Oncorhynchus mykiss, (Fetherman et al., 2014) (Fetherman, 2014)

Fetherman, Eric R. & Avila, Brian W., 2022, Habitat associations of rainbow trout Oncorhynchus mykiss and brown trout Salmo trutta fry, Journal of Fish Biology 100 (1), pp. 51-61 : 56-57

3.4 | O. mykiss fry habitat associations

Stocking had the largest effect on O. mykiss fry single-pass counts (cumulative AIC c weight = 0.95), appearing in all models with ΔAIC c ≤ 5.58. The top model also contained the effects of D 50 (cumulative AIC c weight = 0.5) and temperature (cumulative AIC c weight = 0.33), although the effect of temperature was expected given a similar effect in S. trutta fry counts. Velocity (cumulative AIC c weight = 0.20) appeared in the third model of the set (ΔAIC c = 0.94). Because stocking had a large effect on fry count, D 50 and velocity were compared between sites where stocking did or did not (i.e., natural reproduction) occur. There was no observable relationship between counts and D 50 or velocity in sites in which only natural reproduction occurred, likely due to the lower counts obtained from those sites. O. mykiss fry numbers in stocked sites increased with an increase in D 50 ( Figure 4 ), although S. trutta and O. mykiss fry counts were similar at the maximum measured D 50 of 220 mm. Similarly, increased velocities resulted in increased counts of O. mykiss fry in stocked sites ( Figure 4 ). In sites containing more than five O. mykiss fry (328 fry per km), D 50 averaged 118 (±71) mm, 22% lower than the average D 50 for S. trutta fry, and velocity averaged 0.23 (±0.13) m s 1, 13 % higher than the average velocity for S. trutta fry.

Stocking also had the largest effect on O. mykiss trout fry abundance (cumulative AIC c weight = 0.62), appearing in the top three models of the set. A quadratic effect for velocity appeared in the top model, and when compared across sites that were or were not stocked, a similar positive relationship was observed between velocity and abundance as for the count data ( Figure 5 ). The average velocity in the stocked sites was 0.24 (±0.09) m s 1, but the highest abundance was obtained from sites with a velocity of 0.45 m s 1. Despite lower O. mykiss fry abundance overall, 1.5 times more stocked O. mykiss fry were present than S. trutta fry in sites with a velocity of 0.45 m s 1. Depth had the second-highest cumulative AIC c weight (0.35) relative to stocking. The average depth in stocked fry sites was 0.17 (±0.03) m, and overall, depth had a negative effect on O. mykiss fry abundance in stocked sites ( Figure 5 ). The highest abundances were obtained in sites with a depth of 0.13 m, which contained 1.7 times more stocked O. mykiss fry than S. trutta fry. Unlike O. mykiss fry counts, no effect of D 50 on O. mykiss fry abundance was observed.

Figures

FIGURE 4 Oncorhynchus mykiss fry counts from sites in which O. mykiss were or were not (i.e., natural reproduction) stocked and associations with (a) D50 and (b) velocity. A trendline shows the relationship between each habitat variable and the fry count data for both the sites that were stocked (dotted line) and not stocked (solid line) () Not Stocked () Stocked

FIGURE 5 Oncorhynchus mykiss fry abundances from the four sites in which three pass removals were conducted in July through October 2018 and associations with (a) velocity and (b) depth. A trendline shows the relationship between each habitat variable and the fry abundance data for both the sites that were stocked (dotted line) and not stocked (solid line) () Not Stocked () Stocked