Examination of Biwa salmon (Oncorhynchus masou subsp.) in four rivers discharging into Lake Biwa, Japan, demonstrated the absence of sexual size dimorphism (SSD). Previous studies having reported the direction or extent of SSD as being likely attributable to the relative strength of sexual selection (favouring larger adult males) versus natural selection favouring larger adult females (e.g., fecundity selection) in masu salmon populations, the lack of SSD in this case suggests a balance between those two factors.
