Smolt mortality in the period after transfer to sea has in several reports been connected to smolt quality1,2,3. It is also shown that smolt has a poorer immune system up to 3 months after transfer to sea and that it is more receptive to infections in this period4,5.
In a field study conducted in collaboration with a customer we mapped the infection status of the smolt groups before transfer to sea (Transfer Control) and the consequences after the smolt had been transferred to sea. We sampled 60 dead fish/moribund per smolt group on every freshwater site the last two months before transfer to sea. At sea we sampled 10 dead fish/moribund monthly from each smolt group up to 4 months after transfer to sea.
The Transfer Control showed difference in infection status between the different freshwater sites. One of the seawater sites that we followed in the study received smolt from freshwater site A and B. Site A was infection free, but at site B IPNV and POX was detected. The results show that the IPN virus that smolt group B brought with it from the freshwater site transmitted to smolt group A after 2 months in sea.
Figure: IPNV infection vertically from the freshwater site to the seawater site and then horizontally between smolt groups in the seawater site.
Results from two other seawater sites also showed transmission of infection between smolt groups of PRV, Branchiomonas and probably also Pox. Sequencing and phylogeny of IPNV and PRV isolates from the study confirms this. There were also differences in accumulated mortality between smolt groups with much and little infection from the freshwater phase.
By using Transfer Control you can implement measures to reduce infection at the freshwater site, you can control the composition of the smolt groups at the seawater site and you can make transfers to sea that are taking environmental considerations into account.
Contact PatoGen to discuss how you can use Tranfer Control to improve the fish health and performance of your fish.
References: 1. Project «Overlevelse fisk», 2011 – The Norwegian Food Safety Authority 2. Project «Tap av laksefisk i sjø», 2014 – The Norwegian Food Safety Authority 3. The Fish Health Report, 2018 – The Norwegian Veterinary Institute 4. «Laksens immunforsvar svekkes etter utsett», Christian Karlsen, Nofima, Intrafish 23.03.2018 5. Atlantic salmon post-smolts challenged two or nine weeks after seawater-transfer show difference in their susceptibility to salmonid alphavirus subtype 3 (SAV3), Jarungsriapisit J. et al, 2016, Virology Journal