The formation of non‐anadromous populations of Atlantic salmon,Salmon salarL., in Europe

Abstract

Comparison of the different populations of non‐anadromous Atlantic salmon in Europe has revealed some common traits. The non‐anadromous populations probably became isolated from the anadromous salmon populations during a period of rapid land‐upheaval after the last ice age, about 10 000 years ago. The oceanic temperature in the eastern part of the Atlantic was especially low in this period. This probably diminished the advantage gained from the anadromous habit. During this period the low temperatures in the rivers probably increased the frequency of females which matured without migrating seawards. A large number of individuals in the different populations seems to have been favourable for the formation and survival of such races. The occurrence of a lake in such a river system would probably ease the transition to non‐anadromous behaviour, because the salmon would not then be forced to adjust its entire life‐history when isolated (the salmon could spawn in the river in the usual manner and the smolts then migrate downstream into lakes). Larger lakes would also seem to infer problems for the navigation of the smolts towards the outlet. The only known exception is the river‐living land‐locked salmon population found in the River Namsen, Norway.The factors mentioned above have led to an increase in the probability of occurrence and thriving of mature females without a sea migration or to a decrease in the competitive advantage of anadromous behaviour. The above mentioned similarities can explain the seemingly casual distribution of the non‐anadromous populations of Atlantic salmon in Europe.