64 SEPTEMBER 2023 • WORLD AQUACULTURE • WWW.WAS.ORG This number may be well below the total count of exotic species in Nigeria as the aquaculture industry was not surveyed in the research. In a study to examine the role of humans in the non-native fish introductions and density of non-native fishes in ponds in Essex, England Copp et al. (2005) reported the most commonly encountered and most abundant fishes captured, apart from the native three-spine stickleback, were varieties of fish not native to the British Isles, particularly, brown and red varieties of goldfish and several varieties of common carp. Irresponsible pet-fish owners sometimes discard their fish by abandoning them in parks or releasing them in water bodies. The abandonment (or donation) of goldfish to ponds is not a recent phenomenon, but the incidence of non-native introductions appears to have risen substantially in the last decade in the British Isles with most fish being introduced to ponds in parks close to roads (Copp et al. 2005). This is an unpopular practice of introducing exotic fish to local waters. Some cultures and religions believe the release of fish into the wild brings good luck. This practice poses significant risk to the receiving ecosystem. The ceremony of ‘animal release’ or ‘release of living beings’ is one of the regularly performed rituals in Buddhist practices throughout Asia, and, in recent decades, in the West (Shiu and Stokes 2008). In this practice, pets and other domestic animals are released into ‘natural habitat,’ as a means of compassion and attracting goodwill and good luck. Religious releases are most widespread in Asian countries with considerable Buddhist influence, but also occur in Canada, Australia, and the United States (Agoramoorthy and Hsu 2005, Agoramoorthy and Hsu 2007, Shiu and Stokes 2008, Liu et al. 2013). Aquaculture Development and Introductions Failure to identify important pathways and determine highrisk species and habitats will prevent best use of limited resources where they are most needed for control of unwanted invasive species (Gertzen, et al. 2008). A wide variety of fish species have escaped from culture operations with varied levels of severity (Myrick 2002). Harrell (2002) believes there is no deniability that escapes from aquaculture operations occur and escaped fish can and have interbred with native fish on occasion, but the genetic impact of these escapees on wild conspecific populations is debatable. Pond-raised fish often escape into natural waters due to incidences of flood. Experts opine that transfer of fishes to different habitats within a country should be done with as much precaution as those across borders (Kottelat and Whitten 1996). The African Arowana Heterotis niloticus has become a common fish in the Cross River drainage system of Nigeria following its escape from a farm in Arochukwu into the wild. This species was introduced to culture ponds to increase diversity and improve productivity. The fish, which was originally found in the Niger River drainage system, accidentally escaped into the Cross River Basin from the fish farm during flooding in the 1980s. The species then became established and spread from the upper reaches of the river to its confluence with the Calabar River near the Atlantic. Although the Nigerian aquatic ecosystem is characterized with diverse fish species suitable for culture (Adaka et al. 2013), certain exotic species have been introduced for farming. One of the earliest examples is the introduction of carp at the Panyam fish farms in the 1950s. What was then the Northern Regional Government initiated commercial production of carp (Sikoki 2013) with the introduction of common carp, grass carp, silver carp and mirror carp. Many countries have implemented legislation preventing the importation of potentially invasive aquatic species, including pond plants. Examples include restrictions on ornamental plants, a number of ornamental cold water fishes and almost all crayfish by the United Kingdom, as well as piranhas and snakeheads by many states in the US. Alum-Udensi and Uka (2017) affirmed that the tradition of protection of water-bodies and their aquatic life through cultural practices can guarantee aquatic biodiversity. Impacts of Escaped Fish on Biodiversity, Ecosystems and Humans A great public fear of escaped fish is their potential dominance of new environments (Myrick 2002). Every introduction of an exotic species that becomes established results in changes to the receiving ecosystem (Bhaskar and Pederson 2000). Although some scientists claim that almost every imported species will eventually escape and be found in the wild, there is no hard practical support that this theoretical model will lead to established populations (Ploeg 2008). Organisms must enter a new environment in numbers great enough for reproduction and spread to occur (Gertzen et al. 2008). The overall chance of survival of animals released in new environments may be low (Chan 2006), but the chance that they will transmit diseases to wild populations or native species may be high (Gilbert et al. 2012). Reports of catastrophic impacts of exotic species introductions abound. Castaldelli et al. (2013) reported on the successful establishment of S. glanis, an exotic species in degraded habitats, leading to the decline of native fish fauna in the canals of the lower portion of the Po River basin in Italy. Other catfish introductions are increasing worldwide primarily due to their popularity in Tropical countries face higher risks of from many introduced species because most ornamental fish hobbyists keep tropical fishes. Photo by C. Greg Lutz
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