NatureScot Research Report 972 - Host salmonid specificity of selected pearl mussel populations

Sampling of larval pearl mussels, on the gills of juvenile salmonids, was completed in 2016. It took place in 16 watercourses in pearl mussel SACs on the west coast. The preferred host fish species (either salmon or trout) varied between rivers. The results of this research can have important implications for conservation management.

Confidential Reports 

Scottish Natural Heritage is committed to making the findings of all of its research publicly available whenever possible. However, some animals and plants are threatened by or vulnerable to human activities and therefore require protection. Making some of our research confidential is a way of protecting these species and may help their survival. To be transparent about our work and publications on protected species we have produced a short redacted version of the full confidential report. If you would like a copy of the full report please email [email protected].

Year of publication: 2018

Authors: Baum, D. 

Cite as: Baum, D. 2018. Host salmonid specificity of selected pearl mussel populations. Scottish Natural Heritage Research Report No. 972.

Keywords

Freshwater pearl mussel; glochidia; salmon; trout.

Background 

The freshwater pearl mussel, Margaritifera margaritifera, is a globally threatened species and some Scottish rivers support internationally important populations. Concerns have been raised about levels of recruitment and hence the long-term viability of freshwater pearl mussel populations in many rivers. Freshwater pearl mussels require juvenile salmonid fish to act as a host species during the larval phase of their lifecycle and this may be a factor limiting recruitment. This study looked at the host species preference and availability of hosts in 16 rivers on the west coast of Scotland, all of which are designated for the conservation of freshwater pearl mussels.

Main findings 

• Mussel glochidia were found on the gills of juvenile salmonids in 15 of the 16 watercourses surveyed during spring and autumn 2016. No glochidia were recorded in one river in West Sutherland, although this may be due to the location of the sampling sites on this river. 
• The preferred host species varied between rivers. In six watercourses no salmon were recorded and all glochidia were found on trout. However, both salmon and trout were found in the other 10 rivers. In five of these there was a clear preference for trout, in three others a preference for salmon and in the case of two rivers no obvious difference in infection rates. 
• Host species preferences were generally consistent within a watercourse across both the spring and autumn samples. 
• Salmon and trout fry were more likely to be infected than older age classes, although a total of 136 parr (22% of those caught) did carry glochidia. 
• 59% of the juvenile salmonids caught carried no glochidia, and infected fish generally had fewer than 60 glochidia. However, some high infection rates were observed, the highest being 550 recorded on two trout parr in different watercourses in Sutherland. 
• Juvenile salmonid densities were highly variable across the survey sites. The highest densities were found in a river in Wester Ross.

Document downloads