Strange fish in familiar waters

by Ranjini Murali

Last week we looked at Lantana, an invasive alien plant and its impact on our forests. This week we will look at some invasive alien fish species. To refresh our memories, invasive alien species are species not from a particular country but when introduced to that country spread across the landscape and change the composition of natural ecosystems.

Invasive alien fish species enter the natural water systems through a variety of ways. They may have been brought into the country as ornamental fish for the aquarium, as a food source, as a sport for people to catch them while fishing, and sometimes to control mosquitoes. They then escape, because of poor management and negligence, and establish themselves in our natural ecosystems. Sometimes people even release their aquarium fish into the natural water systems, without realising the problems it can cause. Another way the fish can come into the waterways is through the excess water from ships. They might enter the ship in one place and then when the ship moves they might be released in a completely different place.

Invasive alien fish species are known to cause many problems in the riverine and marine ecosystems in India. Those of you, who have aquariums or eat fish, might be familiar with some of these fish names.

Fish like the African Catfish and the Nile Tilapias, are top predators with voracious appetites and they eat the native species, decreasing their numbers. Scientists have even found birds like the moorhen and parakeet in the stomachs of the African Catfish!

Several species of invasive fish compete for the same food resources native species require. Species like the Three-spot Gourami, an aquarium fish, can be territorial and aggressive. Some other species like the Giant Gourami and Gold fish are known to transmit disease and parasites to native fish species. The African Catfish and the Giant Gourami can establish themselves in polluted waters as they posses modified gills that help them breathe air. Nile Tilapia and Gold fish muddy the waters, decreasing the amount of sunlight filtering through, making the waters unsuitable for native species of aquatic plants and animals. Other such fish running wild include the Siamese fighting fishes, mollies, guppies and swordtails.

‘Sucker fish’ are popularly used in aquariums to keep the glass free from algae. However when they escape they over-eat the algae in the water, disturbing the aquatic food chain balance. They even burrow into the riverbanks to lay their eggs, and this can cause erosion of the banks. This has happened in Thiruvanthapuram lakes.

There are new food fishes being thoughtlessly introduced in culture ponds, and running invasive: these include Red-bellied Pacu (a type of Piranha), Chinese Carps, Grass Carps, Silver Carps, and Bighead Carps. All these are having considerably negative impacts, ousting native species from natural habitats.

They are just some of the range of issues invasive fish species can cause. However, controlling these invasive water species is extremely difficult. Often physically removing them from the water system is the only option. But this can be expensive, time consuming, and often unsuccessful.

As we saw with Lantana last week and these fish species this week, they are often introduced into the country for completely innocent reasons. It’s only when they escape into the wild we come to realise the wide range of problems these species cause.

Controlling the spread of invasive species is a major challenge. It is much easier to prevent its introduction into the country. Recognising this, several countries have very strict laws and regulations on the introduction of exotic species.

This article appeared in the Hindu in School on 14 August 2013.