A variety of algae is used for halibut - about 500 litres a day during May, for example. Algae are pumped into the tank systems containing the larvae. "This is the most important place where we don't want any diseases introduced," says researcher Orjan Karlsen.
Where halibut eggs are kept for about 12 days, the water is filtered to about 0.5 microns, entering the tanks at the bottom and exiting at the top.
Viable flatfish eggs are pelagic and float due to the presence of an oil globule. Infertile eggs sink during incubation.
"The water is of high salinity to separate the dead from the live eggs, so we pump in about 5 litres from the bottom and wait," Karlsen explains. "Then we take the eggs out and count them. This system is quite easy to operate, and not particularly labour intensive."
Halibut eggs are tiny (0.9 mm diameter) compared to those of salmon (6 mm). The larvae are equally small, extremely undeveloped and with only small feed reserves in their yolk sac. "Halibut larvae hatch without any functional eyes, mouth or internal organs," continues Karlsen. "You need about 150 day degrees at 6 C - 25 days - before these things are functional. But we don't start feeding before they are past 200 day degrees - nearly 40 days."
A day or two before hatching, eggs are transferred to tall silos with a water flow - also from bottom to top - of 5 to 7 litres a minute. These are insulated to maintain a stable 6 C.
About 40,000 halibut larvae are produced in each batch and then transferred for start feeding.
When halibut hatch they can swim but in a very uncoordinated way, so any disturbance should be avoided.
"When they are getting past the 150 day degree mark, they are getting extremely phototactic and will swim towards any light source," explains Orjan Karlsen. "We therefore have to avoid any form of light in the system."
Three sizes of silos are used. The larger ones could each produce 150,000 to 200,000 larvae, but the station is working to keep the families together, so it does not mix egg groups.
"We would need a huge egg group of 3 or 4 litres to fill up one of the largest silos, but we are putting in about 1litre," says Karlsen.
During the yolk sac stage larvae grow from about 6 to 12 mm before being transferred for start feeding. They are removed by drawing them to the top with a light and scooping them with buckets.
Last year, researchers improved the oxygen in the silos - hopefully to reduce the 'lockjaw' responsible for many of the larval mortalities over the previous 2 years. Results already look "very good", according to Orjan Karlsen.
"Some 60 to 70% of the larvae can be affected in a bad brood," he explains. "That's a waste of investment because we start feed them all. After 14 days or so those with locked jaws die and we filter them out." By May, the highest number they had was 6%.
In the start feeding tanks the halibut larvae are still pelagic, swimming more or less like cod. A huge milestone is reached when they reach the juvenile/post-metamorphosis stage, and begin to settle on the bottom.
Flatfish start life upright, but when still tiny they turn onto one side, which becomes the belly. The eye and the nostril on that side move up over the head and join the other eye and nostril on what now becomes the back. This major event is called metamorphosis.
Halibut go through metamorphosis from about 25 days after hatch.
Water in the start feeding tanks is increased from 6 to about 12 C over 2 or 3 days, and the larvae fed with live Artemia and smaller quantities of wild-caught zooplankton.
"These larvae each consume hundreds, probably thousands, of Artemia daily, because they don't do much else other than swim around eating," Karlsen explains. "We put 2 million Artemia a day into each tank."
He adds that while Artemia cysts (eggs) are not all that expensive, their production is, because it requires the attention of one person, 7 days a week - "a big investment", says Karlsen.
"We hope we can shorten the period when the larvae require live feed, switching them to dry feed as soon as possible," he continues.
"At present, however, we always start them with Artemia, and after 10 days change over to wild zooplankton to give the correct pigmentation. This cannot be achieved by feeding Artemia alone, even with the different enrichment diets we have tried."
Austevoll's researchers experimenting on the adaptation of larvae to artificial feed, and on various health and disease problems. These include bacterial diseases of eggs and larvae, anti-bacterial treatment of eggs, and the possible use of behaviour for the detection of diseases in larvae.
Some researchers have been working on a certain virus which attacks the eyes and brains of juveniles. It is similar to a virus affecting red drum, seabass and other species in Asia.
"It seems from abroad that this virus is very dependent on the egg batch", says Karlsen. "Some batches have huge amounts of the virus, others do not. So, if we are able to screen the batches before we move them to start feeding, we may control the disease better."
Getting halibut to start feeding used to be a problem. But with the systems now used at Austevoll, more or less 100% start feeding can be achieved within one day.
Feed research includes testing different artificial diets. Researchers check how well the fish grow, how the feed behaves in the water, etc.
"There are a lot of different ways to produce dry feed, and we are trying different products from microencapsulated diets, through yeast-based foods to extruded feeds," explains Karlsen.
"The nearby Herring Oil and Meal Industry Research Institute is also producing a very good dry food which we are trying."
(from article in Fish Farming International, 25(2), February 1998)