Declining Western Alaska Chinook Salmons Stocks- Using Impedance to Look for Answers

Abstract: Western Alaska Chinook salmon stocks are low and our research focus has been on determining processes leading up to the first winter at sea is limiting populations.  Once smolted and at sea, we hypothesize juvenile salmon 1) utilize energy for growth during summer and lipid storage during fall to survive winter and 2) down regulate basal metabolic rates (BMR) during winter to minimize energy storage loss in winter.  Chinook salmon collected early summer and  mid-winter from the Yukon Delta/Bering Sea region were analyzed using traditional energy measurement methods.  Bioelectrical impedance was used to evaluate 1) food deprived juvenile Chinook salmon in laboratory experiments designed to simulate over-winter situations and 2) trawl caught juvenile salmon in the fall.  Three size classes (~100, 160 and 200mm) of salmon were used to describe BMR during low food periods.  Energy densities of Chinook salmon from early summer through mid-fall remained constant (~4.5 KJ/g fish) while growth as measured by length doubled (100 to 200mm) during this same period.  Energy densities and lengths from mid-fall to mid-winter increased from 4.5 to 6.4 Kj/g fish and from 200 to 300mm, respectively.  When comparing energy density at length, energy density did not increase until the fish had reached a length of 200mm.  Over-winter simulations found evidence of salmon reducing BMR by up to 60% during low food periods. Life history strategies utilized by juvenile Chinook salmon to prepare for and survive their first winter include maintaining critical energy densities of 4.5 KJ/g prior to winter while concurrently reaching a critical size of 200mm.  After the critical size of 200mm is reached, the fish then allocates energy into storage until an over-winter energy density level of 6.4 Kj/g is reached.  The subsequent reduction of BMR during over-winter allows minimal loss of the stored energy.