sensitive Danube delta area in the mid-2060s, even without long-distance dispersal. The crayfish plague agent, however, may reach the delta substantially earlier, through dispersal downstream among populations of native crayfish.

Sustainable yield of the red swamp crayfish (Procambarus clarkii) through understanding its population structure and dynamics in Poyang Lake

With increasing commercial demand for its delicacy, Procambarus clarkii has become an important fishery resource of Poyang Lake in recent years. However, habitat loss, overfishing, and the crayfish plague have diminished P. clarkii populations throughout their area of distribution. In consequence, this study systemically analyses the population structure and dynamics of P. clarkii in the wetland of Poyang Lake using the analytical methods of Bhattacharya and Von Bertalanffy. The main purpose was to collect biological data necessary for the management of this species. The present study showed that the population structure consisted of a maximum of 5.58 age classes for females and 3.25 for males, which indicated that P. clarkii is a relatively short-lived species. The Y′/R analysis showed that Epresent > E0.5, which exceeded the optimization (Eopt) criterion of 0.5 for sustainable exploitation of fisheries, so this species was overexploited in Poyang Lake. The Lc (mean length at first capture) equals 67.5 mm. The estimated mesh size for catching the red swamp crayfish is 13.5 cm. The virtual population analysis model (VPA) showed that the largest annual catch of P. clarkii is 3.43 × 104 t. The Maximum Sustainable Yield (MSY) is 1.18 × 104 t. The low natural mortality rate range of 0.68-1.11 year−1 suggested that it is ecologically worthwhile to allow the individuals of P. clarkii to grow to larger sizes in order to contribute to the biomass for biologically optimal exploitation. This could be facilitated by the adoption of measures involving a closed season, which would help to conserve the fishery.

Conservation genetics as a tool for conservation and management of the native Japanese freshwater crayfish Cambaroides japonicus (De Haan)

Unprecedented rates of extinction have lead to the development of fields, such as conservation genetics, in an attempt to better understand biodiversity and consequently devise conservation programs to maintain the genetic integrity of species. We discuss the utility and application of conservation genetics, using examples from Australian freshwater crayfish, with a view to applying this methodology to the sole native crayfish species found in Japan, Cambaroides japonicus (De Haan). This species is threatened by the non-indigenous crayfish P a c i f a s t a c u s l e n i u s c u l u s ( D a n a ) t h ro u g h displacement, competition pressures, and susceptibility to Aphanomyces astaci Schikora (the crayfish plague, carried by the invasive species). Examining the genetic diversity within C. japonicus could allow populations of high conservation priority to be identified (i.e., genetically distinct populations) or, in contrast, show that this species is genetically homogenous. Such genetic continuity may suggest that other conservation methods (e.g., translocations and restocking populations) may be suitable for this species of crayfish. Identifying genetic variability, or lack thereof, is a key step in dictating the future direction of any conservation measures for C. japonicus.

Recovering Turkish narrow clawed crayfish (Astacus leptodactylus) populations carryAphanomyces astaci

Crayfish stocks collapsed in Turkey in mid 1980’s due to crayfish plague epidemics, with up to 25% survival in some cases. The collapsed stocks have then recovered slowly and commercial crayfish fishery was reinitiated gradually within a decade. We examined the prevalence of Aphanomyces astaci DNA in the narrow clawed crayfish (Astacus leptodactylus) collected from two Turkish lakes, Lake Iznik and Hirfanli Dam (N = 40 from both sites). The qPCR results, verified by conventional PCR and sequencing, indicated that nearly all sampled crayfish were A. astaci carries with some of them showing gross symptoms of infection, such as tissue erosion and melanisation, while some did not show any visible symptoms. The prevalence of A. astaci DNA was high in both stocks, being on the average 95% in both Lake Iznik and Hirfanli Dam. Our results show that these stocks are carries of A. astaci , but capable of forming productive stocks which indicates past and contemporary partial resistance adaptation in the host or virulence evolution in the A. astaci .