DAO 71:213-223 (2006)  -  doi:10.3354/dao071213

ABSTRACT: 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°C. This study is a part of a larger project on monitoring the progress of various infectious diseases in farmed cod of wild origin.

KEY WORDS: Atlantic cod · Trichodina cooperi · Trichodina murmanica · Morphology · Co-infection