Why, when and how do fish populations decline, collapse and recover? The example of brown trout (Salmo trutta) in Rio Chaballos (northwestern Spain)

1. Around the year 2000, historically abundant populations of brown trout (Salmo trutta) and Atlantic salmon (S. salar) co-occurring in rivers flowing along the Cantabrian corridor of north-western Spain showed a dramatic decline to alarming levels. For one reason or another, fishing was not banned and fishing pressure continued to reduce the few survivors. Unexpectedly, the populations recovered 'naturally' in a very short time period to the extent that in 2004-06 numbers had attained population sizes comparable to those of the mid 1980s. 2. The population of brown trout in Rio Chaballos showed a boom-and-bust pattern concurrent with those observed across broader geographical scales. This study revisits a 22-year data set to explore the nature of the severe decline and rapid recovery of this population. 3. Recruitment was related to stream discharge in March that covers the emergence period and the earliest search stages for food. Coefficients of variation for discharge and recruitment increased over the years and were highly correlated with each other, demonstrating that increased temporal variability in recruitment is strongly linked to increased variability in stream discharge. In turn, recruitment appears to be the major determinant of year-class strength and hence, of population size. 4. A number of factors appeared to operate as resilience mechanisms, permitting the population to increase rapidly when environmental conditions are optimal. These include strong recruitment-discharge relationships, short life-span, small stocks of eggs or offspring required to fill the amount of space suitable for the youngest juveniles and a few females surviving to spawn for a second time. 5. Implications for fishery management and conservation are discussed in the context of the expectation that these populations will vary over the coming years within ranges similar to those in this study.