Lice burden 'reduces number of one-winter salmon' returning in Norway
A study using a new statistical model in Norway shows that what is believed to be a high number of lice on migrating salmon smolts has a clear correlation with fewer spawning fish returning after one year – although the effect was the opposite on fish that had been in the sea for three years.
Researchers at AquaLife R&D, the Norwegian Computing Centre and the Norwegian Veterinary Institute developed the model to quantify the connection between modelled lice pressure based on lice counts in fish farms, and wild salmon returning to their natal rivers to spawn.
The study shows a significant correlation between high lice counts on migrating salmon smolts and a low number of fish returning as spawners after one year at sea, say the authors.
There was no clear effect on fish that had been in the sea for two years. The effect was the opposite on fish that had been in the sea for three years. One possible reason for the difference between year classes is that lice stress can delay growth and sexual maturation, so that a larger proportion only returns after three years.
'Reasonable to assume' a lice impact
Overall, the study suggests that the greater the sea lice burden, the fewer wild salmon return to the rivers to spawn, in addition to the fact that sea lice affect the age composition of spawning fish.
“We believe it is reasonable to assume that this connection is due to the fact that sea lice cause a significant burden on migrating smolts in the form of increased mortality and delayed spawning,” says Kari Olli Helgesen, a senior researcher at the Veterinary Institute who leads the project that the study is part of.
The model uses calculations from the Norwegian Veterinary Institute of how many sea lice researchers believe the wild salmon was exposed to the year it swam into the sea as a post-smolt, along with spawning fish data from spawning fish counts and reported catches from sport fishing.
Difficult to measure
Both the Norwegian government’s disputed Traffic Light System, which currently determines whether the aquaculture industry is allowed to grow or must contract, and the proposals in the government's aquaculture report on how future growth should be regulated, are based on the impact of sea lice on wild fish.
This is largely based on theory and modelling, as finding hard evidence that lice from fish farms is not easy.
One challenge is that it is difficult to measure how many wild salmon die from sea lice when they migrate to the sea as post-smolts. It is difficult to find the dead salmon smolts, and – even if they are found – it is difficult to say what they died from. The study’s authors say sea lice can be both the actual cause of death, but also partly responsible for the fish dying because they were weakened by the lice load. In addition to mortality, sea lice can lead to reduced growth and possibly also affect other biological functions, such as sexual maturation.
Fish counts and catches
The number of fish returning to rivers to spawn can be measured both as spawning fish counts and as catch counts from sport fishing. In the study funded by FHF, the researchers have combined these two data sources to investigate the extent to which lice burden in migrating smolts to the sea has affected the number and age composition of spawning fish in Norwegian rivers.
“It has long been known that fewer salmon return to spawn after only one year in the sea, and that the decline is greater for these than for salmon that have been in the sea for two or more years. We wanted to find out whether this change in the age composition of the spawning fish could be related to the sea lice load the fish experienced when they migrated to sea,” says Helgesen.
Data from a total of 104 rivers, contributing an average of almost four years of data each, were included. The researchers created a statistical model to quantify how the number of fish returning to spawn was related to the lice load they theoretically experienced as post-smolt. The size of the rivers was taken into account, by adjusting for theoretical smolt production, as well as an estimate of the number of emigrating smolts based on historical catch data.
Correlation between lice load and fish
They found a clear correlation (statistically significant) between a high number of lice on migrating salmon smolts and a low number of fish returning as spawners after one year at sea.
In fish that had been at sea for two years, the same effect was not evident (not significant).
In fish that had been at sea for three years, they found there was a clear correlation between high lice load during smolt migration and increased numbers of fish that returned as spawning fish. One possible reason for this difference in effect between year classes is that lice load can delay growth and sexual maturation. The number of years at sea is derived from the stated size of the spawning fish in the data base and the correlation between weight and number of sea winters is taken from the scientific literature.
“When we can quantify how salmon lice in farming affect how many spawning fish return, we can work more purposefully to make salmon farming sustainable,” says Helgesen.
Can't rule out other factors
Helgesen emphasises, however, that this is a study in which they have found a (statistical) negative correlation between lice from farming and returning spawning salmon and not an experimental study.
“The connection could in principle be due to other underlying factors, and we cannot therefore conclude with a single causal connection,” says the scientist. “We therefore encourage further critical analyses of the data, preferably with a focus on relevant alternative explanations for reduced return migration of spawning salmon to rivers in areas with high lice load from farming, and especially for small salmon.”
The datasets used in this study are available at Monitoring of spawning stocks of salmon and sea trout (NINA) and Salmon catches (SSB).
