Category: Aquatic Species
By Rob McCormack
There are 4 different species of eels you can find in Coastal NSW. Only 2 of them are of interest to us and these are the most common types. Basically we have two types of eels, Long Finned (Anguilla reinhardtii ) and Short Finned (Anguilla australis). The main species cultured in NSW is the Long Finned Eel as this is the one that receives the highest price and demand in this region.
Long Finned eels can be identified by the (top) dorsal fin which is longer than the short finned eel. The long finned eel’s dorsal fin starts well before the anal fin (bottom) which starts just after the anus (vent)(see above image). On the short finned eel the dorsal fin (top) starts just a little before the anal fin (bottom). Short finned do not grow as large as the long finned. Under 10kgs.
Long Finned eels are fascinating animals. They start their lives as eggs in the Coral Sea off New Caledonia and or New Guinea in deep water 300 to 3000 meters down. It’s not exactly known as eels are still a mystery to most researchers as their life cycle has not been repeated fully in the laboratory as yet. Eels are highly fecund and a large female can spawn up to 20 million eggs. She only ever does it once and her whole life is keyed to that one event where she spawns and then she dies. From there they drift with the currents, then hatch into leaf shaped larva drifting, floating and swimming with the currents till they hit the East Coast of Australia. As they reach the coast and the water quality changes the leaf shaped larvae transform into glass eels which are a clear tiny baby eel. When these baby eels drift down the coastline of Australia they enter the river systems and migrate upstream. When they first enter the rivers they are called glass eels as they are very small and clear (see through). Once they enter fresh water and start feeding they become pigmented and darken in colour and are called elvers. These glass eels and elvers migrate upstream in enormous numbers at the right time of year.
It seems unknown why they enter some river systems rather than others but it is supposed that there is some sort of genetic imprinting in the eels from their parents to return to the area their parents came from. Also rainfall is an important factor. Elvers have an instinct to migrate upstream and need flowing water for this to occur. Drought conditions see a dramatic drop in recruitment of elvers into drought affected river drainage basins.
As a general rule it is the male eels that stay in the salt/brackish water and usually the female eels which migrate well upstream from the salt water and are the ones that enter your freshwater rivers, creeks and dams. Eels are a major problem for people who want to use farm dams for aquaculture. When your farm dam fills and then overflows the overflow water may only be a small trickle through the paddock but eventually that water hits a creek or steam with eels in it and they can scent/feel/smell that dam water which is different to the general paddock runoff water and follow that scent back to your dam. Generally they will stick to the water course but if an obstacle is in their path they will go overland around the weir/waterfall etc. They can survive quite well out of water in moist conditions. They may be only small elvers only 100mm long but over time they will grow into massive eels. Generally these eels will stay from 10 – 30 years in your dam and then in flood conditions migrate back to sea to breed and then die. Long finned eels have been known to grow to over 80 kgs in size and live for 60 years. When these adult eels migrate back to the sea they actually change shape. The head flattens and the eyes broaden around the head.
Eels are extremely tough; 10 years ago we did an experiment on a 2 kg eel to see how long it could survive without food. We kept this eel in a 1000 litre tank in good quality water with aeration for 18 months without food before it started getting ulcers and we took pity on it and gave it a feed, cleaned it up and then let her go.
In a normal farm dam type situation it usually takes the small elvers at 100mm long approximately 3 years to become a major predator in your dam. The 3 year olds are in the 250 – 350 gram range and rapidly grow in size from then on and if it’s a yabby dam can completely eliminate all yabbies from the dam as there is nowhere a yabby can go that an eel can not. The yabby of the Cherax variety with a thin smooth shell has no defense against eels and are easy prey. The Eustacus type with a thick, strong and spiny shell can cohabitat with eels. It’s not as critical with native fish as most Australian native fish are fast and smart so most can mostly avoid eel predation. However, the eels will eat the same food as the fish so the more eels the less food available for your fish.
In a yabby dam eels are not a problem until they are over 200 gram in size. That is at that size that they start making a noticeable impact on your yabby population. Most people on the Eastern Drainage who put in a new farm dam and stock with yabbies will usually have a great yabby dam for the first 3 years them year 4 notice a major decline of numbers and by year 5 have no yabbies at all. The dams at most risk to eel infestation are those with a large catchment which quickly fill and overflow regularly. Those dams with small catchments and intermittent overflows are less likely to be heavily infested with eels.
Eels can be captured quite easily from farm dams. Set lines are a traditional way. Just 3 – 4 metres of strong fishing line tied to a tree or stump on the bank with a heavy duty long shank hook (Number 2 or 4) and a piece of fresh meat or fish for bait. Eels are a predator which hunts in the shallows of your dams at night. Just bait up the lines in the afternoon and leave out overnight. Check in the morning and usually you will find the eels have caught themselves. Eels are very strong and smart, they will tangle themselves under rocks and snags etc so make sure your set lines are only short. They also tend to twist and roll which can but extreme pressure on your system so use heavy duty hooks and line.
Traps are also good eel catchers. At Port Stephens we use a large green opera house type trap which we bait with pilchards. This works very well as pilchards are excellent bait as their strong oily smell attracts eels from a long distance. These type eel traps are on sale at RBM Aquaculture. We also use fike nets from time to time and these also work very well. For our commercial harvest of eels we use a home made eel trap approx 600mm square and up to 3mtrs long. These traps catch around 60kgs/trap/night if the eels are present.
In a commercial crayfish farm situation that occasionally has eels, fish and turtles entering the pond and you want to remove everything plus crayfish, we use super yabby traps. These are a long (14 m) net that catches everything in the pond. Super yabby nets are only for use in commercial crayfish ponds or private dams. You should check you states fisheries regulations. Super crayfish traps are available from AABio Aquaculture.
We grow mostly long finned eels here as in NSW this is the one with the ready market and high demand. In Victoria and Tasmania they tend to produce more short finned eels. Caution should be taken if you are purchasing glass eels as you may well be purchasing the wrong type. Don’t get short finned or you may find there is not a ready market for the final product in NSW.
Long finned eels sold by us are sold to an exporter who then sells them to Asia. We just have a 10,000 litre tank here with our eels in it ready for pick up. The exporters have their own transport trucks and come out to our farm. We load the eels up into fine net bags at 30 – 50 kgs per bag, these bags are then weighed and the eels then emptied into the live transport tanks on the back of the truck. We usually do loads of about 100 kgs per go. Only long finned eels are taken and need to be over 650 gram in size. The bigger the better. The exporters pay you at time of pick up at a rate dependent on time of year of $11 up to $15/kg. Small eels and short finned eels they will take off your hands but at $3 – $4/kg.
Both these 2 species of eels have excellent aquaculture potential and many farmers are starting to farm eels. Because of the difficulty of breeding eels in captivity (has not been done commercially as yet despite millions of dollars spent in research in the Northern Hemisphere – some experimental work is showing its possible) all glass eels for culture are captured from the wild. In NSW the NSW Fisheries department is unsure of the glass eel stocks available so has capped the collection of glass eels at 300 kgs/year. To date this supply has been unreliable. Another source of glass eels is to get them from interstate where more are available regularly. There are from 3000 to 6000 glass eels per kilogram.
Eels can be grown intensively in recirculation systems at up to 100kgs/m³ but generally at around 60kgs/m³. They can be grown intensively in ponds at 7,000kgs/ha or just extensively at 500kgs/ha. Eels are cannibalistic and grading to separate the large from the small is wise. In the wild we work on a catch rate of 300kgs/ha of larger over 650gram eels. So if you have farm dams this is what you can expect to harvest from natural stocks in this region.
Many people are looking at eel farming these days due to the relatively high prices paid per kilogram. You can purchase small eels from professional fisherman at around 200 – 300 gram in size and from $2 – $4/kg. These eels can then be trained onto pellet feed and fed daily in special earthen eel ponds can reach a weight of 1kg in 12 months. This is the easiest method of growing eels as using glass eels is a very labour intensive method. 200gram eels can be easily trained onto Silver Perch or Barra Pellets without any great effort. Keep them well fed in good quality water and cannibalism will be greatly reduced.
We do most of our eel fishing over winter at Port Stephens. This is not the best time to do it as eels are cold blooded animals so as the water temperature drops their metabolism slows so they don’t need as much food or feed as much. However if the water is over 15 C then eels can be captured in our traps. We usually fish over winter as we usually receive a better price then and we usually have a bit more time available. We sell the larger eels we capture and keep the smaller ones and grow them up in a pond. We just capture eels as we get spare time and store them in a tank until we have enough for either stocking a pond or for sale.
The only problem with holding eels in winter is that their metabolism is going slow so they are more susceptible to infections. In the traps when captured they run their head along the trap looking for a way out and can loose their slim covering which can let infections attack the eel. We have had big trouble with eels in winter in our holding tanks in the past with fungal infections. We have overcome the problems by holding them in tanks with a prophylactic treatment of hydrogen peroxide and colloidal silver ions in the water. This has been 100% effective and no fungal outbreaks occur now.