Difference between Ion Levels in the Blood of the Brown Trout Salmo trutta from Two Closely Located Rivers before Smoltification.

The concentrations of Cl–, Ca2+, Na+, and K+ in the blood of the brown trout Salmo trutta from two rivers of the Lake Ladoga basin (Alatsoya and Ulmasenjoki) are compared. The concentrations of Cl– and Ca2+ + in the fish blood differ and correlate with the concentrations of chlorides and bicarbonates in the river water. The level of Na+ (in fish blood from both rivers) and Cl– (in fish blood from the Ulmasenjoki River) increases until the age of 2+ in specimens, and then (at the age of ≥3+) remains almost unchanged. Sexual dimorphism is found by the concentration of K+ in the blood: in females its concentration is 13% higher than in males. The concentration of K+ is much higher in females at the age of 1+ from the Ulmasenjoki River than in females of the same age in the Alatsoya River. It is assumed that, along with other factors, changes in the ionic composition of brown trout six months before smoltification can determine the degree of their preparation for downstream migration. [ABSTRACT FROM AUTHOR]