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Tilapia are considered to be a threat to native diversity in many areas where they have been introduced
Tilapia are considered to be a threat to native diversity in many areas where they have been introduced
FAO/20325/J.Spaull

In aquaculture, alien species and genotypes, also known as introduced species and genetically improved species, are a valid means to increase production. However, there is concern that these species will adversely affect local ecosystems.

Four broad categories exist for ecological impacts:

  • basic species interactions such as predation and competition
  • genetic impacts
  • disease impacts
  • habitat alteration

Species interactions

Predator-prey interactions

Top carnivores are often viewed the most significant threat as introduced fishes. Although the generality of this statement is not borne out by an analysis of the records in the Database on Introductions of Aquatic Species (DIAS), predation directly reduces population size of the prey species, and may cause cascading ecological effects, such as increased plant growth when herbivores are removed by top predators. Impacts of predation have been observed from salmonids, however the impact is primarily observed on anadromous and inland fishes and invertebrates. Introduced Pacific salmon and rainbow trout have displaced galaxids, many of which are anadromous, through predation in Australia, Chile and New Zealand. It has been suggested that striped bass that were introduced from the Atlantic coast of the United States of America to the Pacific coast prey on outmigrating juvenile Chinook salmon in the Sacramento-San Joaquin Delta and that juvenile white Seabees may have been displaced from the northern section of their distribution by striped bass predation in coastal lagoons.

Competition

Competition can occur between alien and resident species for food, habitat, mates, or other essential resources. Resident communities have evolved together and can have learned to partition resources; an invader disturbs this partitioning. There is a current moratorium on expansion of salmon farming in British Columbia because of, inter alia, evidence that exotic Atlantic salmon have escaped from cages and have reproduced in the Tsitika River. The fear is that Atlantic salmon will outcompete wild stocks or will contaminate the native gene pool. A similar fear is expressed in Norway where farmed salmon constitute about 30% of all salmon spawning and actually outnumber wild salmon in many Norwegian river systems.

The Pacific oyster, Crassostrea gigas, was introduced to Australia in the 1940s and since has spread to areas where the native C. commercialis and the Sydney rock oyster, Saccostrea commercialis, are farmed. Because of high fecundity and rapid growth rate, the Pacific oyster is crowding out these local species and has been declared a pest in Port Stephens (New South Wales). The Pacific oyster has been introduced into every continent except Antarctica, but only in Australia does there seem to be the concern for competitive displacement of other molluscs. This may in part be due to the fact that the Pacific oyster was often introduced to areas where the existing oyster fishery was in serious decline and therefore resources were not limiting oyster population densities.

Tilapia (Oreochromis spp.) and especially the Mozambique tilapia, Oreochromis mossabicus, are considered to be a threat to native diversity in many areas where they have been introduced and most of the impacts have been reported for inland waters. O. mossambicus in the Philippines and Pacific islands competes for algae and other resources and has displaced preferred species of mullet, Mugil cephalus, brackishwater shrimp, Penaeus merguiensis, and milk fish, Chanos chanos in brackish water fish ponds. O. mossambicus has usually been introduced for fisheries and O. niloticus is now popular for aquaculture. O. mossambicus can tolerate brackish water and there is a strong interest to develop further salt-tolerant strains of tilapia.

Genetic interactions

Possible genetic impacts from alien species include:

  • loss of species integrity from mixing with alien genotypes,
  • reduced reproductive efficiency from hybridizing with alien species resulting in nonviable offspring,
  • decrease in fitness from incorporation of alien genes or the loss of co-adapted gene complexes, and
  • indirect genetic impacts resulting from other ecological interactions e.g. competition or predation reducing a native population to the point where genetic diversity is lost or inbreeding becomes problematic. Salmon, with their homing ability, genetic sub-population structure and complicated life-history provide an ideal model to study the effects of genetic changes.

However, the great majority of studies on fish (and this is mostly on salmonids) merely documents genetic change and not actual change in populations or in fitness parameters as a result of that change. It is much easier to document a change in gene frequency than to document a change in fitness that will adversely affect a population, or ascribe a species decline to genetic factors when many other factors such as habitat loss, pollution, fishing pressure etc, may also be acting on the stock. A review of the salmon literature list numerous examples of genetic changes in farmed salmon and differences between farmed and wild salmonids in biology and in behaviour that are fitness related and under genetic control (at least partially), and therefore would theoretically affect fitness. It was concluded that the mixing of farmed and wild salmonid is generally detrimental to the wild stock, but empirical evidence is not abundant.

Fish farmers must be careful when introducing new species
Fish farmers must be careful when introducing new species
FAO/17429/A.Odoul

Disease impacts

The spread of pathogens along with species transported or traded in aquaculture is a serious concern that is being dealt with by several international agencies such as FAO, World Health Organization (WHO), World Trade Organization (WTO) and the International Office of Epizootics (OIE). Of particular concern to exotic species is that the level of uncertainty will be higher with new introductions on what pathogens may be present and may cause problems in the new environment. For example, along with abalone that the California aquaculture industry imported from South Africa came a sabellid worm parasite that caused no problems in South Africa but has had devastating effects of abalone under culture in California; the impact on other Californian molluscs is unknown.

In Norway in 1975 the monogenaen parasite, Gyrodactylus salaris was found in wild Atlantic salmon parr, probably introduced from infected and resistant Atlantic salmon from Sweden. The causative agent of furunculosis, Aeromonas salmonicida, was also introduced to Norwegian salmonid farming through infected stocks of rainbow trout from Denmark in 1966. The pathogen spread to over five hundred fish farms and to 66 salmon streams by 1991. A. salmonicida has been found in seawater over 20km from infected farms indicating its potential for dispersal. The spread of both Gyrodactylus and A. salmonicida was probably facilitated by stocking programmes that inadvertently used infected fish.

Researchers in Ireland demonstrated that 95% of the production of the nauplius I of the sea lice, Lepeophtheirus salmonis, originated from farmed salmon and speculated that the lice had contributed to the decline in both wild salmon and wild sea trout (Salmo trutta) fisheries.

Disease agents introduced with exotic species or strains may be more pathogenic in their new environment where they may spread to atypical hosts or encounter a more favorable environment (such as a mariculture facility). Whirling disease in rainbow trout is caused by a non-pathogenic myxosporean in brown trout; P. vannamei is a carrier of IHHN (infectious hypodermal and haematopoietic necrosis virus) that can devastate P. stylirostris. The Taiwan, Province of China shrimp industry collapsed after the introduction of diseased animals, e.g. shrimp containing Penaeus monodon-type baculovirus and yellow head virus, and newly discovered viruses caused financial losses of over one billion US$ in Asia in the early 1990s.

Norwegian strains of Atlantic salmon are highly susceptible to the parasite Gyrodactylus salar to which Baltic strains of salmon are resistant. Norway has tried to reverse the impact of Gyrodactylus salar infection to their Atlantic salmon stocks by poisoning entire river systems. The European flat oyster, Ostrea edulis, once imported to the western USA became infected with the blood cell parasite Bonamia which was subsequently spread back to Europe where it caused the demise of the majority of the fishery.

Pathogens can also impact native species by interacting with other species interactions. The introduction of crayfish from North America to Europe also introduced the crayfish plaque. North American species, such as Pacifastacus leniusculus, are resistant carriers that also out-compete native European crayfish due to higher reproductive rates; the plague gives the invaders an additional competitive advantage.

Disease agents hitchhiking in and among the shells of molluscs is another important method of disease transmission that has affected the aquaculture industry and coastal environments. The Pacific oyster's most significant adverse global impact has been in the spread of such organisms. The Japanese oyster drill Ceratostoma inornatum, the oyster flatworm Pseudostylochus ostreophagus, and the coopepod parasite Mytilicola orientalis were all inadvertently introduced with Pacific oyster. However, it has been stated that there have been no catastrophic diseases transported with Pacific oysters.

Habitat impacts

Many species of freshwater animals greatly modify aquatic habitats when placed into a new area, e.g. beavers, crayfish, common carp, and grass carp. The examples of mariculture species modifying coastal environments are more difficult to find. The Asian clam, Corbicula fluminea, was probably introduced into the USA by Chinese immigrants as a food item (though possibly not for intentional farming) in 1938. The clam has since spread widely to inland and coastal areas of 38 states in the USA. The most significant affect is in biofouling of freshwater systems, but the clam can grow in such large numbers as to alter the flow and substrate in streams and lakes and can remove large amounts of phytoplankton from the water column. In New Zealand the common river galaxias (Galaxias vulgaris) is displaced by Chinook salmon predation and competition, but also by disturbing stream bottom habitat when building redds (spawning nests).

As mentioned above, a significant impact of oyster movements has been from epibionts inadvertently introduced along with them. Twenty- nine species of algae, diatoms, protozoans and invertebrates were found in water of oyster shipments, and seaweeds and seagrasses used as packing material for oyster shipments have invaded and transformed thousands of square kilometers of open mudflat of the Pacific coast. The altered mudflat now contains a completely different assemblage of species.

 
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