Components of Brown Trout Age-Class Density Dynamics

Brown trout population dynamics are driven by endogenous and exogenous factors acting together or interacting. Endogenous mechanisms include stock–recruitment relationship and cohort effects, whereas the main external driver is often stream discharge. However, detailed consequences at the cohort and population level of the effects of endogenous and exogenous drivers on age-class density dynamics remain less documented. Here we address the following issues: How do brown trout age-classes quantitatively respond to endogenous and exogenous factors and their interactions? How is this response affected by site-specific habitat characteristics and by stocking and flow regulation? What is the relative importance of these drivers on inter-annual age-class density fluctuations? How does the age-class density fluctuation transfer through the cohort lifetime? To address these issues, we explored a large-scale data sets across a diversity of stream habitats encompassing 82 sampling sites over the years 2002–2018, by fitting a non-additive linear regression model to a set of age-class detrended and standardized density series including flow-regulated/non-regulated and stocked/non-stocked sites. The response variables were 0+, 1+, and 2++ age-class densities at year t + 1. Predictors include seasonal discharge, as an exogenous driver, and 2++, 0+, and 1+ density at the previous year t, respectively, as endogenous drivers. Our findings include: (1) external drivers (i.e., seasonal discharge: high winter and extreme spring discharge) affect early stages more intensely than endogenous factors (i.e., recruitment rate), whereas cohort effects (survival rates) affect older stages more strongly than external drivers. (2) No evidence of negative relationships between densities of two successive age-classes of a cohort, or between spawners stock and the subsequent recruitment, but a positive feedback loop that makes the population density exhibit a sort of inertia over time and, (3) these endogenous effects interact with site-specific habitat characteristics that shift the response of age-class density. Finally, we offer compelling evidence that stocking and flow regulation smoothen the driver’s effects on the age-class density dynamics.