Newly-emerged Salmo trutta Fry that Migrate to the Sea - An Alternative Choice of Feeding Habitat?

摘要

Traditionally, it has been assumed that fry of the anadromous brown trout, Salmo trutta, which migrate seawards when newly-emerged from the gravel, are the losers in an intense intra-specific competition. They are supposed to stop feeding and drift downstream and eventually die. From a field study of a small Baltic Sea population of 'sea trout' it was found that the total body length and physical condition, in terms of body-length-adjusted body-weight, body-length-adjusted mean-otolith-length and mean-otolith-length-adjusted fluctuating asymmetry of the otoliths of newly-emerged fry that migrated seawards were no different from the values for stream-dwelling fry. Although food items found in the stomachs of the two samples of fry were somewhat different, they had eaten the same amounts of food. These findings provide support for the hypothesis that the seaward migration of the fry is an adaptation to to low summer water levels, which regularly occur in many of the small streams entering the Baltic Sea.