Outbreaks of salmon louse during smolt migration reduce the survival rate of the smolt and mean that salmon spend longer at sea before returning to spawn.
This has been shown in a comprehensive study carried out in collaboration between researchers at Uni Research and the Norwegian Institute of Marine Research.
The results, which were recently published in the journal Biology Letters, are based on results from 10 years of field studies observing almost 380,000 smolt (migrating young salmon) from the Vosso and Daleelva rivers in Nordhordland.
Treated for salmon louse
Half of the group were treated for salmon louse in order to protect them against lice on their way out to sea. Survival, age and growth of the salmon returning ready to spawn were subsequently calculated for the two groups.
The researchers found that outbreaks of salmon louse result in a rise in the age of the salmon at the time of return to both rivers.
The results are interpreted as either a postponement of sexual maturation in the salmon as a result of lice, or that lice lead to a particularly high rate of mortality among those salmon that would normally return after one year at sea, so-called young salmon.
The research discovered that salmon louse resulted in an average 32 per cent increase in mortality for Vosso smolt, while there was no increase in mortality for the salmon in Daleelva.
On the other hand, there was a large variation between years in both populations, where there was no mortality due to lice in some years (2009) and yet in other years there was a 70 per cent increase in mortality due to lice (2003 and 2007).
The mortality rate among migrating smolt as a result of salmon louse corresponds to previous findings both abroad and in Norway, including over a longer time period in the Daleelva.
The new study shows that salmon louse can also affect the age demographic of the spawning population.
"The results show the need to discover effective measures to protect the wild salmon population against salmon louse. The large quantity of data material, which shows a wide variation between years, provides the basis for assessing and streamlining measures that farmers have implemented against lice," says research leader Bjorn Barlaup at Uni Research.
Every year, the farmers spend a great deal of resources on preventing salmon louse harming the wild salmon smolt. In this context, the years from the data set showing little or no effect from lice are incredibly interesting when deciding which combinations of measures and environmental factors result in keeping down the numbers of salmon louse and preventing these from affecting the salmon smolt. Research is highly relevant to the ongoing rescue action for Vosso salmon.
"As far back as the turn of the century we saw that salmon groups with the best survival rate were those that were given feed to protect them from salmon louse and were also towed to the coast in a specially-built tank. This knowledge was also an important factor in the rescue work," says Barlaup. Increase in spawning population
Contributions from the farmers organised under the Vosso Union together with municipalities and county councils ensured that approx. 500,000 smolt were towed according to this plan in the five-year period between 2009 and 2013.
"This has led to a significant increase in the spawning population. An increase in the number of natural smolt migrating from the water system is therefore expected this spring. We are hoping to use the large database we have built up, together with the local administration and farmers, to further develop effective measures against lice," says Barlaup.
Researchers in Vosso have primarily received financing from the Norwegian Environmental Agency through a dedicated Vosso project which was started in 2000 with the aim of re-establishing the world renowned big salmon stock in this water system, while the studies in Daleelva received financing from the Institute of Marine Research.
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