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In the first part of the article we summarise some of the experience we have had with smolt production in 2016 and look more closely at some challenges which may arise during smolt production.


Experience in the field

Production of yearlings

In the case of flow-through hatcheries, spring smoltification is marked by low temperatures in the period with light control, but at the same time one has more time to vaccinate and sort without disturbing the sensitive smoltification process. Smoltification in the spring usually takes place using natural light from the New Year or a six-week 12:12 regime followed by 5-6 weeks of 24:0 before the planned release date. It is important to give a clear night signal during the 12:12 regime. Unclear signals or changes under way have been shown to result in erratic smoltification. This is what normal smoltification of yearlings looks like in our ATPase database:

 ATPase smoltifisering 1-åring

The fish population starts with an ATPase value of 9.0 (approx. 290 degree days after the 24:0 light regime), rising to 16.5 and finally to an average of 26.3. At the same time we see that the variation increases, something which is not uncommon in the smoltification period.

Large smolt
20% of all fish we received for ATPase analysis in 2016 weighed more than 150 grams. This is an increase from 2015 when the proportion was 10%. At the same time we see that the average weight of all fish received was 110 grams in 2016 and 96 grams in 2015. Clearly, larger and larger smolt are being produced!

Large smolt may exhibit the same known smoltification profile as smaller smolt. The figure below shows what is probably freshwater ATPase in January, before smoltification commences in February and an average value of 12.5 is reached in March. The average weight of the fish population is approximately 120 grams in January and approximately 200 grams in March. The graph also shows signs of desmoltification at the end of March. Depending on the planned release date we would normally in such cases recommend stimulating the fish using seawater if possible. We have also seen that salt feed can have an effect on smoltification.

 Stor smolt ATPase

Large yearlings
What about populations of subyearlings which are left over the winter to become large yearlings? This is something we see increasingly often, and in this example we see that the fish population has clearly smoltified some time before test sampling commenced. This group is being fed a commercial salt feed and this, in combination with the relatively large size (300-400 grams) of the smolt, appears to be keeping the population within the smolt window.

 Smoltifisering stor smolt ATPase

Disease issues

Things don’t always go according to plan. This fish population should have smoltified in May according to the plan, but instead the ATPase value has stagnated around 5-6 for a period of 8 weeks. Morphologically speaking the fish developed normally, with increasing smolt index and falling K factor, but the seawater tolerance did not increase. Conferring with the client we agreed to examine for various gill agents and it became evident that the smolt had been infected with the pox virus. This is typical if pox virus is present during smoltification.


We see the same when Costia is present during the smoltification process. The graph below shows abnormally slow smoltification. According to the plan this fish population should have smoltifed in early August but smoltification is impeded and the entire process is considerably protracted. Costia was discovered in this tank shortly after we began documenting smolt status and it was evident that both Costia and the formalin treatment has a negative effect on smoltification.


Environmental issues
This is an example of how much the tank environment affects the smoltification process. The fish population here were smolt in the month of May, but the ATPase values dropped dramatically in June. Investigations by the client revealed an unnaturally high CO2 level in the tank, and when the client increased the supply of seawater the fish immediately showed a positive response. In this case the CO2 level was impeding ATPase activity, but since the enzyme apparatus itself was present the population managed to handle the addition of seawater well.


General production issues
Things don’t always go according to plan. This 1.5-year old population was given commercial salt feed to stimulate and maintain smoltification. The population smoltified far too early and showed a drop in appetite which led to immediate desmoltification followed by a slow resmoltification with considerable individual variation in the tank.


SmoltVision – our new tool
In 2016 we carried out comprehensive testing of our new SmoltVision analysis on both yearlings and subyearlings. In difficult cases, as shown above, SmoltVision itself is a very effective tool for the interpretation of smolt status and for informing our advice. SmoltVision enables us to ascertain what ATPase activity is dominant in the gills of the smolt: freshwater pumps or seawater pumps. This has a considerable effect on seawater tolerance.



In the second part of the article we look more closely at what SmoltVision is and how the analysis functions. We will also discuss the most important parameters to keep under control during smoltification and how analysis data will probably play an increasing role in future smolt production.

So what exactly is SmoltVision?

SmoltVision is a real-time RT-PCR analysis. This means that we use the same method as when we screen for different agents in test samples you send in. The analysis measures the gene expression of various markers in gill tissues, which are directly associated with the fish’s chloride cells. We also measure the expression of genes which are associated with smoltification. In the course of smoltification these genes will change, depending on what stage the fish has reached.

The genetic smoltification expression provides us with a picture of what type of ATPase activity is about to dominate in the gills. We can now clearly distinguish between the expressions of the freshwater and seawater components. In our analysis we use THREE genetic markers. The combined profile of these three constituents will provide information which reveals the seawater tolerance of the smolt.

The following figure shows a smolt in pure seawater. It has a low expression of freshwater component (blue), a high expression of seawater component (yellow) and a moderate expression of cofactor (red).


How does SmoltVision differ from other smolt analyses on the market?

The principal difference between SmoltVision and similar analysis methods is that SmoltVision combines data from several genetic markers on completion of the analysis, thereby providing a more complete and refined picture of the fish’s smolt status. Our analysis measures the expression of the freshwater component, seawater component and a cofactor of seawater component.

The cofactor is only found in a type of chloride cell which is exclusively associated with seawater tolerance. An increase in this marker is therefore a strong indication that the fish population is increasing its seawater tolerance. The combination of reduction in freshwater component and increase in seawater component provides us with an incredible amount of data in a single analysis.

How many samples should be taken per population for SmoltVision analysis?

Based on our data we see that there is considerable individual variation in the time when a fish population commences smoltification. To be able to detect this natural variation and to obtain the most accurate result and advice possible, taking a sample of 20 fish from each population for testing is recommended.

The smoltification of yearlings may look like this. Notice the individual spread. Each single point is one individual. It is highly probable that five to ten fish will provide an incorrect picture of the status while smoltification is in progress. Notice also the tendency towards desmoltification in the final sample.



Which parameters is it important to keep under control during smoltification?

Our analysis result will be accompanied by a recommendation, if necessary both in writing and in the form of a conference with our advisors. All our experience shows that there are some key factors we must have under control in order to have a good technical dialogue with our clients. These include temperature, salinity, light control, size and smolt index. We will therefore ask you to provide these data when you send in test samples. We see that clients experiment with temperature, salinity and light control to increase the growth rate of smolt. This is not always ideal, and often has a negative effect on smoltification.

How are the results of the SmoltVision analysis interpreted and presented?

The results of SmoltVision will be presented in various ways. Among other things we look at the relative gene expression, both on average and in single individuals. We can, for example, now clearly see individuals that have passed the top of the smolt window and have commenced desmoltification.

By using the relative gene expression it is possible in practice to compare values according to time, fish populations and hatcheries and this means that a high value is a real indicator of a high gene expression. Typically we see low expression in "maintenance situations" and high expression during smoltification or in connection with other incidents which affect the physiology of the fish.

By testing several samples one can track development over time and therefore draw conclusions about the smoltification process. In an optimal smoltification process, results from SmoltVision will show high relative levels of freshwater components in early samples, followed by a drop in relative levels as the fish population commences smoltification. In parallel with this drop, the amount of seawater component will increase and eventually become the dominant component. The level of cofactor, which is specifically connected with chloride cells which are only responsible for seawater adaptation, will also increase. This means that an increase in this expression is a good indicator of the fish developing seawater tolerance. All this information is summarised in various graphs which give a visual indication of smolt status. Interpretation of data and recommendations are an important part of the product!

The following figure shows an impressive smoltification – of smolt weighing about 17 grams! It is probable that the salmon undergo at least two, maybe three, smoltifications before being released to the sea. This is interesting!



Analysis data from SmoltVision will be an important element of future smolt production

We have not yet reached a point where we can predict the smoltification process very far in advance, but we believe that by building up our database for yearlings and subyearlings at a given hatchery over time, we will be able to create a standard curve for a normal smoltification progression for the hatchery using the same production parameters (temperature, light, feed, salinity, vaccination time, moving, etc.). Once this database is established, very early indication of abnormal smoltification will be possible, and if necessary previously determined release plans can be modified.

We can also make general comparisons with hatcheries using similar production methods and draw conclusions from this. The overall outcome of this will be that SmoltVision will be incorporated into the large volumes of data which our clients will eventually be able to make use of.

Who can use SmoltVision?

SmoltVision is recommended for anybody who wants more objective information about the smoltification process than is available at present. It is recommended when one needs systematically to acquire good data from smoltification and to ensure that the smolt released to the sea are of the highest possible quality.

It is a useful tool for confirming that one is doing things right and is particularly useful for quicker identification of smoltification which may deviate from normal. Such deviations may arise, for example, in the presence of gill parasites, in the event of metal precipitation from the water or if CO2 concentrations are high. Another advantage of SmoltVision is that if follow-up analyses of gill pathogens are needed, we will already have the analysis material available and will save valuable time in a critical period of the fish’s production cycle.

This client was experiencing abnormally slow smoltification with low ATPase values. SmoltVision showed only an expression of the freshwater component and that something must have been impeding smoltification. We carried out an analysis for gill agents and identified Costia. Remedial measures could therefore be implemented and smoltification eventually commenced.



What are the advantages of SmoltVision?

How can SmoltVision be used in practice?

Below we have outlined an example of testing by a hatchery operator producing yearlings in the spring of 2017. This operator intends to monitor two populations throughout the spring, one of large fish and the other of small fish.

Planned delivery is in the middle of May 2017. A test plan has been prepared consisting of three test samples of 20 fish using SmoltVision, and this will serve as documentation of the smoltification status of the fish, both internally and for any customers.

The baseline sample is taken when the 24:0 light is switched on. In production situations without light control baseline sampling in spring should be carried out while there is little or no indication of smoltification in the tank. The next sample will be taken halfway to the release date and the last one about one week before the expected release date.

This hatchery operator will then have carried out a total of six test samples, each of 20 fish, using SmoltVision as a smoltification tool, divided between two size groups.

If indications should arise of deviations from the smoltification progression, it is no problem to take additional test samples in periods when more frequent monitoring is needed. Our advisors will be able to assist you in making such decisions.

SmoltVision analysis at PHARMAQ Analytiq

SmoltVision is now available to our clients and we invite you to contact us for more information regarding prices or if you have any other queries.


Feel free to contact our customer service department or our smoltification advisor:
Elise Hjelle by e-mail at or by telephone, +47 916 33 136



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