The Origin of the Landlocked Habit in Salmon

Abstract

Some river systems contain salmon that never run to the sea. Though regarded as distinct these landlocked or dwarf salmon occur usually in elevated lakes and resemble closely sea-run species. In Norway, Dahl found landlocked varieties of the Atlantic salmon (Salmo salar) ; others exist in Maine, Quebec, and Labrador, and one is reported from New Zealand. On the Pacific Coast landlocked salmon (Oncorhynchus) occur naturally in lakes from Alaska to Oregon and have been planted in other waters. Landlocked salmon of both genera, when introduced into new waters show a tendency to desert them, indicating that inhibitory factors are absent in the new environment. Studies of salmon streams and environmental modifications produced by high dams led the author (1927) to predict the artificial production of landlocked salmon in the reservoirs. Later such fish were discovered. The analysis of the situation showed that young salmon descending the stream were led by the warming up of surface waters in the reservoir to abandon the down stream migration, seek deeper cool waters in the reservoir, thus becoming physiologically trapped, and remain there until the approach of sexual maturity inaugurated the up stream movement to the spawning ground. Naturally landlocked races have been attributed previously to the existence of barriers in the stream. However, these would prevent the access of adults coming from the sea while not preventing the down stream movement of the young. This would result in eliminating the fish rather than in holding them in upper waters. The author suggests temp. with climatic changes as the controlling factor. When the migrations were first established after the ice age, both young and old came and went in cold water. As the climate became warmer, surface temp. rose in lakes until the young were led to desert the warm layer in favor of deeper waters. In the slow changes following the disappearance of the ice temp. conditions were gradually produced in the lake which resembled those suddenly created in the artificial lake formed by the erection of the dam. In both cases the warm surface layer formed a blanket that prevented the young salmon from leaving the water body.