Figure 4.

Thermal tolerance in egg clusters. (a) The thermal tolerance of salmon embryos developing in conditions matching their natural habitat—clusters of 200 eggs buried in gravel PVC pipes—was substantially reduced at flow velocities typically observed in natural redds (less than 0.1 cm s⁻¹ [13–15]). Shown are the proportion of surviving Chinook embryos reared from fertilization to hatch in five replicate experimental units as a function of experimental temperature and flow velocity. Lines and bands denote the means and 95% CI at each of the 6 treatments. (b) Within experimental replicates, survival was lower in the downstream or back half of the egg cluster, likely due to O₂ depletion as water flows through the egg cluster (figure 3b). Paired points represent logit-transformed proportion survival in each experimental replicate. (c) Flow and temperature conditions in this experiment also affected the mean size at hatch, with embryos hatching at smaller sizes in warmer slower flowing water. (Online version in colour.)