Saprolegnia PhD

Summary

In Scottish rivers many Saprolegnia species can be detected including Saprolegnia parasitica, Saprolegnia diclina, Saprolegnia delica and Saprolegnia ferax. These species are endemic to all fresh water habitats around the world and are known to be partly responsible for declines in natural populations of salmonids and other fresh water fish. In general, we now know that S. parasitica is most pathogenic on fish, in particular salmon, whereas S. diclina and S. ferax prefer to infect eggs, although all species have been found on all freshwater stages of salmon.  

Previously S. parasitica was thought to be a secondary pathogen, with little economic or environmental importance. However, this is far from the truth and it is now well established that it often acts as a primary pathogen. In fact, Saprolegnia is capable of actively suppressing host immunity and produces nucleases and proteases that are translocated into fish cells where they degrade RNA and proteins and undermine defence responses in the fish.  

At least six taxonomically distinct strains of S. parasitica have now been found. Most interestingly during outbreaks of saprolegniosis we tend to find only one strain type – “phylotype-2” isolates. The phylotype-2 isolates that are associated with outbreaks are very pathogenic and kill 100% of Atlantic salmon in experimental challenge studies, whereas the other five strain types are much less pathogenic. 

To date, there have been no detailed investigations to determine which phylotypes are found in our rivers. Many fish sampled by the FHI on several Scottish rivers all mentioned Saprolegnia present but they have not, thus far, considered it a primary pathogen. The phylotypes present in these cases were therefore never assessed.  

We seem to have much more aggressive isolates present in our rivers then about ten years ago and in some rivers we can see mortalities of 80-90% of returning Atlantic salmon due primarily to saprolegniosis. This is disastrous for the fisheries impacted and if this rate of mortality continues it would be a matter of deep concern. 

Members of Fisheries Management are therefore funding a PhD studentship at the University of Aberdeen to address the following main objective: what type of Saprolegnia strains (phylotypes) are found in Scottish rivers that are suffering mass mortalities and in rivers that do not have any major issues? 

Given the potential impact of these pathogenic isolates, an increased focus on biosecurity, for all water users should be a priority. Disinfection of gear before and after use should be a key focus. It is well understood that prevention is better than cure, and therefore a precautionary approach should be adopted to avoid the risk of spread. Ultimately, we aim to develop recommendations based on the results collected through the PhD to manage our rivers to reduce the impact of saprolegniosis in collaboration with District Salmon Fishery Boards and Rivers and Fisheries Trusts. 

Funding and Partners

This project is funded by the following members of FMS: 

Argyll FT 

Ayrshire Rivers Trust 

Caithness DSFB 

Dee DSFB 

Esk DSFB 

Findorn DSFB 

Helmsdale DSFB 

Kyle of Sutherland Trust 

Ness DSFB 

Nith DSFB 

Northern DSFB 

River Tweed Commission 

Spey Fishery Board 

Tay DSFB 

Urr DSFB 

West Sutherland FT 

Wester Ross DSFB 

Western Isles DSFB 

We are also grateful to the Scottish Government for grant support towards this important project.