Massive Glass Eel run in the Navua River, Fiji

Valentine’s Day this year witnessed a major run of Glass Eels (Anguilla sp.) in the Navua River, Fiji. Judy Tilbury, who resides close to the mouth of the Navua River witnessed a “river of glass eels” migrating up the river early in the morning on February 14th, and this was followed by a second, possibly larger run two days later. What are glass eels, and what brings about this remarkable phenomenon of nature?

Glass eels (or elvers) are the post-larval stage of adult eels that occur in many of Fiji’s rivers. When mature, the adults head to the sea where they spawn, although exactly where this takes place is not known. The hatching and metamorphosis of the babies into the glass eel stage is well synchronised, and when conditions are right they migrate in phenomenal numbers back to the rivers where they will stay until mature. Studies elsewhere, such as New South Wales in Australia, have shown that the elvers stay close to the river mouth as they adapt to the transition from salt water to fresh. They remain hidden in the sediments or places like seagrass beds until conditions are right for their migration. Their mass migration is linked to the new moon, a strong rising tide and, in the case of Fiji at least, heavy rainfall. These are exactly the conditions that occurred early on February 14th this year.

In some parts of the world a lucrative aquaculture industry exists, based on capture and grow-out of the  elvers to market size in ponds. The ponds need to be furnished with an overhanging bank or fence, to prevent the eels from escaping, as they are readily adapted to travelling short distances over land. To date, no attempt has been made to grow eels this way in Fiji. There is a growing market for sale and consumption of the elvers, especially in China.
There are several species of eels in Fiji; identification of the elvers requires counting the number of vertebrae when they are still at the transparent glass eel stage.

G. Robin South
Institute of Marine Resources
University of the South Pacific