The importation and spread of non-native pathogens and parasites are major concerns for the U.S. Department of Agriculture/Animal and Plant Health Inspection Service (USDA/APHIS), state agriculture agencies, and federal and state natural resource agencies.
Non-native pathogens and the diseases they cause are often called emerging or foreign animal diseases.
Some of the recent, high-profile diseases of fish are Infectious Salmon Anemia (ISA), Spring Viremia of Carp (SVC), and Viral Hemorrhagic Septicemia.
In mollusks there have been problems with Perkinsus marinus (perkinsosis or Dermo), Multinucleated Sphere X (MSX), and Quahog Parasite Unknown (QPX) diseases. Other aquacultured species have similar examples.
Pathogens may be imported along with fish or other aquaculture species, moved from facility to facility, or moved between cultured and wild stocks. Producers should follow recommended biosecurity practices, such as quarantining new stock, to prevent pathogens from entering culture facilities.
It is sometimes advisable to screen for specific pathogens before bringing new stock into culture facilities. Failure to plan for biosecurity can result in reduced production, unhappy customers, agency intervention, and economic losses.
Many pathogens are widespread and already endemic in the U.S., and many are not. When emerging diseases are detected, the government may respond with dramatic action. For example, entire fish farms have been depopulated to control SVC and the movement of live fish from areas with VHS has been prohibited to prevent its spread.
Actions such as these can severely disrupt operations and could halt sales for extended periods of time, in some cases years. There are varying opinions on the danger introduced pathogens pose to cultured and wild stocks. Certainly, some pathogens have caused major losses.
Viruses have caused the most concern in recent years as methods of detecting them improve and as more examples of viruses infecting cultured or wild fish accumulate. It is particularly diffcult to assess the risk from viruses because uncertainty is high and the losses from some viruses have been considerable.
Opinions are influenced by the history of human and livestock epidemics, the often dire warnings in the media of human influenza pandemics, the diffculty of treating viral infections, and a general “unknown factor” associated with viruses.
The “unknown factor” includes the fear that there are many highly pathogenic viruses in fishes and other aquaculture species that have yet to be identified. Even with biosecurity programs such as health certification and screening for specific pathogens, the fear that all aquatic species carry dangerous viruses can lead to many restrictions on the importation, culture, or interstate shipment of aquatic species.
Various state agencies and land-grant universities also have programs on emerging pathogens and aquatic animal health.
Genetic alterations:
Aquaculture producers are frequently advised to raise native species to avoid the potential problems associated with culturing non-native species. However, the genetics of captive populations of a species will be different to some degree than the genetics of wild populations of the same species.
Then, if captive individuals escape and interbreed with wild stocks, there may be genetic change in the wild populations. Interbreeding may be especially problematic for small, wild populations of imperiled species.
One kind of genetic change is the introduction of genes not found in the wild population, such as occurs if cultured hybrids breed with wild fish. More often, though, there are changes in the frequencies of variants or alternate forms of genes called alleles.
Some genetics experts consider these changes, which they call genetic contamination by native aquacultured species, to be worse than the effects of non-native species. Although this opinion is debatable, the issue of genetic contamination is increasingly important for aquaculture.
Genetic contamination is most likely to occur where large numbers of captive individuals may escape, as with net pens, floodplain ponds, or shellfish leases, or where captive individuals are stocked into public or private waters. An example would be the escape of native species such as cobia (Rachycentron canadum) or snappers from net pens located in coastal or offshore marine waters.
Concerns about genetic contamination have led to restrictions on the commercial culture of native species even where it is unlikely that captive individuals would escape.
An example is the prohibition on the commercial culture of native sturgeons in Florida to prevent the escape of individuals into breeding populations in the northern portion of the state.
Even though the probability of escape under sturgeon culture Best Management Practices (BMPs) is low, only non-native sturgeon may be cultured in Florida. So when is a native species not a native species? When it is raised in aquaculture.
That is the essence of the genetics contamination issue as it pertains to the interbreeding of captive and wild stocks. The genetic composition of cultured populations will differ from that of wild populations even if broodstock originate in the local, wild population and care is taken to use a reasonable number of broodstock.
These procedures do not prevent changes in allele frequencies if enough captive individuals inter-breed with wild individuals. Broodstock may have only a subset of the genetic variability present in a wild population (a small sample may miss some alleles) and likely will differ in overall allele frequencies due to sampling error.
It should be noted that genetic interchange between hatchery and wild stocks has been occurring in many species for a long time, in some cases more than 100 years, especially because of stocking programs for popular sport fish. Allele frequencies also change naturally in wild populations, so the baseline genetic composition of these stocks varies across time. Still, most agencies attempt to minimize genetic changes caused by stocking.
Alleles are variants of a gene and produce slightly different versions of the protein that is the gene product. Alleles arise naturally in populations over time because of gene mutations. The importance of different alleles and allele frequencies is less well known.
Some argue that alleles are adapted to the local environment and that having native alleles gives individuals an advantage. Therefore, if cultured individuals interbreed with wild individuals, the resulting offspring will be less fit and fewer of them will survive to reproductive age, causing the wild population to decline.
Loss of fitness may occur because affected individuals grow more slowly, are less tolerant of environmental extremes, are less attractive to mates, have lower quality eggs or sperm, or have other disadvantages. Others argue that most genetic variation is neutral and does not affect fitness.
Still others are less concerned about genetic exchange if there are no noticeable negative effects, as with many sport fish stocking programs. This debate is important because it greatly affects how the escape of cultured native species is viewed.
Several factors determine the frequencies of alleles within a population, including selection, random drift, immigration from other populations, and genetic bottlenecks. Selection occurs when alleles are favored by some process.
These may be natural processes within the environment, as when an allele gives increased survival or preferential mating. Selection may also occur in captivity because of differential survival in the captive environment, captive mating reducing or eliminating mating preferences, or human selection for desirable traits (e.g., faster growth or color).
Allele frequencies also change randomly over time if selection forces are not strong. Random changes occur more frequently if the effective population size—the number of individuals that actually produce offspring—is small.
Individuals from other populations that immigrate into and interbreed within a population can affect gene frequencies, especially if there are enough immigrants. Lastly, drastic declines in abundance or effective population size can create genetic bottlenecks where there is loss of genetic variability, especially the loss of rarer alleles.
The same principles hold true when hybrids or intergrades (crosses between subspecies; Box 1) escape and interbreed with wild stocks of one or both parental species. Channel catfish x blue catfish (Ictalurus punctatus x I. furcatus) hybrids are becoming more common in aquaculture and are frequently cultured within the native range of both species.
Hybrid sunfish (Lepomis spp.) can backcross with wild stocks. If one of the species making up the hybrid is not native to the region, concerns about genetics and non-native species may be greater. For example, green sunfish (Lepomis cyanellus) is a prohibited, non-native species in Florida, and hybrid sunfish containing green sunfish genes are prohibited.
Producers should be aware of the potential for genetic interchange between captive and wild stocks when planning new aquaculture operations. Carefully follow any genetics policy required by regulatory agencies.
Compliance requirements may include reducing or preventing escape, using broodstock from the region, rotating broodstock often, and using breeding schemes that explicitly address genetic diversity and artificial selection.
Author:
Jeffrey E. Hill
