Development of Salinity Preference in Pre-Smolt Coho Salmon, Oncorhynchus kisutch

When tested in a horizontal salinity gradient, freshwater-adapted coho salmon exhibited a bimodal preference response throughout most of the pre-smolt period. Preference modes were located at fresh water and at a salinity intermediate between fresh and sea water. The concentration of the higher preferred salinity changed both with time and instantaneously with the dimensions of the test gradient. Fish tested in a gradient that originated at 4‰ exhibited a single preferred salinity at 7‰ in June, which increased gradually to 13‰ by February. The same fish tested in a gradient originating at fresh water showed a corresponding preference for an intermediate salinity in addition to their preference for fresh water. However, this second preferendum was at a somewhat lower salinity than that observed in tests in which fresh water was excluded from the gradient.With the approach of the smolt transformation, preference for fresh water was greatly reduced and the response distribution became unimodal. At no time during the period prior to the smolt transformation did the preferred salinity exceed 14‰.