A head start for salmon smolt pigmentation

21 December 2015

Skretting reveals pigmentation breakthrough

Atlantic salmon producers are now growing freshwater smolts to a larger size in order to achieve optimum growth, performance and health. Consequently, the fish are spending less time in seawater before harvest but are exhibiting decreased levels of pigmentation upon saltwater harvest.  

The salmonid pink colour is highly valued and originates from carotenoid pigments, mainly astaxanthin and canthaxanthin. Carotenoid pigments can be obtained naturally from plankton, algae, crustacean shells or produced synthetically. In some cases pigment is derived from bacteria or fermented phaffia yeast.

During the seawater stage salmon have been fed pigmented diets, however pigment is not standard in freshwater diets.

Historically it was not unusual for salmon farmers to grow 40 grams smolts in fresh water and ship them to salt water pens. Today it is not unusual to transfer smolts at 150 grams and even larger. Today’s smolts are larger and further along in their growth cycle.

“With the sea production period 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,” says Leo Nankervis, Team Leader Salmonid Nutrition at Skretting Aquaculture Research Centre (ARC).

To overcome this particular pigment challenge, Skretting, as the global leader in aquaculture feeds, has formulated freshwater-specific feeds containing carotenoids, enabling pigmentation to begin prior to transfer.

“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 pigment in the fillet by the time the fish have transferred,” says Nankervis.

(Findings from these trials are shown in the supplied Pigmentation figure.)

Modelling studies

 Skretting ARC’s freshwater trials, led by in-house researcher Guido Riesen, were conducted on salmon juveniles of approximately six months of age and were continued through the seawater phase of growth (see the supplied Experimental design figure). The fish were transferred to seawater sites at a size of 115g and given identical amounts of pigment when fish size was 230 grams.

Our findings indicate that diet formulations containing 70ppm pigment offered a freshwater model that is most aligned with the seawater model.

“The uptake utilisation of astaxanthin in freshwater was very similar to that in seawater. This was excellent news as it meant we could model for both stages,” says Nankervis.

Market benefits

Roar Sandvik, Global Product Manager for Freshwater & Transfer Feeds at Skretting, says the new freshwater feeds fulfil an increasingly important requirement in the production system for salmon. Together with the knowledge gained through the R&D process, the feeds can provide invaluable support for fish farmers wishing to establish a clear strategy for achieving optimal pigmentation.

“The pigment astaxanthin is an essential component of the diet of salmon; among other things, it influences the growth and health of the fish. 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,” says Sandvik. “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.”

Stress management

Skretting ARC’s pigment studies have acknowledged the equally important area of sea lice treatments and their effects on pigmentation.

Hydrogen peroxide has become widely used as a delousing agent. In Norway, its use as a bathing treatment against sea lice and other challenges has increased several-fold in recent years. However, with hydrogen peroxide being an oxidising agent and astaxanthin and canthaxanthin being antioxidants, Skretting ARC researchers felt it was important to learn to what degree the former decreases the pigment level in the flesh of the fish. These investigations were led by senior researcher Gunvor Struksnæs.

They found a certain number of fish will break down some of their carotenoid pigments into idoxanthin (a metabolite of astaxanthin) when faced with a stressful event. Interestingly, some individual fish 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 Skretting ARC has been conducting with larger fish in the seawater stage – looking at the effects of hydrogen peroxide bathing, explains 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 pigment to idoxanthin.”

“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.

 

Pigmentation figure

 

Experimental design figure