Scale dependent temporal and spatial patterns of population change in anadromous sea trout (Salmo trutta) in Scotland over seven decades

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 a period of 72 years and over 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 (67% of Regions and 80% of Districts exhibited a different pattern from the National pattern). At the National level, very clear and substantive declines over the study period were evident. At Regional levels (N=9) change in rod catches was more nuanced with some Regions showing no change, one Region showing an increase in sea trout caught and 6 (67%) exhibited a different pattern from the National one. At the District levels (N=64) opposing patterns of change were observed over the study period, even in adjacent Districts with 80% of Districts exhibiting a different pattern from the National one. A conclusion of this study is 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 across all geographic Regions nor all Districts. Two logical inferences derive from these results. Firstly, for best effect, the application of limited resources for management should be focused at smaller scales than at countrywide. Secondly, that monitoring population change at small spatial scales, for example in several index rivers, is unlikely to capture the highly nuanced patterns of change in species with high levels of intra-specific structuring as is commonly exhibited by the Salmonidae.