Study underway to learn more about Guam’s native freshwater eels

Karina Mejia, a graduate biology student at the University of Guam within the National Science Foundation EPSCoR program, and her research mentor, UOG Associate Professor of Biology Daniel Lindstrom, are looking to answer some unknowns about Guam’s most common river eel — the giant mottled eel, or marbled eel (Anguilla marmorata) — and they will be using some innovative techniques and technologies in the process.

Anguilla eels are found widely through the tropics, and their popularity in Japanese, Chinese, South Korean, and Taiwanese cuisine has contributed to four of its 16 species becoming endangered. While the species in Guam is not endangered, it could be sustainably managed as a food source, but surprisingly little is known about it.

The knowns and unknowns

River eels spend some of their lifecycles in both salt and fresh water. In particular, A. marmorata and related species spawn in the ocean, where the eggs and larvae drift until they become small glass eels and enter freshwater rivers.

For the A. marmorata, the spawning grounds are not well-defined, and it’s not clear what time of year they journey upstream — both critical pieces of information in order to sustainably fish them in the wild or to raise them in aquaculture facilities.

A deep dive into DNA

Former UOG graduate student Sean Moran discovered that the marbled eels in Guam’s rivers have significant genetic differences. Mejia’s main focus, she said, is to build upon his findings by showing that there are genetic differences because they’re coming from different spawning grounds.

She will do this by performing a high-resolution DNA analysis using a new PCR-based genetic sequencing technology called MIG-seq developed at Tohoku University in Japan. This will allow her to group Guam’s eels with other genetically documented eels in the Indo-Pacific.

Rings that tell a life story

The second analysis Mejia and Lindstrom are hoping to do is a technique known as otolith microchemistry. It is commonly used on species around the world to trace their migration patterns, but it has not yet been conducted on the A. marmorata species in Guam or elsewhere.

The process assesses chemical concentrations within an ear bone, or otolith, of a fish. Much like a tree, otoliths add rings over time, capturing the chemical elements of their environment. The elements found can be compared to the chemical signature of different parts of the ocean, providing a daily timeline of the fish’s migration.

“So we’ll be able to say, ‘OK, this eel floated around in the ocean this many days,’ and if we’re lucky, we can say, ‘This is where it was on Day 27 — this is really close to where it was spawned,’” Lindstrom said. “We’re hoping it’s possible.”

Information for conservation

With research lacking on the tropical Anguilla species, Mejia is hopeful that her work will not only contribute to the regional conservation efforts, but will also inform local conservation decisions so Guam’s population doesn’t become endangered.

“If we’re able to say, ‘You’re only allowed to catch this amount,’ we can keep the population going and keep them from becoming endangered and then extinct,” she said.

Lindstrom said their findings will also be applicable to aquaculture. No one has been able to spawn and rear these or related eels completely in captivity, he said, so eel farms rely on the collection and captive growing of juvenile eels, or glass eels, as they swim into rivers. By knowing the locations and timing of spawning, those places could be better protected, and wild-caught fisheries could be more sustainably managed.

Mejia hopes to have enough data by next fall to draw conclusions and complete her thesis paper by Spring 2024.