Fig. 6. Quantification of Na⁺/K⁺-ATPase-immunoreactive mitochondrion-rich cell numbers (A,B) and size (C,D) in the gills of lake trout, brook trout and Atlantic salmon kept in 0 p.p.t. (FW) and those transferred gradually and kept in 30 p.p.t. for 3 weeks (SW). Mitochondria-rich cells on the primary filaments (A,C) and secondary lamellae (B,D) were counted and analyzed separately. Each value represents the mean ± s.e.m., N=4. Asterisks indicate significant differences between FW and SW (P<0.05, Wilcoxon rank-sum test).