Fishpond predators astonish University of Hawaii researchers

Native Hawaiians have been removing barracuda, jacks and other predators from their fishponds for centuries to preserve prized mullet, milkfish and other aquatic food species for harvesting.

But new research from the University of Hawaii at Manoa suggests the predators today might actually have a beneficial role to play in fishponds — that of biocontrol agent against the Australian mullet, an invasive species.

Since it was accidentally introduced into Hawaiian waters in the 1950s, the Australian mullet has been competing for resources with the native mullet, a traditional fishpond food fish known in the Hawaiian language as amaama.

The problem is the invasive is probably winning.

Earlier research indicated the Australian mullet has a growth rate advantage over the native mullet, and that the invasive population is able to adapt to changing conditions faster. The long-term impact on the Hawaiian aquatic ecosystem remains unknown, according to the 2019 study.

In the new study, published last month in the journal Aquaculture, Fish and Fisheries, UH researchers examined the diets of the predators caught in the Heeia Fishpond in Kaneohe and identified a preference for the Australian mullet.

“We were surprised that the invasive was part of their diet,” said study co- author Erik Franklin, associate research professor with UH Manoa’s Hawaii Institute of Marine Biology.

The native mullet was not found in the stomachs of the predatory fishes, confirming the results of a diet study conducted before the introduction of the Australian mullet.

The inclusion of the Australian mullet in the contemporary diets of the predatory fishes demonstrates a shift toward the new prey and represents a potential benefit to native mullet populations, according to the study.

While the paper suggests the jacks and barracuda can be used for biocontrol, it doesn’t say exactly how that can happen.

Franklin said that still needs to be figured out by fishpond managers. It may not be as simple as removing the predators from the fishpond less often, he said, because at some point they could turn their attention on other food species.

This is believed to be the first study using genetic barcoding and stable isotope analyses on the diet of predatory fishes in Hawaiian fishponds, and the first diet study on these fishes in over 70 years.

Franklin said he expects to see additional research about the food web of Hawaiian fishponds. Traditional Hawaiian fishponds are a potentially important aquaculture resource, yet the fish populations within them remain woefully understudied, he said.

A 1990 survey of Hawaiian fishponds identified 488 sites, many of them in degraded condition or beyond repair. According to the nonprofit Kua‘aina Ulu ‘Auamo, there are now 38 fishponds being cared for across the island chain following a resurgence in interest in the traditional practice.

These fishponds, known as loko ia, have been identified as a potential source of food security. More than 80% of food consumed in Hawaii is imported at a cost of nearly $3 billion annually, and even a small replacement of imported food could end up generating substantial income.

In 1900, fishponds across the islands yielded 682,464 pounds of fish. Over the years, however, it became cheaper to catch fish than to raise them, and short-term gains from ocean fishing became more profitable than the long-term investment of operating the fishponds.

By 1994, only six of the original fishponds were still operating commercially in Hawaii, resulting in a 95% reduction in yield from 1900.

“The restoration of functional Hawaiian fishponds represents a promising opportunity to increase local seafood production but there is still much to learn about the ecology and dynamics of fishpond communities,” said lead author Anela Akiona in a news release.

Akiona is a UH Manoa marine biology graduate student who was supported by a Hau‘oli Mau Loa Foundation Graduate Fellowship at the university.

Also listed as co-authors are Keli‘i Kotubetey and Hi‘ilei Kawelo from Paepae O He‘eia, the nonprofit that runs the Heeia Fishpond. A group of volunteer fishers contributed to the tag- recapture experiment, while volunteer interns from the Laulima A ‘Ike Pono program helped with the collection and preparation of samples used for analysis.