Effects of (Margaritifera margaritifera) glochidial infection on performance of tank-reared Atlantic salmon (Salmo salar)

Experimental infection of pearl mussel glochidia on the performance of Atlantic salmon hosts was examined. Infection intensity was 1392±641 SD glochidia per fish six weeks after challenge but declined significantly 15 weeks after infection to means of 50 and 112 fish−1 in two trial tanks and, by 6 months numbers were <1 fish−1. This loss was attributed to fungal treatments with a combination of malachite green which was still the authorised treatment during the study in 2001 and formalin. The weight of infected fish was significantly lower than controls at 15 weeks but this was not significant after 1 year. The condition factor of infected and naïve fish was not significantly different. Lactate was measured as a possible indicator of stress in infected fish but there was no significant difference with controls. Fish that had been previously infected with glochidia were re-infected in their second year and comparison made with infection of naïve fish to determine whether glochidial infection elicits an immune response. Although there was no significant difference in glochidial numbers in both groups at around 9000 glochidia host−1 3 weeks after infection, numbers of glochidia in naïve fish did not change to 15 weeks after challenge, whereas there was a significant reduction in previously infected fish to 116 fish−1. Various treatments were used to provoke closure of the glochidial valves. It is concluded that infection of salmon with glochidia levels in the current study had no significant effect on salmon performance, condition and stress as measured by assay of lactate.