Sunday, 6 March 2016

Glass Eels are Arriving in our Creeks

On 6 March 2016, it was a dark still night (~8 pm) and the tide had just peaked when I took a look in a tiny creek at the northern end of Palm Cove, Cairns, Queensland Australia.  The small fish that leap when a torch is shone on the water where not to be seen today and I had to look for creatures.  After a few minutes, it became apparent that every few seconds a tiny, clear living thread would zoom past. They were hard to catch and it took a while to get the first one.

Anguilla reinhardtii elver or glass eel

Anguilla reinhardtii
Glass eel or elvers of longfin eel (Anguilla reinhardtii) - click to enlarge
Most of them were following the thin film of freshwater that lay at the surface of the tidal creek. They were mostly moving through the thin film of the very margins of the creek, where the creek cut across the sandy beach.  In all the photos that follow, the gravel that is visible is actually some of the finest whitest sand in Cairns and it is just the small size of the eels that makes the sand look coarse.

Glass eel moving up the very boundary of the water - probably following a salinity gradient
Three glass eels are present in this photo
The glass eels rested momentarily in the mangrove detritus before striking out again.  It proved to be easier to capture the resting eels.  A highly venomous box jellyfish was also bumping its way along the bank.

Chironex fleckerii
Box jellyfish with 2 cm bell
It was so hard to photograph these small creatures in the surging water, I made a video.  The clarity is not great as fresh and saltwater are mixing near surface creating haze.

I captured a few glass eels to photograph and later placed them in an established freshwater fish tank with a few neon tetras.  The elvers seemed to handle an instant transition from saltwater to freshwater well and were alive in the morning.  Instant transitions kill most fish as their gills have to switch from excluding salt to absorbing it and kidneys from excreting salt to recovering it.  I guess that animals which are desperately seeking freshwater can handle sudden transitions.

Glass eels become large freshwater longfin eels when they grow up.  The creek at Palm Cove is very small and it is a wonder that glass eels were attracted to it.  There was a noticeable salinity gradient with relatively fresh water present at the very surface.  Upstream there is only 100 m of mangrove creek before the vegetation transitions into a paperbark/rainforest swamp forest.  These areas tend dry out in the dry season so how do the eels survive?  Back in April 2014, I was looking around in this area for interesting biodiversity when I found a mature eel half out of the water in the mangrove roots. I thought the eel was dead but when I got close, it took off.  Perhaps the eel was surviving by taking refuge in a thin film of freshwater floating above the seawater.

Mature longfin eel using mangrove roots to lift its body to/above the surface.
Place where I found the eel

I took a few eels and let them go in my fish tank.  Two weeks later, most seem to be still alive.  After about 4 days they started to develop a dark skin, but their flesh is still clear.  During the day they hide under objects but at night they swim frenetically, perhaps 40 cm per second.  So they are not just trying to reach freshwater, they want to penetrate upstream as fast as possible.


  1. Fascinating account mate. I've seen these glassy fish east coast beaches (NSW) before, but assumed there could be hundreds of fish larvae which look like this, am I incorrect? How are your captured eels doing now? Do you think the migrating instinct has/will leave them?

    Cheers, Steve

  2. Hi Steve, the glass eels are pretty distinctive, particularly as they swim right at the surface to better sniff out the freshwater. The captured eels survived for at least four months and would probably be doing well now if I had taken better care of them. They quieten down after about a week in the tank but up to that point, they swim like crazy (at night).