Between 2001 and 2019, Brook Trout and Brown Trout have declined by 44% and 42% respectively, in the North Branch Au Sable River. Because age 0+ individuals account for a significant fraction of population estimates, it is not unreasonable to assume that the populations declines are primarily the result of reduced 0+ age classes. However, there currently is no clear explanation for this decline. We conducted mark-recapture surveys at three locations spread over approximately 8 river miles during summer and fall of 2024 to estimate age 0+ population size and to estimate trout survival, growth rates, and condition factor. We also monitored several environmental parameters and estimated available habitat for 0+ trout. Temperature and habitat availability accounted for most of the variation among sample locations. Brown trout density was consistent across all 3 locations while brook trout density was significantly higher at the coolest location. Furthermore, trout had significantly lower survival rates during periods of warmer temperatures, with brook trout being the most affected. Mean growth rates for Brown Trout and Brook Trout increased over time and were 0.12-0.15 lower at the coolest location. Mean K-factor values were between 1.06 and 1.94, indicating that age 0+ trout are relatively healthy, and were consistent. However, K-factor values declined from July to September. In conclusion, mid to late summer survival most likely accounts for low age 0+ abundance, and temperature may partly be the cause of declines in age 0+ trout in the North Branch Au Sable River.