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Invasive Alien Fish Species in Freshwater of the Continents

This review article presents the updated informationof freshwater Invasive Alien Species (IAS) of fishes of the continents to understand the homogenate progression in global freshwater ecosystems and the leading pathways of freshwater IAS fish introduction throughout the continents. North America contains 45 freshwater invasive fish species within 18 families, which is the most profuse off all other continents, whereas, Australia possesses the lowermost amount of freshwater IAS fish, 18 species under 8 families. Besides, Asia retains 41 freshwater IAS fish under 14 families and Europe acquires 32 species under 18 families. The uppermost spreader IAS fish family is Cyprinidae followed by Cichlidae subsequently. Nine common fishes (Carassius auratus, Cyprinus carpio, Onchorynchus mykiss, Oreochromis mossambicus, Poecilia reticulata, Salmo salar, Salmo trutta and Salvelinusfontinalis) turned out invasive in all the continents (without Antarctica). Among IAS introduction pathways, aquaculture causes the highest percentage of freshwater IAS introduction in all continents. Aquaculture root 25% of IAS fish introduction in Africa, 21% in Asia, 19% in Australia, 20% in Europe, 19% in North America and 19% in South America. There is hardly any documentation on freshwater IAS fishes of Antarctica. Therefore, future research consideration may be designed.


Introduction
The unintentional and deliberate introduction of species to ecosystems, along with other frequent anthropogenic goings-on swaying our environment, has taken place since antiquity (Hughes, 2003;Francis, 2012). When such introductions cause economic and/or ecological harm or adversely affect human health, are classified as IAS. Therefore, in the 21 st century, IAS are categorized as "eco-warrior" amongst the noteworthy coercions of the global biodiversity (Pimentel et al., 2000(Pimentel et al., , 2005Pejchar and Mooney, 2009). Likewise, the biodiversity of global freshwater ecosystem is defined alarming because of its declining rate; which is mostly allied with the existing and potential impacts of IAS (Sala et al., 2000;Vörösmarty et al., 2010). Amongst aquatic organisms, freshwater fishes are one of the absolute introduced taxa around the world (624 species: Gozlan, 2008) and this introductions as well as corresponding ruthlessness of influences can be occurred to a countless extent in ecosystems (Zedler and Kercher, 2004). In accordance with Moyle and Leidy(1992), approximately 20% of the freshwater fish species of the world (ca. 1800 species) are already extinct or in serious declining phase due to IAS introduction. Irrefutably, the vicissitudes caused by IAS are so massive, ecologists have commented that we are inward bound a different era, "The Homogocene" (a term apparently coined by Gordon Orians in the mid-1990s; Rosenzweig, 2001), in which all continents will be connected as a ""New Pangaea"" through the alike biota. Both or either establishment of non-indigenous species and loss of native biodiversity are lessening dissimilarities among regional biota, which can be referred to as biotic homogenization (Rahel, 2002). However, to look at the broad image of global IAS fishes, it is perceived that the remaining information on freshwater IAS in a continental screen is not adequate to picture the overall process of homogenization. There are hardly any documents, for instance, for Antarctic region and only an insufficient number of data of freshwater IAS for South America. Hence, a compilation of current status of freshwater IAS amongst continents is obligatory at this instant. To fulfill the aforementioned issue, this paper summarized IAS fish in freshwater (including diadromous) in the continents of the world including their introduction pathways.

IAS freshwater fishes of Africa
Africa harbors a well-diversified freshwater ichthyofauna, resulting from a long history of complex climatic and geological events (Lévêque, 1997). Here, the phenomenon "IAS" is recent (approximately 150 years old), compared to other continents (Chenje & Mohamed, 2006), this time factor may, however, be a reflection of the deficiency of records. Africa has received 14IAS fish from all continents except Australia and Antarctica ( Table 2). The most abundant invasive fish family presents in this continent is Cyprinidae (consists of 8 species); subsequently Cichlidae (consists of 6 species); Salmonidae (5 diadromous species); and Poeciliidae (4 species) ( Table  3). Invasive fish had been introducing here by several pathways. Of them, "aquaculture" is a very operative purpose (25%), followed by pet aquarium trade (17%), regional faunal improvement or "acclimatization society" (12%) and the least is smuggling (2%) (Fig.1).

IAS freshwater fishes of Asia
Asia is host to an enormous amount of freshwater icthyofauna with tremendous species richness and high levels of endemism (Braatz et al., 1992). Despite home to 36% of the world"s freshwater resources (UNEP database), Asia"s most freshwater ecosystems are, however, under grave threat, and so does the biodiversity in it. Introduction of IAS, which is almost ubiquitous, is one of the root causes of this threat (Dudgeon, 2000). According to World Invasive Species Database, 41 IAS fish within 14 families remains in Asian freshwater (Table 4). Family Cyprinidae and Cichlidae entail with 8 species each, which are the most abundant invader fish family in this continent (Table 3). A wide range of human activities are responsible for IAS fish expansion here, arising from intensive aquaculture (21%), followed by the pet/ aquarium trade (17%), live food trade (12%) and fish stocking (10%) (Fig.2).

IAS freshwater fishes of Australia
Australia possesses an adequate quantity of diversified and unique freshwater fish fauna, although the components of the fauna are fairly small compared to other continents (363 species within 35 families) (Allen, 1989;Page & Brooks, 1991). However, from the 18th century, Australia has been subjected to species arriving from elsewhere. Ecologists have often revealed that Australia has received far more invading species than the number of Australian species invaded elsewhere in the world (Table 2). Australia possesses 18 freshwater invasive fish species within 8 families (Table 4). Alike other continents, "aquaculture" configures the maximum percentage (19%) of introducing IAS fishes. "Pet/aquarium trade" holds little lesser position (17%) than aquaculture (Fig.3).

IAS freshwater fishes of Europe
Amendments of freshwater systems through species introduction have an elongated history in Europe, on tracked before the 20 th century (Freyhof & Brooks, 2011). But this issue intensified here after mid-19 th century by several "acclimatization societies" (Copp et al., 2005) that was mostly liable for bringing IAS fish from outside of the continent for faunal improvement. As conscripted by World Invasive Species Database, the number of freshwater invasive fish species in Europe is 32 contained by 13 families (Table 4). The most intensive family of invader fish is Cyprinidae (comprises 10 species), followed by the diadromous family salmonidae (5 species) (Table 3). In description of introduction paths, it can be summarized that the most ostentatious pathway of invasive fish introduction here is "aquaculture" (21%), followed by "pet/aquarium trade" (17%) (Fig.4).

IAS of freshwater in North America
North America has a broad array of freshwater ecosystems within a multitude of habitats resulting rich assemblages of fishes reside; more than 1200 freshwater fish species within 50 families (Page & Brooks, 1991;Abell, 2000). The freshwater habitats and the astonishing biodiversity in it of this continent are, however, facing unprecedented threats from a range of sources; one of which is introduction of IAS from all over the world (Abell, 2000). It can be concised from Tables 1, 2 and 3 that North America consists of 45 freshwater invasive fish species within 18 families, which is the most profuse off all other continents (Table  4). Among 9 different ways, the most frequent pathways of fish introduction here include "aquaculture" (19%), followed by "pet/aquarium trade" (17%) and "live food trade" (15%) (Fig.5).

IAS freshwater fishes of South America
As with many other bunches of flora and fauna, freshwater endemic and invader fishes of South America is far from fully known, as because decades of poor monitoring and the lack of specific studies have limited our knowledge in this regard (Pompeu et al., 2012). However, introduction of IAS are identified as one of the widespread consequences across the continent for incessant loss of freshwater fish species here (Reis et al., 2003;Barletta et al., 2010), even before the whole species diversity of this continent is acknowledged. This continent acquired most IAS fish from Asia and North America, 10 and 9 species respectively ( Table 2). The most abundant invasive species exists from both the family Cyprinidae and Loricariidae (6 species from each family) and consequently, 5 species from each family Salmonidae and Cichlidae (Table 3). "Aquaculture" is the most dominant pathway of fish introduction in this continent (19%), followed by "pet/aquarium trade" (18%) and "live food trade" (15%) (Fig.6). n/a n/a n/a -Clupeidae n/a n/a 1 n/a n/a n/a -Cobitidae 1 1 1 n/a n/a 1 -Cyprinidae 8 10 9 6 8 4 -Esocidae 1 1 1 n/a 1 n/a -Gobiidae 3 1 3 n/a n/a 2 -Ictaluridae 1 1 2 1 n/a n/a -Latidae n/a n/a 1 n/a 1 n/a -Loricariidae 4 1 5 6 n/a n/a -Moronidae n/a n/a 1 n/a n/a n/a -Perciidae 2 2 1 n/a 1 1 -Petromyzontidae n/a n/a 1 n/a n/a n/a -  This is may be because some of them are fast-growing (e.g., Cyprinus carpio), so vastly used for aquaculture, some of them have robust flexibility (e.g., Salmo salar), so they can survive in adverse environment and become hostile. Hypophthalmichthys spp. and Oreochromis niloticus circulated in five different continents without Australia and Antarctica. The species Gambusia spp. is introduced in every continent mostly for biological control purpose (controlling mosquito population). It is revealed that the most effective introduction pathway of freshwater invasive fish species is "aquaculture", which causes the highest percentage of introduction in every single continent (Fig 1). Aquaculture rooted 25% IAS fish introduction in Africa, 21% in Asia, 19% in Australia, 20% in Europe, 19% in North America and 19% in South America. Thereby approaches "pet/aquarium trade" and "live food trade" possessed the subsequent potent pathway.

Future research potentials
Firstly, research on Asian freshwater invasion by IAS fish has been largely neglected in comparison to other regions (Peh, 2010). To improve biological invasion research in Asia, it is important to address questions on natural history, taxonomy and ecology of the IAS fishes of this region (e.g. Peh, 2002;Peh & Sodhi, 2002;Yap et al., 2002;Yap, 2003). Basic natural history information and supporting baseline data are still urgently needed as a ground work before strategies for surveying, eradicating and managing biological invasion (Peh, 2010). Secondly, in Antarctic region, there are closely nil research based on freshwater fish diversity, distribution and fish invasion. To understand the trends of freshwater fish diversity and distribution, investigation and a compilation of recent data is a burning issue at this instant. Thirdly, there is an arising indication in invasion ecology that beyond the biological invasions in any ecosystem, there are stimuli of native species diversity and abundance (Peh, 2010). From the Table 4, within continents where there presents high abundance of freshwater fish, IAS of fish emerged there in. Therefore, together with the information of IAS, vigorous documents of diversity and distribution of native species from every continent as well as every biogeographic region is crucial.

Conclusions
The world these days faces a foremost challenge. That is, on the one hand, public outcry at the undesirable detrimental consequences in ecology of several IAS; on the other hand, there is the irrefutable and significant contribution of some IAS fishes to an undeniable need, for instance, food fish requirements. The growing field of invasion science at the moment, poised at a crossroads where ecology and public perception meets, meanwhile is increasing. Therefore, other than focusing on culturing hybrid (e.g. carp), predatory (e.g., bass and trout) and fast-growing fishes (e.g., tilapia), we should emphasis on culturing, trading, stocking and consuming fish, which has potential to balance the ecosystem. Side by side, we necessitate a number of workout strategies that could address both needs; i.e. cope with widereaching hunger and poverty, and simultaneously, deal with and pledge us sustainable use of world"s freshwater resources. Thus, the homogenization process could be defended and delayed.