Nutreco-owned feed giant Skretting has just achieved a breakthrough in salmon smolt pigmentation, a revelation the company says will provide many market benefits for fish farmers.

In recent years, in order to help fish being raised in open seawater sites achieve optimum growth and health, Atlantic salmon producers often elongate the time that juveniles spend in the freshwater grow-out phase. This reduces the time the smolts spend in the seawater and decline the pigmentation period.

“The pigment in salmonids that provides their prized pink color is the carotenoid astaxanthin,” Skretting explained. “Obtained naturally from plankton, algae and crustacean shells as well as in nature-identical synthesised form, astaxanthin is also an antioxidant and a precursor to vitamin A. In cases where organic feeds are being formulated, the pigment is derived from bacteria.”

While salmon in the seawater stage are given pigmented feeds, it’s generally not enough for proper pigmentation and regulations on feed additives stipulate that synthetic astaxanthin cannot be given to salmonids in the first six months of life. This particular rule has recently been lifted in the European Union, but it is still very much in place in Norway.

“With the sea production time now several months shorter, the salmon are missing out on significant pigmentation time ahead of harvest. Year-on-year, we have seen pigmentation decline in various markets, including Norway, Canada and Chile,” said Leo Nankervis, a leader in salmonid nutrition at Skretting Aquaculture Research Centre.

To combat this issue, Skretting underwent extensive trials with astaxanthin and formulated freshwater-specific feeds containing the chemical compound, enabling pigmentation to begin prior to transferring the fish from freshwater to seawater.

“These feeds give salmon farmers the opportunity to get a head start on the pigmentation process, which can give in excess of 0.5ppm extra astaxanthin in the fillet by the time the fish have transferred,” explained Nankervis.

Skretting researcher Guido Riesen conducted trials at its Aquaculture Research Centre, starting with six-month-old salmon juveniles through the seawater growth phase. The salmon were transferred to seawater sites at sizes of 115g, which is when they started receiving feeds containing “the customary levels of pigment.”

Riesen and his team’s findings established that diet formulations containing 70ppm pigment offered a freshwater model that is most aligned with the seawater model.

“The uptake utilization of astaxanthin in freshwater was very similar to that in seawater. This was excellent news as it meant we could model for both stages,” said Nankervis. “It was also about this time that the EU took away the six-month limit for pigmentation, making it commercially possible to use pigment throughout the freshwater stage. Although, at the moment, the regulation still exists in Norway.”

“The pigment astaxanthin is an essential component of the diet of salmon; among other things, it influences the growth and health of the fish,” Sandvik explained. “It also gives them the appearance that end-consumers look for. Therefore, an important goal for fish farmers has always been to achieve good, even pigmentation.

“However, it is clear that the reduced time that salmon are now spending in the seawater stage of growth has been challenging the pigmentation process in most production regions. These new formulations are a big advance for the marketplace because they address that imbalance.”

The pigment studies have an added benefit: They have furthered Skretting’s knowledge in sealice treatments and their effects on pigmentation, as well.

Skretting decided to test the widely-used common delousing agent, hydrogen peroxide, an oxidizing agent, to learn how much it decreases the pigment level in fish when combined with astaxanthin, which is an antioxidant.

Skretting Senior Researcher Gunvor Struksnæs led the studies, and found that “a certain number of fish will break down some of their astaxantin into idoxanthin [a metabolite of astaxanthin] when faced with a stressful event. Interestingly, some individuals are affected to a much larger degree than others, which now enables better understanding of the variation that we see in the pigmentation response between individual fish.”

These findings complement the research the Aquaculture Research Centre has been conducting with larger fish in the seawater stage – looking at the effects of hydrogen peroxide bathing, explained Nankervis.

“We have found a downturn of pigmentation following hydrogen peroxide bathing, but it is not as high as we initially thought it might have been. Additional simulations that haven’t included bathing but have lowered the water levels have also triggered the breakdown of astaxanthin to idoxanthin,” he said. “We have confirmed that stress, particularly crowding stress, is a major contributor to the transfer of astaxanthin to idoxanthin in salmon. This knowledge has given us another important avenue of further research as we look to establish a bigger picture understanding of the mechanisms that are controlling the degrading of pigmentation in larger fish in seawater systems,” says Nankervis.