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.
|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.
|Box jellyfish with 2 cm bell|
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.