Investigating the selective survival of threatened steelhead juveni

Background/Question/Methods

In intermittent streams, summer can be a critical period because reductions in stream flow lead to habitat fragmentation. Anadromous species, such as steelhead trout, are especially sensitive to these reductions because juveniles rear in freshwater prior to migrating to the ocean. We hypothesized that summer conditions may affect juvenile survival as well as adult returns, i.e. population dynamics. Our goal is to understand the factors influencing steelhead survival during the summer . We monitored steelhead populations across three recent summers (2009-2011) through a mark-recapture study in a small Californian stream. Each year, we captured juvenile steelhead in the early summer and individuals were measured and implanted with PIT tags. We then used a portable antenna to track the fates of tagged individuals across each summer. We adopted a bayesian hierarchical modeling approach to separate the demographic process of biological interest, which integrates the individual heterogeneity as well as physical and biological covariates, from the observation process through the detection of marked individuals. For each summer, we calculated weekly estimates of probability of detection (p) and summer survival (Ф). To visualize the form of selection through fitness surfaces, we used nonparametric methods to display selection acting on multivariate phenotypes or covariates.

Results/Conclusions

First, our results emphasize the importance of imperfect detection when estimating survival. In our preliminary analysis, we highlighted that the range of probability of detections varied across summers (2009: p=0.77-0.89; 2010: p=0.69-0.87; 2011: p=0.87-0.97). Ignoring uncertainty due to imperfect detection promotes a false sense of confidence about the selective survival. We showed that the range of summer survival estimates varied across years (2009: Ф=0.80-0.96; 2010: Ф=0.90-0.99; 2011: Ф=0.89-0.99). These results suggest that water level may have influenced summer survival as 2009 was relatively dry compared to 2010 and 2011. Second, we demonstrated that combining mark-recapture framework and nonlinear selection is a flexible way to visualize the relationship between survival and multiple factors. We quantified the strength and form of selection acting on steelhead juveniles while exploring variation in fitness surfaces over time. We highlighted selective survival depending on time, body size, and species abundance. Our work contributes to an improved understanding of the influence of temporal habitat fragmentation on the dynamics of a threatened stream fish.