Northwest Fisheries Science Center

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Document Type: Report
Center: NWFSC
Document ID: 7770
Title: Some physiological changes in coho salmon (Oncorhnychus kisutch) during smoltification and seawater adaptation
Author/Editor: Leroy C. Folmar
Year: 1981
Institution: Report of the National Marine Fisheries Service, Northwest and Alaska Fisheries Science Center. Seattle, Washington

Changes in gill Na+–K+ ATPase specific activities and plasma concentrations of T4, T3, Na+, and Cl- were monitored during the periods of smoltification and seawater adaptation, and then at 30 and 180 d after seawater entry in 10 groups of yearling hatchery-reared coho salmon.  During the period of smoltification (April-May), dramatic increases in both plasma T4 concentrations and gill Na+–K+ ATPase activities occurred; these changes were found to be significantly correlated in their timing. 

Plasma T3 levels slightly increased during this period and were significantly correlated with both plasma T4 concentrations and gill Na+–K+ ATPase activities.  Plasma concentrations of T4 and T3 were significantly correlated.  Some of the hatchery groups demonstrated a significant decrease in plasma Na+ and Cl- concentrations during the same period.  Plasma concentrations of Na+ and Cl- were found to be significantly related, but were both unrelated to gill Na+–K+ ATPase activities or plasma T4 and T3 concentrations.  Plasma K+ concentrations changed significantly in some groups, but not in others. 

Transportation-from hatcheries and transfer of yearling coho salmon from freshwater to seawater resulted in significant increases in gill Na+–K+ ATPase activity during the first 8 d of seawater residence.  In contrast, plasma T4 concentrations exhibited a variety of responses which included increases, decreases, and no change.  Plasma T3 concentrations showed similar variable responses to seawater transfer.  Plasma T4 and T3 concentrations showed a significant correlation, but neither T4 nor T3 were related to gill Na+–K+ ATPase activities. 

All of the hatchery groups showed an initial increase in both plasma Na+ and Cl- during the first or second day of residence in seawater.  By the eighth day in seawater, Na+ and Cl- levels were stabilized in some of the groups, but continued to rise through 180 d in seawater in others.  Plasma Na+ and Cl- concentrations were significantly related, as they were in freshwater.  Plasma concentrations increased slightly sometime during the first 6 d in seawater, but general remained uniform throughout the seawater sampling period.  There were no significant relationships between any of the plasma electrolyte concentrations and gill Na+–K+ ATPase activities or plasma concentrations of T4 or T3.

The success of smoltification for the various stocks was assessed by determining the proportions of surviving fish and smolted fish after 6 months in seawater.  The percentage of surviving fish ranged 27-80%.  The percentages of smolted fish among the survivors ranged 48-90%.  Percent smolts and percent survival were not significantly related.

The peak of plasma T4 activity in freshwater coho salmon was comparable to that of anuran amphibians undergoing metamorphosis, which suggests that thyroxine may play a similar role in the parr-smolt transformation. 

Notes: Likely the original manuscript for Folmar, L. C., W. W. Dickhoff. 1981. Evaluation of some physiological parameters as predictive indices of smoltification. Aquaculture 23(1-4):309-324.