FIGURE 3 Diel-scale migration timing of female (red) and male (blue) Sakhalin taimen spawners (a) and post-spawners (b) in 2019 during ascent at MS1 and descent at MS4, respectively. Grey shading indicates approximate night-time (1837–0420).

3.3 Biological and behavioral effects on migration timing

Sex was the only significant fixed effect influencing seasonal migration timing during MS1, with male spawners arriving at Site L several days earlier than females in both years (P = 0.000, Table 1, Fig. 2). Males also arrived earlier than females at tributary sites (MS2,P = 0.000). Arrival timing at this stage also depended on which tributaries spawners entered first, with those entering T2 and T4 being the earliest and latest, respectively (P = 0.012, F -test). Contrary to MS1 and MS2, departure from the tributaries at MS3 appeared to be earlier for female than male post-spawners (Fig. 2); this was true only for the daytime migrants and led to a significant interaction between Sex and DN in the model (P = 0.041). There was a small but significant shift from daytime to night-time departure at MS3 by both sexes as the season progressed (P = 0.043). Individuals that spawned in two tributaries tended to depart later from the last (second) tributary than those spawning in one tributary (P = 0.043, Fig. S2). Sex was again the only fixed effect to explain downstream migration timing at MS4, with the female post-spawners preceding the males (P = 0.009).
TABLE 1. Best linear mixed-effects models to explain the seasonal migration timing of Sakhalin taimen during four migration stages (MS). Sexm and DNn denote Sex = male and DN = night, respectively. First2, First3, and First4 are the effects of entering tributaries T2, T3, and T4 relative to entering T1, respectively.