Drivers of stream-living brown trout populations in Spain: An assessment at a large geographical scale

Resumen

A declining trend in the brown trout (Salmo trutta) abundance has been observed in several datasets available. Quantitative data from 723 stream sites (3915 occasions) provided by a consortium of fisheries authorities and simultaneously collected explanatory variables (hydrology, temperature, impacts and pressures, geology, physiography and angling activity) were analyzed to answer the following issues: (1) what factors drive temporal variability in brown trout abundance? (2) Do human-induced alterations affect sites where declining populations have been observed? and (3) in which proportion of the study sites temporal abundance of brown trout concur with potential driving factors?. Results provide no evidence of density-dependent recruitment regulation, but a recruitment limitation due to hydrological (mostly) and thermal (less consistent) events. Decreasing abundance was observed in one third of the sampling sites concurrent with hydrological and thermal drivers. No evidence of human-induced alterations apparently drive the observed patterns. Nevertheless, the realized carrying capacity appears significantly (p<0.05) lower in impacted than in unimpaired sites. Moreover, populations in impacted sites show significantly higher recruitment and mortality rates, behaving like artificially 'accelerated' populations. In the impacted sites, populations are more volatile, and most likely due to their limited carrying capacity these sites appear prone to reach ?viability thresholds? during episodes of stochastic fluctuations or naturally-driven abundance trends. According to global change predictions, these effects are likely to remain over time or even intensify within the large scale geographical scales assessed.