A fish is taking over Taiwan''s lakes. The sailfin catfish is a freshwater species native to South America. It was introduced to Taiwan in the 1980s, as a pet that eats algae and can keep aquariums clean. But these household pets become insidious problems when they''re released into the wild. Today, Taiwan is in an uphill battle to control its catfish invasion. Our Sunday special report.

This is Bihu Park in Taipei’s Neihu District. Surrounded by mountains, this is a popular spot to relax and fish, or just to sit back and escape the city’s hustle and bustle.

But here at this corner of the park, it’s a storm of activity, cutting a stark contrast to the lake’s serenity. These conservationists work together to carry a boat to the water, and then they set up a gill net — but what exactly are they working on?

Lin Chun-han

Taiwan Ocean Conservation and Fisheries Sustainability Foundation

Today we’re attempting to clear the lake of a type of fish called the sailfin catfish. This is an invasive species that has taken over the lake.

Bihu Park is a great place for Neihu residents to relax and go on walks, but few residents are aware of what lurks beneath the surface of its lake. The calm surface hides an unwelcome guest — the sailfin catfish, also commonly known as an algae eater.

After placing the gill net for two hours, the group is ready to pull it in.

In just two short hours, they have captured 37 sailfin catfish. Looking closer, one is greeted with the fish’s small round eyes, and its labyrinth-like skin pattern. Its coarse skin is covered in hard scales that form a sort of armored plating.

Lin Chun-han

Taiwan Ocean Conservation and Fisheries Sustainability Foundation

The sailfin catfish is not indigenous to Taiwan. It’s from South America. There is a suitable environment for it there, as well as natural predators. But they don’t have natural predators in Taiwan.

Some 30 years ago, the sailfin catfish was considered a must-have for aquariums, and it was imported into Taiwan. It was known as a garbage collector or algae eater.

Chen Hung-pang

Aquarium retailer

The main purpose of algae eaters is to eat the algae that thrive in aquariums due to the lighting. Aquarium owners don’t need to worry about frequently cleaning the aquarium — the algae eater does a very good job of that, saving people lots of time.

Algae eaters were imported for use inside aquariums. How did they end up out in the wild?

Less than 2 kilometers away at Dahu Park, the same problem exists – an invasion of algae eaters.

Chang Chia-hung

Taiwan Conservation Association of Native Fishes

When these algae eaters grow up, many aquarium owners discard them due to their offensive appearance. Others get rid of them due to their aggression toward other fish in the aquarium.

Out in the wild, the fast-reproducing sailfin catfish can quickly become a problem. During its breeding season from May to August, a multitude of eggs are laid, creating an ecological catastrophe.

Chang Chia-hung

Taiwan Conservation Association of Native Fishes

There was a study done by a master’s student at National Taiwan University. It found that sailfin catfish can lay up to 6,000 eggs at a time. In my own calculations, I found that they laid up to 2,500 eggs.

About an hour’s drive from Taipei, Bade Pond Ecological Park in Taoyuan has also fallen victim to the sailfin catfish.

Of Taoyuan’s 2,000 or so ponds, about 800 are used for irrigation. Some of those ponds are also used for aquaculture. In recent years, the sailfin catfish has been a source of grief for the aquaculture industry.

Chen Chin-heng

Aquaculture farmer

Algae eaters use up oxygen and consume feed — they eat other fish’s feed. This creates losses for those of us who cultivate fish, but we don’t have a solution for it.

This 20,000-ping pond in Taoyuan’s Sinwu District is teeming with sailfin catfish. According to aquaculture farmer Chen Chin-heng, his net once drew up 1,000 catties of the fish in a single go. On the day we were there, we didn’t catch many, but we could see signs of them in the pond.

Sailfin catfish are omnivores. If they eat food meant for other species, they invariably add to the operating costs of aquaculture farmers. But what’s of even greater concern is the harm that sailfin catfish can do to farmed fish.

Hu Jung-chan

Taoyuan Aquaculture Union

They have an impact on the fish we raise, like the tilapia and the Taiwan sea bream. These fish are beautiful starting out, but then they get caught up in the net together with the sailfin catfish. They rub against each other in the net and that helps us scale the fish, but the problem is that there’s no market for these fish after they’re scaled —