The Ghost of Density-Dependence: Environmental (Hydrological) Factors Drive the Numerical Changes of Young Migratory Trout Salmo trutta in a Lake District Stream (UK), 1966–1996

Finding empirical support for the “paradigm of density-dependence” has been a major focus of ecological and fishery research. Quantifying relationships between the abundance of spawners and the subsequent recruitment is essential for testing the key prediction of density-dependent population regulation: that the number of recruits is mechanistically, but non-linearly, dependent on the number of reproducing individuals. Long-term data are required to explore such relationships, but such data are rare. Elliott and colleagues used a 30-year study of brown trout, Salmo trutta L. in a small UK stream to construct a stock–recruitment relationship suggesting remarkably severe density-dependent mortality of recruits at high spawners’ abundance. In marked contrast, more recent studies on other brown trout populations, suggest environmental (hydrological) factors play a principal role in driving variation in recruitment. These disparate results underscore the more general controversy regarding the relative roles of density-dependent versus density-independent population regulation. The objective of this study was to revisit and re-analyze the data reported by Elliott in light of recent results from other trout populations. The results suggest that variation in stream discharge soon after emergence drives variation in recruitment and early survival rates, and produces the same two-phase, threshold-like recruitment patterns observed in other brown trout populations. These results cast doubt on the original interpretation of the data, and add to a growing body of evidence that environmental (hydrological) factors are the principal drivers of recruitment variation in stream-rearing salmonids.