A Salmon farm on the Scottish west coast

One pathogen strain seems the main culprit in damaging salmonid disease

A distinct strain of the main fungus-like pathogen which causes saprolegniosis — thought to be responsible for around 10% of economic losses in the salmonid industry — looks likely to be responsible for the majority of outbreaks of the disease on Atlantic salmon fish farms.

A team of researchers carried out sampling work across 14 aquaculture sites in Scotland to paint a clearer picture of the diversity of oomycetes and fungal organisms present. They sampled during disease outbreaks with a high number of cases as well as outside of these periods, and from the skin of randomly selected fish along with the water where they were farmed.

They found that Saprolegnia was dominant across the study, accounting for 66% of all oomycetes. S. parasitica was the most prevalent species, supporting the results of other studies in salmonid production. This was especially pronounced in the samples from fish.

“The variation in Saprolegnia species abundance between water and fish suggest adaptive strategies to increase the probability of attachment to target hosts,” the researchers wrote in Journal of Fungi.

One strain has key impact

Within S. parasitica, one strain stood out. The subtype known as phylotype S2 accounted for 93.5% of the samples from fish, 91.1% of samples during widespread disease outbreaks, and 62.7% of Saprolegnia overall. Of the other five phylotypes identified in the study, types S5 (6.7%) and S3 (2.2%) were the only others isolated during the significant outbreaks of disease, while S1 was not seen at all.

The results suggest that S. parasitica phylotype S2 is the primary cause of Atlantic salmon saprolegniosis, the researchers said, noting that there have been similar observations in Chile and Canada.

The findings of the study have possible repercussions for disease surveillance on fish farms. Due to the prevalence of the subtype in samples taken from fish, “water sampling and spore counts alone may be insufficient to predict Saprolegnia outbreaks in freshwater aquaculture,” they stressed.

Species with lesser roles spotted

The scientists isolated other species of Saprolegnia over the course of the study, but the majority of S. australis, S. delica and S. ferax were found at just two recirculating facilities, where water treatments may have favored certain species to survive at the expense of others.

They also observed further oomycetes in the sampling efforts, including from the Achlya, Leptolegnia, Phytophthora and Pythium genera, although they concluded that the disease risk from these was limited due to either their limited presence or lack of known tendency to infect either Atlantic salmon or fish more generally.



Read the full research report in Journal of Fungi.