Progress of co-infections of Trichodina cooperi and Trichodin murmanica parasitising farmed Atlantic cod Gadus morhua juveniles in Iceland

Two year classes of wild cod juveniles, caught for on-rearing in shore-based tanks, were examined for Trichodina spp. infections at regular intervals for 9 mo. The prevalence, density and proportion of each species found was determined. Additionally, biomass, seawater temperature and salinity in the tanks was recorded regularly. Two species were identified: T cooperi and T murmanica. Their prevalence, density and proportion varied between sampling times, T cooperi being the dominant species, more prevalent and in higher density in the beginning, In the following months, T murmanica gradually increased its prevalence, density and proportion, reaching an almost total dominance in both year classes after 7 mo of rearing. The results indicate that the rearing environment on this farm favoured T murmanica. Changes in salinity could explain this to some extent; a drop in salinity, due to an inflow of geothermal freshwater to control the temperature during the coldest months, coincided fairly well with an increasing proportion of T murmanica. Transfer of wild fish into the rearing environment, with a subsequent increase in biomass, resulted in a proliferation of trichodinids. This may have altered the interaction between species and the competition for resources. It is conceivable that a higher host density favours one Trichodina species over the other. Temperature could also play a role. During the first 3 to 4 mo of rearing, the temperature in the tanks followed the temperature of the open sea, which dropped by 3 to 5 degrees C. This study is a part of a larger project on monitoring the progress of various infectious diseases in farmed cod of wild origin.