Farming of freshwater crayfish is growing in popularity around the world. Currently the major producers are China and the USA and the main species is the redswamp crayfish (often referred to as "crawfish"), or Procambarus clarkii, which is native to the USA and has been introduced throughout much of the world. The majority of redswamp crayfish production is in extensive culture systems (meaning low stocking densities and little management of stock) or from fisheries. Production in these systems is highly variable, and though observed variables (such as weather conditions) are often suggested to be the cause, the actual reason is often never determined.
Semi-intensive aquaculture of freshwater crayfish has been practiced for at least 30 years. In Europe, native freshwater crayfish such as the noble crayfish (Astacus astacus) and the introduced signal crayfish (Pacifasticus leniusculus) are produced in semi-intensive systems, most commonly for stocking into natural waterways to replenish stocks lost or threatened due to crayfish plague or environmental alteration. Typically brooding females (females with eggs under the tail) are removed from ponds in late summer and are placed in indoor tanks. Eggs are stripped from the female and placed in a hatchery. The juveniles hatch from the eggs at the beginning of summer and are then placed in small, shallow outdoor ponds or tanks. Most juveniles are sold for stocking into natural lakes and waterways at the end of that summer, though some may be held through winter for sale as advanced juveniles in the next summer. Rarely, the crayfish are grown in captivity for sale for human consumption this takes several years.
There are a number of Australian freshwater crayfish species which have shown considerable potential for aquaculture. Species considered to have the greatest potential include the marron (Cherax tenuimanus), the yabby (Cherax destructor) and the redclaw (Cherax quadricarinatus). The potential of these species has been widely publicised and consequently they have been introduced to many countries for semi-intensive aquaculture (click here to see a recent review of Australian freshwater crayfish aquaculture species and industries by colleague Max Wingfield). It is unfortunate that their introduction has frequently been accompanied by considerable promotion, with proponents often making unrealistic or unsubstantiated claims about production and marketing. In some cases, this has lead to a boom-bust cycle in the development of the industry, eg. redclaw in Ecuador. Nonetheless, these species are clearly exciting candidates for semi-intensive aquaculture, and there is little doubt that there will be a significant global industry in semi-intensive aquaculture of redclaw. Redclaw are currently farmed in Australia, China, most South American countries, Israel, and other countries (click here to see some pictures of redclaw farms). Current annual production (2001) in China may be as much as 1000 tonnes. Uptake of culturing of redclaw in the rest of Asia has been slow, and perhaps this is due to it being confused with the redswamp crayfish which is considered a pest in many countries as it is a prolific burrower and has caused problems in rice paddies. The redclaw crayfish does not have a burrowing habit.
Disease in Freshwater Crayfish Aquaculture
Disease is a normal aspect of the ecology of an animal, and together with other ecological factors, such as resource availability (food, oxygen, water, habitat) and competition with other animals, regulates population numbers. Equally so, the ability of a pathogen (such as a virus or bacterium) to cause disease, and thus affect the population (by killing animals or reducing their ability to breed), is dependant on other ecological factors and on the animal itself. In other words, whether or not a host (in this case a freshwater crayfish) becomes diseased after infection by a pathogen is dependant on factors relating to the pathogen itself (eg. pathogenicity, transmissibility how readily it infects another host), the host itself (eg. disease resistance), and the environment (eg. the presence and level of environmental stressors). Therefore, the presence of a pathogen in a population may have little consequence if the environment is optimal. However, if there is an alteration in one of these factors, eg. a rapid change in the environment such as rapid drop in pH, or mutation in a pathogen to make it more pathogenic, then disease may occur and this may have a significant affect on the population. Similarly, a pathogen which normally causes disease may mutate to a less pathogenic strain or the animal may develop resistance or tolerance to the pathogen. Perhaps the best example that comes to mind is the case of white spot syndrome virus (WSSV) in shrimp aquaculture. WSSV is the causative agent for white spot disease which has caused billions of US dollars in losses in shrimp aquaculture in Asia since 1993 and in South America since 1999. When introduced to an area, WSSV typically causes 100% mortality in populations (ponds of shrimp) within 3-5 days. However, many people have noted that after this initial epizootic (high mortality) phase, WSSV may be present in subsequent crops without causing significant mortality. Therefore, something has altered within the virus or in the shrimp so that the result of infection is not always an epizootic. Nonetheless, rapid alterations in the environment, which is common during the monsoon season in Asia, will often lead to an epizootic in the population.
It is obvious to state that an aquaculturist will attempt to provide optimal conditions for culturing their stock. Clearly this is required for good feed conversion ratios and the ultimate bottom line... profits. As discussed above, maintenance of an optimal culture environment is paramount from a disease perspective. Having said that, it is important to understand that the culture environment is not the natural ecosystem in which the cultured animal evolved over millions of years together with other organisms, including some which might occasionally cause them to become diseased (such as viruses, bacteria, parasites). Moreover, the progression from extensive to semi-intensive to intensive to super-intensive systems is usually characterised by a culture environment that is less and less similar to their natural environment. Usually the environment of semi-intensive and more intensive systems is man-made, the animals are often stocked at densities much higher than occur in nature, and usually in monoculture (almost certainly predatory species are eliminated), and feed is provided. Hence the relationship between pathogen-host-environment is affected. The higher stocking densities may lead to more frequent transmission of a pathogen to new hosts, and the removal of predatory species increases the chances that a sick animal will be cannibalised by another host, factors which lead to a higher prevalence or incidence of disease in the population. In addition to this, just simple management becomes critical in more intense systems, that is to say that the loss of aeration for 1 hour in a super-intensive system may lead to major losses, whilst in less intensive systems the effect may not be so great. Add the potential for quicker spread of pathogens on top and it is clear just how vital good management becomes in intensive systems.
Another important factor is feed. Typically intensive systems utilise feeds from external sources often hatchery operators use fresh feeds (such as other crustaceans or molluscs) or processed feeds may be used. Processed feeds may contain protein derived from similar animals to that which are being farmed, and are often caught far away from the farm (sometimes from another continent). The significance of this is firstly that the more closely related the feed species is to the cultured species, the more likely that pathogens in the feed species are able infect the cultured species. The second significant point is that the greater the distance to the source of the feed species from the cultured species, the less likely it is that the cultured species has been exposed to any pathogens that the feed species may carry. Remember that an organism and its pathogens co-evolve over millions of years, and they reach some sort of stable state. It is not in the best advantage of the pathogen to kill all of its hosts; then it would actually cause itself to become extinct. The affects are sometimes devastating when a host is exposed to a pathogen with which it has never previously been in contact. The best example of this in the freshwater crayfish world obviously is the crayfish plague fungus Aphanomyces astaci which was introduced to Europe from USA with American crayfish. The American crayfish are highly resistant to A. astaci and it rarely causes disease in them because they have co-evolved with the fungus; that is, the crayfish have developed mechanisms to effectively deal with the fungus. However, the European crayfish had no prior exposure to A. astaci, and once exposed they were completely unable to cope with the fungus. Even quite unrelated feed species can represent a risk. For example, bivalve molluscs such as oysters, clams and pipis are filter feeders (meaning they filter food particles from the water) and have been known to harbour pathogens of other animals, including of humans (such as hepatitis virus). Please note carefully that WSSV also kills many freshwater crayfish species. In fact, there is at least one report of WSSV causing serious losses in captive freshwater crayfish after they were fed infected shrimp. All known pathogens of crustaceans are inactivated (killed) by cooking. The toughest pathogens known to infect crustaceans are parvoviruses which are inactivated by heating for a short period at 80C. Therefore, feeds containing aquatic animals are safest when properly cooked.
It is important to realise that, although crayfish have biological traits that make them different from other aquatic animals, there are many characteristics of aquaculture and indeed agriculture which are constant regardless of the cultured species. I would strongly encourage anyone thinking of farming freshwater crayfish to learn about aquaculture systems in general. If you're interested in growing redclaw in ponds in northern Queensland, talk to the Government field staff and the redclaw farmers in the area (they're all good people, I know many of them well), but also talk to the prawn farmers and barramundi farmers in the area. They have a wealth of experience which will be of help. Most importantly, learn from the experiences of the other industries.
When you do go ahead and research the development of aquaculture industries, you will soon realise that disease is an important issue, and one that must be considered from the very initial stages of designing your farm through to sending your product to market. It is not uncommon to read in early promotions for aquaculture species that they are disease resistant or not susceptible to disease this is misleading as less is known about diseases of these animals new to farming systems after all, how could we possibly know as much about diseases of an animal that may have been farmed for 10 years or less when compared to sheep or cattle which have been farmed and closely observed for thousands of years? No, disease is an extremely important issue for all farmers and one that must never be trivialised. So here are a few important points to consider. Please note that this is not an exhaustive list and I would be happy to include additional points from others who email. In making these points, I am aware of the constraints often faced in designing and running operations (eg. often crayfish farms are developed on an experimental basis in conjunction with another primary agricultural activity, and so the site characteristics may not be perfect) in most cases there will be a compromise between what is ideal and what is realistic or possible.
Facility design: Ensure that facilities can be properly drained (ie. completely) and quickly if necessary. You need to be able to capture your stock rapidly in the case of an outbreak of disease for effective management (treatment or emergency harvest and sale as is common in shrimp aquaculture). Individual ponds/tanks should be isolatable if an outbreak of disease occurs in only one production unit. From a disease perspective, flow-through designs are not ideal as disease may spread rapidly throughout entire facilities. The farm should be located away from other businesses which may introduce harmful chemicals to the farm via air, water or other means.
Water quality: The water quality parameters required for most farmed freshwater crayfish species have been described and it is advisable to follow these guidelines when selecting your site for the establishment of the facility. However, there are other important water quality issues that will not be found in such a description. An important consideration is the other businesses utilising the same water source, and particularly, what other aquaculture facilities. It is best to avoid sourcing water for your facility from a water body which receives effluent from an activity which is likely to introduce pathogens of the species which you are culturing, and this includes aquatic animal processing plants and other aquaculture facilities culturing the same species. This lesson was learned quickly in prawn aquaculture in Asia and South America where farms are clustered on estuaries and coastlines. Serious diseases spread rapidly because the effluent water from one facility was the intake water of the next facility on the same estuary. Once disease struck at one facility, it soon spread to the others. Similarly, sourcing intake water near your own outlet is not advisable as it will lead to recycling of pathogens. It is often advisable to construct a holding or settling pond on your property for treatment of water prior to release.
Stock selection: Obviously it is desirable to select healthy and vigorous stock. It is best to familiarise yourself with the industry and identify those enterprises which sell good quality broodstock or juveniles it is worth paying extra for good quality broodstock. The more you handle stock, the easier it is to discern very healthy individuals from less healthy animals (eg. strength of the tail-flick response). Hatcheries are not typically used for the production of stock for the Australian species. Though the economic reasons are easy to understand, in my opinion there are some positives to running a hatchery which may not be fully appreciated eg. for selective breeding programs and for stocking crayfish of the same age and size. From a disease perspective, properly run hatcheries can be used effectively to significantly reduce the likelihood of disease outbreaks occurring in freshwater crayfish aquaculture. This is due to the biology of freshwater crayfish and particularly the fact that, unlike many marine crustaceans, the larval stages of freshwater crayfish are not free-living. They occur within the egg. Therefore a freshwater crayfish hatchery can be very simple but effective. The eggs can be easily stripped from under the abdomen of the female, disinfected in a solution of formalin or some other disinfectant, and hatched in clean water. As none of the known pathogens of freshwater crayfish are transmitted within the egg, done properly this process will result in the production of specific pathogen-free (SPF) freshwater crayfish (see Edgerton and Owens 1997, in my list of publications). Of course, there can be no guarantees that no pathogen of freshwater crayfish will be transmitted within the egg, and extra precautions must be taken to prevent the introduction of pathogens with the water, feed or with subsequent introductions of stock. However, a farm established with stock from a hatchery which follows these procedures, and which implements proper quarantine controls, is far less likely to have an outbreak of disease. SPF stock have become very important in the development of other farming industries, including shrimp, and will likely become important in the development of semi-intensive and intensive freshwater crayfish aquaculture. Importantly, SPF stock should be sourced when introducing a freshwater crayfish species for aquaculture to a country or region.
Pond/tank preparation: Regular drying is essential for a healthy system. In earthen pond aquaculture, it is usual to dry ponds (for several weeks, until the soil crusts and cracks) between crops and often lime is added to restore the pH of the pond bottom which becomes acidic due to excess organic matter from feeding. This treatment inactivates pathogens present in the sediments and restores a healthy habitat remember this is where your crayfish spend all, or at least a lot, of their time. It is for this reason that I am somewhat sceptical of the use of gravel in earthen ponds. Tanks and filters need regular cleaning to prevent the build-up of harmful bacteria and other pathogens. This includes holding and purging facilities it is very important to carry out regular cleaning of these facilities.
Stock management: Monitor the health of your stock. Take notice of whether they're feeding (you may wish to consider using a feeding tray as is used in prawn aquaculture, or some other device) and other external features such as colouration and fouling. Though fouling is not detrimental unless it is severe, it may indicate an underlying problem usually it is related to poor water quality, but the stock may have a more serious infection resulting in less frequent preening. Also, be careful when shifting stock around the farm. A common practice in some operations is to relocate the smaller crayfish collected during harvest into other ponds for on growing. These animals may actually be smaller due to the presence of a pathogen, so by shifting the animals you will also spread the pathogen within the farm. It may be better to find a market for those smaller crayfish or to dispose of them.
The greatest weapon in combating disease is your own diligence. Though these animals are in unnatural conditions, which may lead to greater transmission and expression of disease, you are also closely monitoring the system to ensure that it is functioning properly. At the same time, you are monitoring the stock for signs of distress. If you suspect that your stock are diseased, promptly discuss the problem with an aquatic animal pathobiologist who may ask for some samples to be collected for examination.
To conclude, here are the major points:
- As the intensity of culture increases, a progression which is common as an industry develops and managers become more confident in farming the species, the potential for severe disease outbreaks increases.
- Learn from the experiences of other aquaculture industries you don't need to reinvent the wheel, and you can learn a great deal from the successes and misfortunes of others.
- Good management reduces the likelihood of a disease outbreak unfortunately it cannot completely eliminate it because some pathogens are simply too pathogenic and too easily transmitted.
- Disease management begins from the earliest stage of setting-up an aquaculture facility design the facility mindful of contingency plans in the advent of a disease outbreak.
- Select a site with a low density of farms culturing the same species.
- Select very good quality stock if available, buy specific pathogen-free stock or consider setting-up a hatchery to produce your own.
- Be particularly careful when selecting feed try not to use fresh feed, and if you must, cook it first. Do not feed shrimp to freshwater crayfish (this point is equally important for aquarium enthusiasts). Take note of the protein source in processed feed, and ensure that it has been properly cooked if the feed contains protein from aquatic animals.
- Rehabilitate production units between each crop for earthen ponds, allow them to dry and till the soil. Regularly clean holding and purging facilities.
- Monitor the condition of your system and stock continually.
- Promptly contact a qualified aquatic animal pathobiologist when it is suspected that your stock is diseased.