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INTRODUCED AND INVASIVE FISH SPECIES
Key words: Invasion, Exotic species, Native species
Fig – 14.
INTRODUCTION
"Invasive species" is a phrase with several definitions. The first definition expresses the phrase in terms of non-indigenous species (e.g. plants or animals) that adversely affect the habitats they invade economically, environmentally or ecologically. It has been used in this sense by government organizations as well as conservation groups such as the IUCN (International Union for Conservation of Nature).
The second definition broadens the boundaries to include both native and non-native species that heavily colonize a particular habitat.
The third definition is an expansion of the first and defines an invasive species as a widespread non-indigenous species.This last definition is arguably too broad as not all non- indigenous species necessarily have an adverse effect on their adopted environment. An example of this broader use would include the claim that the common goldfish ( Carassius auratus ) is invasive. Although it is common outside its range globally, it almost never appears in harmful densities.
Conditions that lead to invasion:
Scientists propose several mechanisms to explain invasive species, including species-based mechanisms and ecosystem-based mechanisms. It is most likely a combination of several mechanisms that cause an invasive situation to occur, since most introduced plants and animals do not become invasive.
Species-based mechanisms:
Species-based characteristics focus on competition. While all species compete to survive, invasive species appear to have specific traits or combinations of specific traits that allow them to outcompete native species. Sometimes they just have the ability to grow and reproduce more rapidly than native species; other times it's more complex, involving a multiplex of traits and interactions.
Studies seem to indicate that certain traits mark a species as potentially invasive. One study found that of a list of invasive and non-invasive species, 86% of the invasive species could be identified from the traits alone. Another study found that invasive species tended only to have a small subset of the invasive traits and that many of these invasive traits were found in non- invasive species as well indicating that invasiveness involves complex interaction not easily categorized. Common invasive species traits include:
The ability to reproduce both asexually as well as sexually Fast growth Rapid reproduction High dispersal ability Phenotypic plasticity (the ability to alter one’s growth form to suit current conditions) Tolerance of a wide range of environmental conditions (generalist) Ability to live off of a wide range of food types (generalist) Association with humans Other successful invasions
Typically an introduced species must survive at low population densities before it becomes invasive in a new location. At low population densities, it can be difficult for the introduced species to reproduce and maintain itself in a new location, so a species might be transported to a location a number of times before it become established. Repeated patterns of human movement from one location to another, such as ships sailing to and from ports or cars driving up and down highways, allow for species to have multiple opportunities for establishment (also known as a high propagule pressure).
An introduced species might become invasive if it can out-compete native species for resources such as nutrients, light, physical space, water or food. If these species evolved under great competition or predation, the new environment may allow them to proliferate quickly. Ecosystems in which all available resources are being used to their fullest capacity by native species can be modeled as zero-sum systems, where any gain for the invader is a loss for the native. However, such unilateral competitive superiority (and extinction of native species with increased populations of the invader) is not the rule. Invasive species often coexist with native species for an extended time, and gradually the superior competitive ability of an invasive species becomes apparent as its population grows larger and denser and it adapts to its new location.
Facilitation is the mechanism by which some species can alter their environment using chemicals or manipulating abiotic factors, allowing the species to thrive while making the environment less favorable to other species with which it competes. One such facilitative mechanism is allelopathy, also known as chemical competition or interference competition. Facilitation also occurs when one species physically modifies a habitat and that modification is advantageous to other species. For example, zebra mussels increase habitat complexity on
ecosystems with high species diversity seem to be more susceptible to invasion. This debate seems largely to hinge on the spatial scale at which invasion studies are performed, and the issue of how diversity affects community susceptibility to invasion remains unresolved. Small-scale studies tend to show a negative relationship between diversity and invasion, while large-scale studies tend to show a positive relationship. The latter result may be an artifact of invasive or non-native species capitalizing on increased resource availability and weaker overall species interactions that are more common when larger samples are considered.
Invasion is more likely if an ecosystem is similar to the one in which the potential invader evolved. Island ecosystems may be prone to invasion because their species are “naïve” and have faced few strong competitors and predators throughout their existence, or because their distance from colonizing species populations makes them more likely to have “open” niches. Alternately, invaded ecosystems may lack the natural competitors and predators that keep introduced species in check in their native ecosystems. Lastly, invaded ecosystems have often experienced disturbance, usually human-induced. This disturbance may give invasive species, which are not otherwise co-evolved with the ecosystem, a chance to establish themselves with less competition from more adapted species.
Vectors:
Fig – 14.
Non-native species have many vectors, including many biogenic ones, but most species considered "invasive" are associated with human activity. Natural range extensions are common in many species, but the rate and magnitude of human-mediated extensions in these species tend to be much larger than natural extensions, and the distances that species can travel to colonize are also often much greater with human agency.
Invasive species also come from organisms stowed away on every type of transport vehicle. For example, ballast water taken up at sea and released in port is a major source of exotic marine life. The invasive freshwater zebra mussels, native to the Black, Caspian and Azov seas, were probably transported to the Great Lakes via ballast water from a transoceanic vessel. The arrival of invasive propagules to a new site is a function of the site's invasibility. Species have also been introduced intentionally. For example, to feel more "at home", American colonists formed "Acclimation Societies" that repeatedly released birds that were native to Europe until they finally established along the east coast of North America. Economics play a major role in exotic species introduction. The scarcity and demand for the
valuable Chinese mitten crab is one explanation for the possible intentional release of the species in foreign waters.
Invasive freshwater fish species in Australia include carp, brown trout, rainbow trout, redfin perch, mosquitofish ( Gambusia spp), weather loach, and spotted tilapia to name a few. Some introduced freshwater fish species have had devastating impacts on Australia's endemic freshwater fish species and other native aquatic life. For example in much of south eastern Australia's freshwater systems introduced carp (often incorrectly called "European" carp) dominate the lowland reaches, while introduced trout species almost completely dominate the upland reaches. While the damaging impact of carp is well recognised, little in the way of control measures have been employed to control their spread. Their ability to colonise almost any body of water, even those previously considered to be beyond their physical tolerances, is now well established.
The Northern Snakehead: An Invasive Fish Species
During the summer of 2002, several individuals of an exotic fish species called the Northern Snakehead were found in a pond in Crofton, Maryland, about 20 miles north east of Washington, D.C. The potential impact of this introduced species was considered so damaging that the event made national headlines. Officials posted signs encouraging anglers to kill any snakeheads that they caught.
Fig – 14.3 - Adult Snakehead Source: Maryland Department of Natural Resources
The Northern Snakehead, Channa argus , is no ordinary fish, biologists explain. It is a voracious top-level predator, meaning that it has no natural enemies, and could decimate populations of native fish. About 90% of its diet consists of other fish, though it also eats crustaceans, insects, and plants. In its native range it can live in water with temperatures ranging from 0 to 30 degrees C; it is found in muddy or vegetated ponds, swamps, and slow- moving streams. Snakeheads can breathe air and survive for up to four days out of water, and can survive for longer periods of time when burrowed in the mud. They are capable of traveling over land to new bodies of water by wriggling their bodies over the ground. These features are adaptations to the seasonal drying of shallow bodies of water in the snakeheads native habitat in China and allow it to disperse widely should local conditions become unfavorable. It is capable of surviving in much of North America should it become established. An established population of snakeheads in Maryland could have long-term disastrous consequences for the ecology of the region.
The Central and Shannon Regional Fisheries Boards have reported that chub, an invasive fish species, has been successfully eradicated from the Inny River system in Co. Westmeath. This invasive species was reported to have been illegally introduced to the country by anglers attempting to establish a viable chub population in Ireland. Chub were identified in the River Inny system in 2005 by Dr Joe Caffrey, a Senior Research Officer in the Central Fisheries Board.
Between 2006 and 2009, the Central and Shannon Regional Fisheries Boards implemented a rigorous monitoring and control programme in an attempt to eliminate this invasive fish species from Ireland. In 2006, a removal operation was conducted on a forty kilometer stretch of the river, between Lough Derravaragh and Lough Ree. At Shrule Bridge, seventeen mature chub, capable of breeding were captured and killed. Subsequent chub removals saw four fish removed in 2007 and two in 2008. In a further removal operation in 2009 two male chub were removed.
The policy of the Central & Regional Fisheries Boards has been to prioritise the removal of invasive species. In the case of the recent River Inny survey, it was imperative that the fish were removed before they had a chance to spawn and when water levels were suitable for conducting such an exercise. The Boards have considerable experience in fish removal and relocation operations and conduct all surveys in accordance with best practice and incur minimum fish mortality.
The Fisheries Boards are confident that the potentially invasive chub has been eradicated from this river system and from Ireland. However, it is proposed to monitor the status of the River Inny fishery over the coming years. Non-native invasive species have a negative impact on native Irish fish and their habitats. They can cause considerable irreversible environmental and socio-economic impacts at genetic, species and ecosystem levels.
Impact:
Ecological impacts
i. Land clearing and human habitation put significant pressure on local species. This disturbed habitat is prone to invasions that can have adverse effects on local ecosystems, changing ecosystem functions. ii. Invasive species can change the functions of ecosystems. iii. Invasive species that are closely related with rare native species have the potential to hybridize with the native species. Harmful effects of hybridization have led to a decline and even extinction of native species.
Genetic pollution Natural, wild species can be threatened with extinction through the process of genetic pollution. Genetic pollution is uncontrolled hybridization and introgression which leads to homogenization or replacement of local genotypes as a result of either a numerical or fitness advantage of the introduced species. Genetic pollution can bring about a form of extinction either through purposeful introduction or through habitat modification, bringing previously isolated species into contact. These phenomena can be especially detrimental for rare species coming into contact with more abundant ones where the abundant ones can interbreed with them, creating hybrids and swamping the entire rarer gene pool, thus driving the native species to extinction.
Attention has to be focused on the extent of this problem, it is not always apparent from morphological observations alone. Some degree of gene flow may be a normal, evolutionarily constructive process, and all constellations of genes and genotypes cannot be preserved. However, hybridization with or without introgression may, nevertheless, threaten a rare species' existence.
Economic impacts: Benefits Often overlooked, economic benefits from "invasive" species should also be accounted. The wide range of benefits from many "invasive species" is both well- documented and under-reported. (In most cases invasive species have benefits, but the negative effects almost always outweigh the positive.) Costs Economic costs from invasive species can be separated into direct costs through production loss in agriculture and forestry, and management costs of invasive species. Estimated damage and control cost of invasive species in the U.S. alone amount to more than $138 billion annually. In addition to these costs, economic losses can occur through loss of recreational and tourism revenues. Economic costs of invasions, when calculated as production loss and management costs, are low because they do not usually consider environmental damages. If monetary values could be assigned to the extinction of species, loss in biodiversity, and loss of ecosystem services, costs from impacts of invasive species would drastically increase. Economic Opportunities For many invasive species there are commercial benefits, either existent or capable of being developed. For instance, Silver Carp and Common Carp where heavy metals are not excessive in their flesh can be harvested for human food and exported to markets already familiar with the product, or into pet foods, or mink food. The depletion or exploitation of any unwanted species is dependent on officials who recognize the need for a solution. Commercial enterprises need assurances that the exploitation can continue long enough for a reasonable profit to be generated and that taxation of the 'resource' is given a sufficiently long period of grace that an enterprise is attracted to the proposition.
Introduced species:
An introduced , alien , exotic , non-indigenous , or non-native species , or simply an introduction, is a species living outside its native distributional range, which has arrived there by human activity, either deliberate or accidental. Some introduced species are damaging to the ecosystem they are introduced into, others negatively affect agriculture and other human uses of natural resources, or impact on the health of animals and humans. Introduced species and their effects on natural environments is a controversial subject and one that has gained much scrutiny by scientists, governments, farmers and others.
Nature of introductions:
By definition, a species is considered “introduced” when its transport into an area outside of its native range is human mediated. Introductions by humans can be described as either intentional or accidental. Intentional introductions have been motivated by individuals or
this species almost disappearing. Its successful translocation into Lake Issyk-kul in Kyrgyzstan, although having negative consequences for the indigenous Issyk-kul fish fauna, probably saved this fish from extinction. Introduced pelagic whitefish ( Coregonus lavaretus ) then became the major commercial fish replacing Sevan trout and Sevan khramulya ( Varicorhinus capoeta ).
High altitude rivers and streams mostly have a low fish production and hence no importance for fisheries, except at subsistence level. Fishery management of rivers and streams has concentrated on introductions of fish for recreational and sport fishery, with mainly brown trout being stocked. In some streams this exotic fish has established self-sustaining populations. In the Western Ghats, rainbow trout has also been successful in some streams with the same result, but in other mountainous areas of Asia where conditions are suitable, rainbow trout is cultured. Common carp is another exotic which has been widely introduced, and is now stocked regularly in a variety of cold waters, such as Tarbela Reservoir on the Indus River in Pakistan, high altitude lakes in the Western Ghats, and the high altitude ( m) ricefields of Arunachal Pradesh in India. Production of marketable size trout on fish farms is still low, as the final product is too costly for wider consumption. The best results with farm production of market-size trout have been achieved in northern Pakistan and in Kashmir (India).
Accidental introductions:
Fig – 14.
Unintentional introductions occur when species are transported by human vectors. For example, numerous marine organisms are transported in ballast water, one being the zebra mussel. Over 200 species have been introduced to the San Francisco Bay in this manner making it the most heavily invaded estuary in the world. Increasing rates of human travel are providing accelerating opportunities for species to be accidentally transported into areas in which they are not considered native.
Damming of the Chu River in Kyrgyzstan provided new habitat for reservoir fish and resulted in higher fish production than in the wild river. On the other hand, diversion of water for
irrigation reduced the river's flow downstream where it fed a series of lakes in the desert. As a result, these lakes, formerly rich in fish, are now largely desiccated. The profit from the reservoirs just makes up for losses from the lakes. Damming of the Himalayan rivers in India has flooded many spawning grounds of schizothoracines and mahseers (Tor spp) in Pakistan and India, or made it impossible for them to reach spawning grounds.
Introduced species on islands:
Perhaps the best place to study problems associated with introduced species is on islands. Depending upon the isolation (how far an island is located from continental biotas), native island biological communities may be poorly adapted to the threat posed by exotic introductions. Often this can mean that no natural predator of an introduced species is present, and the non-native spreads uncontrollably into open or occupied niche. The field of island restoration has developed as a field of conservation biology and ecological restoration, a great deal of which deals with the eradication of introduced species.
Genetic pollution:
Purebred naturally evolved region specific wild species can be threatened with extinction in a big way through the process of genetic pollution i.e. uncontrolled hybridization, introgression and genetic swamping which leads to homogenization or replacement of local genotypes as a result of either a numerical and/or fitness advantage of introduced plant or animal. Nonnative species can bring about a form of extinction of native fishes by hybridization and introgression either through purposeful introduction by humans or through habitat modification, bringing previously isolated species into contact. These phenomena can be especially detrimental for rare species coming into contact with more abundant ones where the abundant ones can interbreed with them swamping the entire rarer gene pool creating hybrids thus driving the entire original purebred native stock to complete extinction. Attention has to be focused on the extent of this under appreciated problem that is not always apparent from morphological (outward appearance) observations alone. Some degree of gene flow may be a normal, evolutionarily constructive process, and all constellations of genes and genotypes cannot be preserved however, hybridization with or without introgression may, nevertheless, threaten a rare species' existence.
Mountain rivers of Tien Shan and Pamir have largely unexploited fish stocks, as they are poor in species and numbers. Many of such streams and rivers remain a welcome reserve of indigenous fish. Only a few introductions of exotics have been done. The introduction of rainbow trout in some Tien Shan lakes initially resulted in spectacular growth rates, but within several generations the growth rate slowed down, fecundity of females declined, and eventually cannibalism took place. Coregonids (six species) introduced into lakes in northeastern Kazakhstan with the purpose of increasing commercial catches also showed a decline in growth rates in response to the depletion of food supplies.
Multispecies introductions took place especially in water bodies of Kazakhstan. For many years the fish faunas of Lake Balkhash, its major tributary Ili River and Kapchagay Reservoir, situated on the Ili, have been manipulated by introductions and commercial fisheries. The result is that the indigenous fish species now form a meaningless proportion in total fish catches (Petr and Mitrofanov, 1998). In addition, the manipulation of fish fauna in
