When everything becomes salt feed

Written by Richard Torrissen Written by Richard Torrissen

Product manager for SuperSmolt at STIM,

Salt feed does not smoltify fish.

The problem arises when HI consistently refers to the so-called TrueSmolt project (Striberny et al.) as relevant documentation in this context. The feed used in TrueSmolt was not a traditional salt feed, but an experimental feed developed as a copy of SuperSmolt FeedOnly.

The HI researchers themselves describe Striberny et al. as a trial with “similar salt concentration” and “similar experimental design”.

But how similar were the treatments and trials really?

The feed used in the TrueSmolt project contained NaCl, CaCl₂, MgCl₂ and free L-tryptophan. The feed in the HI trial contained only NaCl and CaCl₂.

This is a significant difference, as smoltification with SuperSmolt is based on the interaction between all four of these ingredients. The fact that both feeds contain NaCl and CaCl₂ does not mean that they affect the fish in the same way.

This is also interesting when looking at fish behaviour. The HI researchers did not record migratory behaviour in the fish that received the salt feed. This differs from what we see when using SuperSmolt FeedOnly.

Migratory behaviour, where the fish begin to swim with the current, is one of several smolt indicators we have used for nearly 20 years when following commercial groups of fish through the smoltification process. This is a clear behavioural change that the fish farmers themselves observe in the tanks.

The fact that this behaviour was not recorded in fish given HI's salt feed is interesting. It points to a biological difference between the response to HI's salt feed and what we observe when using SuperSmolt FeedOnly.

There is also a significant difference in experimental design. The HI trial had no seawater phase, whereas TrueSmolt followed the fish for two months after transfer to full-strength seawater. The title of Striberny's article, «More than one way to smoltify a salmon?», says much about what the researchers actually wanted to investigate.

In the TrueSmolt project, researchers at the University of Tromsø compared, among other things, fish subjected to traditional light control with fish kept under continuous light and given an experimental feed developed as a copy of SuperSmolt FeedOnly.

The fish given the SuperSmolt copy performed well after sea transfer and at the same level as the traditionally light-controlled fish – but they were larger.

This is a key value of SuperSmolt: smoltifying fish without the need for a traditional short-day period while significantly reducing production time.

The researchers at UiT nevertheless concluded that the fish given the SuperSmolt copy could not be called «true smolt», partly because they did not show the classic decline in condition factor seen in the light-controlled fish.

From a production perspective, the significance of this definition is more academic, given that the fish performed well after transfer to sea.

If fish farmers are to make informed choices about their smoltification strategy, academia must be precise about what its experiments actually document.

In research terms: documentation of one specific biological intervention cannot be used to document another biological intervention. The same applies to experimental design. At the very least, the differences must be described and an explanation provided as to why the results are considered transferable.

We found neither the differences nor the explanation in HI's article.

Read the documentation:

• Striberny et al. - «More than one way to smoltify a salmon?»
  Compares SuperSmolt-based smoltification with traditional light control.

• The TrueSmolt project
  The research project on which the Striberny study is based.

• Mechlaoui et al. - commercial production through to harvest
  Follows SuperSmolt FeedOnly-treated fish through 19 months at sea until harvest.