Downstream migratory behavior of salmonids and its relationship with cortisol and thyroid hormones: A review

Abstract

The migratory behavior of several species of salmonids raised in hatcheries or artificial streams together with its hormonal control were studied. Masu salmon yearlings were territorial during the parr and pre-smolt stages, less aggressive during parr-smolt transformation, but aggressive after the peak of smolting. Subordinate fish had high levels of plasma cortisol, but the cortisol levels of dominants were as low as those of control fish. Thyroxine (T 4)-treated pre-smolts showed less aggressive behavior. Before the migratory period, chum salmon fry and coho salmon yearlings preferred shaded areas more than open areas. Triiodothyronine (T 3) or T 4 treatment induced a preference for open areas. During the premigratory season, chum salmon fry swam against the flow of current. T 3 treatment changed their swimming direction to downstream (with the flow of current). Underyearling sockeye salmon showed a natural surge of T 4, 2 weeks before downstream migration occurred. Similar fish reared in artificial streams were induced to migrate following T 3 and T 4 treatment; however, the effect was not consistent between years. Underyearling coho salmon showed greater preferences for seawater or undirected searching behavior after treatment with growth hormone, T 4 and cortisol. These findings suggest that thyroid hormones, growth hormone and cortisol play an important role in smolting and in the patterns of behavior which favors downstream migration in salmonids.