Hope for Turkish crayfish farming
How many times have you heard that in former times the rivers used to be full of crayfish? And no matter how you look, you actually have never seen one? Even today there are places in the Czech Republic where there are plenty of crayfish, but unfortunately not just the same species as before. The original, noble crayfish (Astacus astacus) is now a critically endangered species. This is due, among other things, to introduced species of crayfish that are resistant to the crayfish plague they carry and also have a high reproductive and resistance potential. The issue of crayfish infection by crayfish plague was addressed in a recent study, which was co-authored by Adam Petrusek and his colleagues from the Faculty of Science of Charles University in Prague.
Together with his PhD student Michaela Mojžišová from the Department of Ecology and Dutch intern Michiel Tangerman, Adam Petrusek contributed to the research mainly with laboratory analyses and they joined the project under the supervision of Prof. Pavel Kozák from the Faculty of Fisheries and Protection of Waters of the University of South Bohemia in Vodňany. The experimental part took place at one of the largest lakes in Turkey, Eğirdir. In cooperation with the local Research Institute of Fish Culture and Hydrobiology, they studied the transmission of the crayfish plague agent, the oomycete Aphanomyces astaci, from mothers to offspring and investigated how this transmission could be prevented in rescue or commercial crayfish farms.
Crayfish plague was introduced to Europe, including the territory of the present Czech Republic, from North America during the 19th century and even today this disease is decimating populations of native crayfish species, especially in Europe, but also in some Asian countries. The disease pathogen is spread by invasive crayfish of North American origin, which can be infected without showing symptoms of the disease. This gives them a major competitive advantage over local crayfish, which are very susceptible to the disease. Two non-native American species, the signal crayfish (Pacifastacus leniusculus) and the spinycheek crayfish (Orconectes limosus), are particularly widespread in this country. They have invaded, usually with abundant human help, not only large rivers such as the Vltava and Elbe but also smaller streams and backwaters. Unfortunately, the crayfish plague is spreading along with them.
The disease has also affected populations of the Turkish crayfish (Pontastacus leptodactylus), which is widespread from Eastern Europe to Turkey and Armenia and occurs in islets in our territory, where it was introduced at the end of the 19th century. However, in many places in the species' range, populations have survived after coming into contact with the crayfish plague pathogen. As a result, some individuals in the population are chronically infected, the disease persists in the population, and if stress levels increase, it can break out in full force and the crayfish die. This is also the case with populations in Turkey, which collapsed dramatically after the accidental introduction of crayfish plague in the 1980s. Since then, crayfish have increased in some places, but they are far from returning to their previous numbers.
In Turkey, the crayfish used to be a locally economically important species, largely exported. As a result of chronic crayfish plague infections and thus reduced crayfish abundance in lakes, current catches are much lower. This is one of the reasons why there is nowadays an effort to artificially culture these crustaceans in the country. However, these artificial cultures face a major problem - infected crayfish caught from the wild often experience disease outbreaks and mortalities after transfer to the culture. This happens not only in adults but also in juveniles who spend the first weeks of their lives attached to their mothers. Unfortunately, infected crayfish can no longer be cured, so the only way to establish a "healthy breeding" might be to prevent vertical transmission of the pathogen from mother to offspring. However, it has not yet been clear whether this occurs via the egg or whether the hatchlings become infected by waterborne spores of the pathogen.
The study, published this year in the journal Aquaculture, was designed to contribute to the understanding of the mechanisms of transmission and to test possible technologies that could enable the successful establishment of healthy broodstock under the conditions of the Turkish Fisheries Research Institute.
The authors of the study investigated whether A. astaci is vertically transmitted to the young ones during maternal incubation and to what extent hatching of crayfish eggs under artificial conditions, together with treatment with antifungal baths, reduces infection with this pathogen. Female crayfish carrying eggs were captured for the experiment in Lake Eğirdir, Turkey, where A. astaci infection has been recorded for a long time. The captured females were collected shortly before hatching and subsequently divided into experimental groups. Juvenile survival was monitored, and the DNA of the crayfish plague pathogen was detected by molecular analyses (quantitative PCR method) at several intervals from hatching to the juvenile stage.
And what were the results of the experiments conducted? They confirmed that almost all wild-caught females tested for A. astaci by PCR were indeed infected, and infection was often confirmed in naturally developing individuals on their mothers, both in the wild and in the experiment. Moreover, these individuals showed higher mortality in the experiment, which could be related to the development of the disease.
In contrast, no clear infection with A. astaci was confirmed in any individual hatched from eggs collected from the mothers. Treatment with antifungal baths further reduced the mortality of juvenile crayfish, possibly due to a reduction in the development of other opportunistic pathogens. Thus, the study concludes that the separation of eggs from infected mothers before hatching already substantially reduces, perhaps even completely prevents, vertical transmission of the pathogen. While it cannot be completely guaranteed from one study that all young crayfish hatched and developed under artificial conditions are completely free from A. astaci, this approach seems to be suitable for routine use in aquaculture in areas where chronic infections with this pathogen are prevalent.
The results of this experiment confirm that infection with the crayfish plague pathogen in natural conditions can often occur early in a crayfish's life. However, transmission from mothers to their babies is probably not via eggs but, as among other individuals in the population, is caused by spores of the A. astaci pathogen from the surrounding aquatic environment. Other routes of infection, e.g., direct contact between the young and the mother, cannot be completely ruled out. However, it can be stated with certainty that the separation of eggs from infected mothers, even just before hatching, dramatically reduces the likelihood of transmission of the pathogen to them. Immature crayfish produced by artificial incubation of eggs, even if they come from infected mothers, may be suitable for the establishment of artificial farms as well as for potential reintroduction into open waters in areas where crayfish plague has decimated native populations.
In the conditions of the Czech Republic, however, the return of crayfish to Czech waters from artificial breeding would not make much sense. Even if the population dies out in some places because of the crayfish plague, the plague pathogen will still be present in the landscape because it has a suitable breeding ground in the form of American crayfish species. The pathogen is therefore waiting for its European host and, if it returns, the disease may strike again. Experiments exploring the biology of the crayfish plague pathogen, such as this one, may perhaps contribute to new solutions for the protection of crayfish on our territory in the future. Today, however, it is essential to take particular care to avoid unintentional contamination of the waters and the spread of invasive species, since humans are usually responsible for the colonisation of new sites by non-native crayfish. Finally, as with other endangered species, the protection of their natural habitat plays a key role in the survival of crayfish.
Tereza Žirovnická
Published:
Dec 12, 2023 02:30 PM
Document Actions
