Scale Dependent Temporal and Spatial Patterns of Population Change in Anadromous Sea Trout ( Salmo trutta ) in Scotland Over Seven Decades

Vollständiger Abstract

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ABSTRACT The spatial scale at which temporal change in a population is measured has the potential to skew the inferences drawn on patterns of change from monitoring data. This study analysed data from rod catches of anadromous sea trout ( Salmo trutta ) in Scotland over 72 years and three spatial scales (National, Regional and District). The pattern of temporal change evident at the national scale mostly did not clearly manifest at Regional or at District spatial scales, which were also generally decoupled. At the National level, very clear and substantive declines were evident. At Regional levels ( N = 9) change was more nuanced, with some Regions showing no change, one Region showing an increase in catch, and 6 (67%) exhibited a different pattern from the National one. At the District levels ( N = 64) opposing patterns of change were observed, even in adjacent Districts, with 80% exhibiting a different pattern from the National one. We conclude that, as a national resource, sea trout numbers have declined substantively over the last seven decades (by around 50%). However, this general picture is not applicable at Regional nor District level. The lack of correlation in response amongst small spatial scales indicates a Portfolio Effect indicating that the observed national pattern of change represents the combination of more variable trends at smaller spatial scales. Two other logical inferences derive from these results. First, for best effect, the application of limited resources for management should focus on smaller scales. Second, that monitoring population change at small spatial scales alone, for example in several index rivers, is unlikely to capture the nuanced patterns of change in species with high levels of intra‐specific structuring as is found in the Salmonidae.