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Nile tilapia threaten mullet habitat in Hilo

State officials are trying to decide what to do about an “invasion” of tilapia in Hilo’s Wailoa River system.

The culprit is the non-native Nile tilapia, large schools of which can be seen with the naked eye in the river system, including its tributary Waiakea Stream and Waiakea Pond, which sits in the middle of Wailoa River State Recreation Area.

The Wailoa River system is a prime mullet fishery, and biologists in the state Department of Land and Natural Resources Division of Aquatic Resources worry it is being threatened by the proliferation of Nile tilapia.

“Our biggest concern right now is the Nile tilapia is competing with the native mullet (‘ama‘ama) for habitat and resources and potentially affecting the fishery,” said Hilo-based DAR aquatic biologist Troy Sakihara.

Sakihara and his team have been taking in live and dead tilapia caught by fishermen, and he marvels at how fast and large the Nile species grows.

“What makes them particularly troublesome as a potential invasive species, is they can survive and take over in a wide variety of habitat conditions,” Sakihara said. “Their very hardiness makes them an issue.”

The reasons behind the recent increase in the Nile tilapia in Hilo are unknown, but biologists caution that while aquaculture farming facilities are “closed systems,” all it takes is one lapse in biosecurity to accidentally start an invasive population.

Two years ago, the Nile tilapia was reclassified by the state Board of Agriculture as a restricted species only for research — to permit importation for aquaculture facilities to farm raise them. While there’s no evidence the recent invasion of the tilapia in fresh water streams has anything to do with commercial farming operations, it confirms fears DLNR and Division of Aquatic Resources leadership and biologists expressed when board members were debating whether to allow the DOA to revise its rules to permit aquaculture importation of the fish.

“Unfortunately, Hawaii has a long history of bringing in species, thinking that they’ll provide some commercial or ecosystem benefit, to discover later that these same species outcompete native species,” said Brian Neilson, DAR administrator. “We are now seeing stark evidence of this. While Nile tilapia were present in the Hilo waterways before aquaculture operations began, reports of Nile tilapia or hybrids are on the rise, indicating that their population may be increasing, and their range may be expanding. That’s the real downside of bringing non-native species into the state for any reason.”

Kim Fuller, a DAR aquatic invasive species biologist, said that Nile tilapia are valuable for commercial food production for the very same reasons they can become pests when they’re present in natural fresh water environments.

“They reproduce quickly, grow quickly and are very competitive for food resources with native fish,”she said. “Worldwide, Nile tilapia have a history of invasion, and when we did our risk assessment for the introduction of these fish we found them to be fairly high risk of becoming invasive in Hawaii.”

The DAR team in Hilo is in the process of determining what to do to try and control the burgeoning population of Nile tilapia, including the possibility of an open fishing tournament.

There are no restrictions or bag limits on tilapia. For now, biologists say, the public can catch and eat as many as they want.
Source: Hawaii Tribune Herald

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