Ornamental fish trade is introducing invasive species into local ecology

Scroll.in

26-04-2025

Aquarium trade is a multibillion-dollar business with the value of trade in ornamental fishes alone estimated at $6 billion in 2023. Japan, Indonesia and Singapore export the maximum quantity of ornamental aquarium fish, while India ranks 19th among the world's ornamental fish exporting nations.
There are indications that ornamental fish-keeping may have originated in China with the domestication of common carps and goldfish more than 1000 years ago. Compact home aquariums as we know today however became popular only during the 19th century. The British were among the earliest European fish-hobbyists. They started to collect and transport tropical freshwater fishes from the Commonwealth to their temperate homes, which was likely the precursor to today's global multibillion-dollar enterprise of tropical fish-keeping.
Early fish-hobbyists chose only the small and medium-sized species of tropical freshwater fishes for their aquariums. These were predominantly from south and southeast Asian countries and the tropical American waters. Some of the favorites were barbs, rasboras, danios and tetras.
A few species of cichlids, including the still very popular angelfish, were also domesticated and selectively bred. Air-breathers of south and southeast Asian origin like paradise fish, betta (Siamese fighter) and gourami and tropical American live-bearers such as the guppy, platy, swordtail and mollies also joined the bandwagon.
Two species of Indian freshwater fishes that were widely domesticated and traded are the zebra danio and dwarf gourami. These two species are relatively small in size and are found in the wild throughout the country. Although at present many other Indian freshwater species (especially those from the Western Ghats and Northeast India) have found a place among the ornamental tropical fishes in worldwide trade, zebra danio and dwarf gourami have excelled in their popularity that we find several domesticated variants in the market today.
Interest in large freshwater fish
Goldfish and koi carps are somewhat large-sized freshwater fishes. The latter can grow to around two feet in length and has greater longevity. And as koi carps are not only brilliantly colored but also well adapted to cold water, they are popular in outdoor ponds throughout the world.
There has also been growing interest over the past 40 years in keeping large-sized cichlids, catfishes and other fish. Home aquariums first started keeping large-sized cichlids popularly called 'oscars'. Then came in catfishes such as 'freshwater sharks' and 'plecos'. Around the same time red-bellied piranhas were also in the aquarium trade. Looking back, during the 1980s, a pair of palm-sized red-bellied piranhas were sold for Rs 4,000 in the famous Russell Market in Bengaluru.
The most noteworthy trade in large-sized freshwater fishes however began when feng shui, an ancient Chinese practice of arranging spaces to create harmony and balance with the natural environment, became popular. Feng shui brought with it many kinds of indoor plants and some fishes that supposedly brought good luck and fortunes to their keepers.
Popular feng shui fishes were an American cichlid called 'flowerhorn' and another very large but unrelated fish from the same region called 'arawana'. These two species were heavily traded and people were ready to pay huge sums of money to possess one. A standard flowerhorn fetched up to Rs 3,50,000.
Back in the wild
Fancy for large-sized fishes like the flowerhorn and arawana was short-lived as they neither brought the coveted fortunes nor were they compatible with home aquariums. They either outgrew their containers or, being predators, they proved too cumbersome to maintain due to their demands of live fish as food. It did not take long before owners found it more 'prudent' to release them in natural water bodies, such as wetlands and rivers, in the vicinity.
Unfortunately, however, such irresponsible actions put these large predatory fishes 'back in the wild' albeit in regions far beyond their native waters, transcending continental boundaries.
Aquarium fishes thus introduced locally soon naturalised in their new habitats where some have already become invasive. Among the most invasive species of aquarium fishes are the tropical American sailfin catfishes. These armored catfishes are also popularly called 'plecos' and 'suckers' in trade. They were first traded as 'tank cleaners' because they are obligate grazers and aquarists found them handy in keeping their tanks free of alga. Nevertheless, due to their large size and sometimes aggressive behaviour, they were sent back to the wild.
Plecos proved to be invasive even in the United States, not very far from their native neotropical habitats. Plecos are invasive in other parts of the world too. In many parts of India, such as Kerala, fishers are frustrated by catches that are heavily loaded with these armoured catfishes that even damage their valuable nets. Shores strewn with discarded plecos are not an uncommon sight in south India.
Range extends through trade
Earlier, it was easier to transport live freshwater fish than marine species. Freshwater fishes were therefore largely involved in early trade as that began during the colonial era. Currently, 80% of all ornamental fish exports, worldwide, is of freshwater species.
Larger species of freshwater fishes were transported and introduced outside their geographical range primarily to enhance commercial fish production. While some like trout were also introduced to promote the elite sport of angling, most others such as the Mozambique tilapia were brought in under the guise of food security. Notably, the latter is now placed among the world's most invasive species of fishes.
Another of the colonial era's botched fish introductions involved mosquito fishes. These small tropical American live-bearers, also called Gambusia, were carried far and wide and introduced locally in an attempt to eradicate the dreaded malaria. These surface-swimming fishes were believed to feed on the aquatic larvae of mosquitoes. Despite their small size, due to their very adaptive nature and capacity to rapidly multiply, mosquito fishes have proven to be highly invasive wherever they were introduced.
The most important pathway by which freshwater fishes have extended their geographical range across continents is however ornamental fish trade. The number of species in trade is also increasing in leaps and bounds. And, apart from owners wantonly releasing exotic fishes into local water bodies, there are accidental introductions as that happen during floods. The crisis is further aggravated by development projects such as those that concern interlinking of rivers.
Local invasions
Species that are popular in the ornamental fish trade are very adaptive by nature. This virtue helps them easily adapt to newer habitats wherever they are introduced. Most introduced species have shown the ability to quickly naturalise and some like the plecos have already become invasive.
During colonial rule, Mumbai, Kolkata and Chennai were the three most important hubs of ornamental fish trade in south Asia. The three cities continue to monopolise local and international trade in ornamental fishes.
Ornamental fish trading hubs have also had a major influence on the local freshwater fish diversity in countries including India. In Chennai for instance, 30% of its freshwater fish species are not native. Majority of the introduced fishes have come in through aquarium trade catalysed by local and regional transfers of drinking water through canals.
All naturalised fish species have the potential of becoming invasive under favorable conditions. Once they become invasive, introduced fishes pose threats to native freshwater biodiversity. Even rare and endemic species cannot be given the benefit of doubt in this regard.
Unregulated trade, conservation challenges
Unregulated ornamental fish trade poses major challenges to the worldwide efforts in conservation of freshwater biodiversity. In the Western Ghats, for instance, many species of endemic fishes have entered the fray in more recent times and, some like 'Miss Kerala' (Dennison's barb) are under immense pressure.
The desire to add novelty to their fish exhibits is driving hobbyists to travel far and wide, collecting rare, endemic and even threatened species of freshwater fishes. Species that have not been scientifically described and named are already there in private aquariums and even in international trade. The Rohan's barb, for instance, was known to hobbyists for more than sixty years. It was however known to science only in 2010.
It is therefore important that globally binding conventions such as Ramsar, CITES and CBD together developed guidelines and protocols for regulating international trade in ornamental freshwater fishes. Regions such as south and southeast Asia, where trade in ornamental fish is poorly regulated, should be designated as hotspots. In the absence of adequate regulations, and greater awareness of the dangers of fish invasions, conservation of freshwater biodiversity will remain an unsurmountable challenge.