Recirc
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A recirc specialist makes the case for all-female production in RAS

Early maturation in net pen farmed salmon was identified as an economic liability decades ago, and can still be a problem today. The ability to improve salmon growing conditions using RAS has compounded this problem. Indeed, optimal culture conditions for salmonids in RAS is still not clear. That was the message from a few presenters during the Aquaculture Innovation Workshop (AIW) held in Vancouver, Canada last November. In particular, it is still unclear what environmental conditions will minimize early maturation in salmon grown to harvest size in RAS.

Steve Summerfelt from the Freshwater Institute in Virginia, USA, presented a study by his group comparing maturation rates of two strains of Atlantic salmon grown in RAS under continuous light for 24 months post-hatch.  In this trial the Gaspe strain grew faster, reaching 4 kg. However, a serious challenge for the RAS farmer became clear in that greater than 50% of the male St. John River strain fish matured early.  The Gaspe strain were all females and no females matured from either strain. Could it be that RAS is just for girls?

Precocious maturation

Early maturing Atlantic salmon are called grilse, while early maturing Pacific salmon are called jacks (boys) or jills (girls). By the time the mature ones can be identified, the quality can be slightly or severely compromised, depending on how good the farmer is at identifying them early. During the maturation process pigment comes out of the flesh and pigment goes into the skin; oil content in the flesh decreases as energy goes into gonad production.

When determining what percentage of the fish are maturing early, the farmer must decide whether or not to grade out these maturing fish. Additional labour is required to grade, and there is an additional handling of the fish with the associated stress and reduced feeding. If the farmer decides to leave the fish and not grade, the quality of the maturing fish will continue to decline and they will start to die off.

Why do males mature early?

Maturing a year or two earlier than the rest of their cohorts is one of the mechanisms used by salmon to preserve their genetic diversity. In the early 1990s I remember learning about Chinook salmon known as “sneakers” in the wild. These are a very small percentage of males which sometimes stay in freshwater instead of going to sea. They mature at 50 grams or so. When the sea run Chinook return to spawn and a big female starts releasing eggs, the little 50 g sneaker darts in to fertilize the eggs before the male that is 500 times bigger. Genius. For Atlantic salmon, some males mature a year or even two years earlier than their cohorts.

Arrested development

Though many factors likely play a role in determining the timing of maturation in salmonids, four significant factors are genetics, temperature, photoperiod, and sex. Longer term breeding programs can decrease the percentage of early maturing fish. Farmers growing salmon in RAS can work with their egg suppliers to incorporate that selection into their genetic breeding programs. When considering possible shorter term solutions, by controlling the other three factors (temperature, photoperiod, and sex), early maturation can be minimized or eliminated in RAS. Here are the three key options:

• Temperature - Lower temperature can help, and in RAS, any temperature can be set; however, the farmer must be aware of the risk of compromising growth by lowering temperature. At the Aquaculture Innovation Workshop Garry Ullstrom from Kuterra described their RAS for Atlantic salmon grow-out in British Columbia: Early production cohorts had maturation rates of more than 10%. They were able to decrease that rate down to 2.3% by changing three things: 1) decreasing the culture temperature from 15ºC to 13ºC for part of the production cycle; 2) decreasing water turbidity which may have allowed for better photoperiod control; and 3) restricting feeding.

• Photoperiod - Net pen salmon farmers use lighting over the winter to reduce maturation. For RAS grow-out, studies still need to be done to optimize photoperiod. At the workshop, Colin Brauner from the University of British Columbia presented results on the effects of photoperiod and salinity on maturation in both Atlantic salmon and coho salmon grown in RAS. His group found that after growing Atlantic salmon in RAS for 10 months from smolt, maturation was 25 - 30% using 24 hour lighting. They were able to significantly reduce that by using a 12:12 photoperiod. Salinity (2.5, 5, 10, 30 ppt) did not impact maturation. With the same culture conditions, coho maturation was 1 - 1.7% using 24 hour lights and zero under a 12:12 photoperiod. Note that the coho were all females and feeding was restricted in both trials.

• Sex control -Trials producing monosex female salmonid populations were first carried out in the 1970s using several species including rainbow trout, chinook, coho, and Atlantic salmon. Female populations may solve the early maturation problem for Atlantic salmon grown out in RAS, though owing to limited availability of monosex female stock, this solution is yet to be realized.

For coho, all-females are already being cultured in RAS with no early maturation. . . or maybe it’s all early maturation: With all-female coho using optimal temperature and photoperiod, the entire population can be pushed to reach harvest size and subsequently mature just 18 - 20 months from first feeding. Essentially, all of the females are triggered to mature a year early (two year life cycle instead of three). Coho have a strong correlation between growth and maturation, where the fish that trigger to mature grow significantly faster than those that will mature a year later. In this case, it can be fruitful for the farmer to delay harvest until the meat and roe together are at peak value.

My recommendation for salmon grow-out in RAS would be to produce only monosex female populations and use a temperature and photoperiod profile that will allow you to control when the fish mature. I suppose that I could have added “reduce feeding” as another factor that controls maturation, but I hope that the industry can fine-tune the other facts and avoid that one. One thing that is clear is that much more research needs to be carried out before we understand optimal culture conditions for salmonids in RAS.

At Northern Divine Aquafarms, we first used RAS to grow salmon smolts in1998. This was before the term “RAS” was coined and we just called it “recirc”. We soon realized that this technology allowed us to produce smolts six months earlier and three times as big when compared to our flow-through system. I remember thinking “why would you grow fish in anything else?” Lots of people had the same idea which led some to start growing fish right through to harvest in RAS.

Early days, early challenges

It was 17 years ago that we started to harvest our first RAS-grown sturgeon for meat. Today, the sturgeon, caviar and coho salmon grown at Northern Divine have the cleanest taste possible at harvest; however, that is not how it started 17 years ago.

Farmers growing fish such as catfish in ponds have been battling this problem for years, though many others who grow fish in net pens or flow-through systems have, for the most part, not historically encountered this challenge. When RAS came along there were initially no problems with off-flavour since most of the production was for smolts which no one was eating.

As farmers realized the benefits of growing fish in RAS, they started to experiment with growing several species to market size. Unfortunately, in many instances, the market knew there was a problem before the farmer.

One of our first harvests of sturgeon for meat went to a banquet at an aquaculture conference 15 years ago. Various types of seafood were served at different stations throughout the venue. The first sturgeon that came out was delicious; this was the first time that many people had ever tried sturgeon. An hour or so later, they refreshed the platter with meat from the next fish they cut. And oh oh. This one had a slight muddy off-flavour. I didn’t know why it tasted that way, but we quickly realized that we needed to find a fix.

Nature of the problem?

A muddy off-flavour in fish is bad for business, and fish coming out of many RAS have off-flavour. The amount of off-flavour will vary with the system, water quality parameters, species and size of fish, and even individual variations. Wild fish from certain lakes and rivers also have this off-flavour. Consumers don’t want to eat fish with that flavour and they won’t likely buy it a second time. An even bigger problem is that they may stop buying that genre of fish altogether.

What is the cause?

Compounds causing off-flavour in the water also get into the fish. The main compounds are the naturally produced organic chemicals geosmin and 2-methylisoborneol (MIB). These compounds are produced by certain bacteria (cyanobacteria and actinomycete). If you want to read all of the gory details, check out the excellent summary by Davidson et al in Aquaculture Engineering 61 (2014) 27-34 or other published papers from the Freshwater Institute.

By design, recirculating aquaculture systems grow plenty of bacteria. The systems need nitrifying bacteria; thus, biofilters are designed to optimize surface area and living conditions for bacteria. The bacteria which produce geosmin and MIB also live there and the off-flavour causing compounds enter the fish via the gills and then accumulate in the fat. The fattier the fish, the worse it will be. The more geosmin and MIB in your system, the worse it will be.

Solving the problem


There is only one solution and it is very simple - depurate, purge. (But let’s not call it that anymore for the sake of our customers). Let’s call it “conditioning” or “finishing” which sounds a little more delicious and safe. Whatever you call it, it requires taking your fish out of the RAS and putting them either on flow-through water or onto another water source that does not contain off-flavour compounds.

Many methods don’t work. We can’t just kill all the bacteria in a RAS because, as we discussed, we actually need some of those bacteria. There has been research on ozonating the culture water which has not been successful. Nam-Koong et al Aquacultural Engineering 70 (2016) 73–80 demonstrated that treating the water with ultrasound will remove off-flavour compounds from RAS, but the energy required would not be economical.

For now, farmers have to transfer the fish into a “finishing” system. But for how long? That depends on many factors including the level of geosmin and MIB in the culture water, the species, size, and lipid content of the fish, the water temperature, etc. You can read reports that suggest anything from less than one day up to three weeks of conditioning. To finish caviar takes longer . . . way longer. When I attend trade shows where caviar companies have their booths I go from one booth to the next sampling their caviar. “How does it taste?” they ask. “Like mud,” . . . I think. The next booth, “like dirt”. The next booth, “like earth”. Wow. How can you grow a fish for so long and then harvest it with an off-flavour?

Basically, producers have to condition their fish until they can’t taste off-flavours anymore and that will be different at every farm, varying with every variable. And there will also be variability from fish to fish, so don’t limit taste tests to just one fish. Terry Brooks, of Golden Eagle Aquaculture in British Columbia, Canada has the process down to a science. He conditions the fish in two stages where he first reduces the levels of geosmin and MIB in the RAS for a period of time, and then moves them to a new flow-through tank for a very short period of time. By tasting his RAS-grown coho salmon, you could never tell if they were from the wild, from a saltwater net pen, or from his RAS farm in Canada.

“If people get a hint of off-flavour, you’re done,” he says.

So, why is there still a problem?

Some farmers still send off-flavoured fish to market. …Please stop. It’s bad for business. For all of us.

But why does this still happen? There are a number of reasons: some newcomers to the industry have not read the literature; sometimes it takes longer to get rid of off-flavours than the literature indicates; or some farmers have just received bad advice.

How do you know that off-flavour is gone? There is one way and it is very easy: taste it. Taste a few fish. Taste them often. Get good at identifying even the tiniest hint of off-flavour. And then sometimes you have to make the difficult choice to delay harvest. I know that sucks, but ignoring it is probably the main reason that off-flavoured fish still ends up in the market. Delaying means delayed cash flow, change of processing schedule, maybe sending the harvesters or processors home, and telling your customer that they are not getting any fish that day.

If you send the fish anyway, before they have clean flavour, then you are killing your business, and harming the rest of the RAS farmers.

Justin Henry is the former general manager of Northern Divine Aquafarms, a sturgeon and salmon RAS in British Columbia, Canada. He now heads up Henry Aquaculture Consult Inc, an international advisory and consulting service for the aquaculture industry.   He can be reached at: This e-mail address is being protected from spambots. You need JavaScript enabled to view it

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