Individual variation in distribution, activity and growth rate of Arctic charr kept in a three‐tank system

The movements and distribution of groups of Arctic charrSalvelinus alpinuswere examined in a rearing system that offered a choice between two different feeding tanks separated by a larger non‐feeding tank. The passages of individual fish were monitored continuously during a period of 3 weeks using the PIT (passive integrated transponder)‐tag technique. The primary aim was to examine if only some charr were occupying the feeding tanks, thereby excluding other individuals, and whether differences in visit activity explained within‐group variation in individual growth. On average, about 35 of the 40 charr in each group shoaled in the large non‐feeding tank leaving only five individuals in the feeding tanks. Charr that spent a long total time in one of the feeding tanks made frequent excursions to the other tanks resulting in a continuous exchange of individuals. Individual growth rates were correlated positively with visit activity rather than with the total time spent in the feeding tanks. Thus, individuals with low growth rate spent as much time in feeding tanks as charr with high growth rate. However, less successful fish tended to visit the feeding tanks at night when the feeders were switched off. Based on behavioural and growth results obtained in this experiment, the use of multitank systems in the cultivation of Arctic charr is discussed.