Wrasse breeding breakthrough

The research aims to improve the survival rate of ballan wrasse that are being bred in captivity in order to be used as cleaner fish in salmon farms.

“The problem with breeding is keeping fish fry alive,” Kari Attramadal, a postdoc at NTNU’s Department of Biology explains. “Salt water fish larvae are tiny and vulnerable when they hatch and leave the protective environment if the egg for the first time.”

“Wrasse larvae are very underdeveloped when the hatch, they are more or less still foetuses,” says Kjørsvik.

Fish fry that hatch in fresh water, such as salmon, are much stronger and more robust, ready to start on the dangerous journey outside the egg. For wrasse, the transition from the relatively sterile environment of the egg out into open water is huge. The fish larvae quickly encounter a multitude of bacteria.

“When the eggs hatch and the fish larvae come in contact with water, they are colonized by bacteria. Because of this, the environment that the fish larvae is in when it first opens its mouth is essential to its strength and chances of survival,” Attramadal explains.

The intestinal flora is just as important in these larvae as it is in humans, and researchers estimate that the immune system in wrasse larvae isn’t fully developed until about three months.

Bacteria in the tanks tend to be much more abundant than in the natural environment of the ocean. Disinfecting and cleaning the water in these tanks is therefore important. But, by doing this, the natural bacterial flora dies, which allows for other, more aggressive and opportunistic bacteria to spread. These bacteria are the ones that the fish larvae are most vulnerable to.

To prevent invasion by these opportunistic, bad bacteria, the environment in the tanks has to be such that the good bacteria are allowed to flourish. Researchers are using artificial selection to make the aggressive, bad bacteria give way to the good ones.

“The solution is to create a stable environment. We do this by adding a bio-filter to the system that contains good bacteria that are well adapted to the environment. These bacteria will fill up the environment so that the bad bacteria don’t have any way to spread,” Kjørsvik says.

The fish larvae need to be protected against bacteria that could potentially harm them, but they need to be exposed to certain bacteria to be able to develop their immune system.

“We have tried to figure out what bacterial flora is best for the young fish to be exposed to, to find the right cocktail,” Attramadal says.

“The way the water is treated makes a huge difference when it comes to survival. The survival rates in environments where we’ve used artificial selection are much higher— the difference was even greater than we’d anticipated,” she concluded.

New diet

The other fundamental element that researchers have taken into consideration is the food that the young wrasse are fed. Small salt water fish larvae like this can’t eat dry food, they need to eat live prey, which are easier to digest. There are two species that are used as food for young farmed marine fish: rotifers and Artemia salina, a species of brine shrimp.

There are, however, other organisms that more closely resemble the natural food sources of these fish and the researchers have observed that giving wrasse larvae access to copepods during early development practically works miracles.

“Previously, it has been difficult to produce copepods on a large scale, however. But now, we have a found a method to make this possible, which is a fantastic breakthrough when it comes to raising marine fish on the whole,” Kjørsvik says.

Great results

“When the fish larvae are given the correct food and are raised in the proper water quality, the results are great. The young fish develop much better, and have higher growth and survival rates. They are better at swimming and catching prey, have quicker bone development, and are less likely to develop mutations. They also handle stress much better,” Kjørsvik said. “If we can raise robust fish, maybe we can train them to be even more interested in eating salmon lice. Fish can actually be stimulated to learn. If we can do this, we will have effective, happy working fish.”