'Over 90% had deformities'

In an experiment, researchers followed a group of rainbow trout from an average weight of 36 grams to 5.5 kg, as Norway's Institute of Marine Research writes in a report on its website.

“Over 90% of these had spinal deformities when they reached 5.5 kg,” says researcher Per Gunnar Fjelldal.

This is significantly more than is common in farmed salmon, for example. 

"Skeletal deformities cause welfare problems in farmed fish, and also reduce growth and quality. It is therefore important to have knowledge about how deformities develop throughout the entire production cycle." 

Introduced to become farmed fish 

Rainbow trout were introduced from North America to Europe in the late 19th century with the intention to farm them. 

“During the over 100 years that rainbow trout have been farmed, there has probably been significant selective breeding,” says Fjelldal.

Now researchers have taken a closer look at whether this can cause problems for the fish. 

"The rainbow trout that swim around Norwegian fish farms have a high appetite and grow quickly."

It has previously been shown in other species that these characteristics can have a negative effect on the skeleton, or what scientists call the fish's bone health. 

"Considering that rainbow trout were introduced over 100 years ago with the aim of farming, and that the species is also widely used as a research fish, there is surprisingly little knowledge about bone health in farmed rainbow trout."

Acquired deformities as it grew 

When the fish were small, the researchers found very few traces of deformities, but this changed as they grew. 

“We saw that the fish developed more and more deformities in different areas of the spinal column throughout the experiment,” says Fjelldal. 

The experiment lasted 13 months, and included X-ray examination of 80-100 fish in four rounds.

The fish were kept in freshwater tanks until they weighed 160 grams in June, then transferred to cages where they were kept until May of the following year, when they had reached 5.5 kg.

"This is approximately the same growth rate as in regular fish farms." 

Many had changes to the tail fin 

During the period in the cage, the fish developed deformed vertebrae in the neck region, anterior part of the abdominal cavity, tail and caudal fin region. 

"Sixty five percent of the fish had deformities in the rear vertebrae in the caudal fin region," says Fjelldal.

These vertebrae are part of the caudal fin complex, and contribute to the normal function of the caudal fin during swimming.

In parallel with these changes, the fish also developed a degenerated and abnormally small tail fin. 

Both of these changes affect the fish's ability to swim normally. 

Also checked fish not included in the experiment 

To see if the results from the experiment also applied to rainbow trout in commercial fish farms, the researchers examined three groups from one farm. 

"We found similar deformities in these groups, so the results appear to be transferable from the experimental fish to ordinary farmed fish, but that does not mean that all rainbow trout in all farms are as severely affected as those we examined," says Fjelldal.

Negative effect on animal welfare 

Development of deformities in this experiment were of an extent that the researchers observed a negative effect on animal welfare. 

"When the fish develops deformities in the spinal column, it also affects the ability to move. A deformed spinal column loses flexibility, which in turn prevents normal swimming behaviour. Combined with a degenerated small tail fin, this is exacerbated, "says Fjelldal.

In addition, the researchers see that the more deformed the fish, the poorer its growth.

Little research has been done on deformities in general and what leads to such changes in rainbow trout, but there is a lot of similar research on salmon. 

"We don't know what causes rainbow trout to develop so much deformity, but based on knowledge from salmon, nutrition and farming conditions may play a role," says Fjelldal.