Biology subjects

Rodger, J. R.

Publications and source records attributed to Rodger, J. R..

3 recordsLinked to original sources

bioRxiv2025

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.

ecology↗

bioRxiv2023

Individual genotype but not phenotype predicts river migration success in Atlantic salmon

Migratory species typically undertake demanding long-distance journeys, across different habitat types during which they are exposed to multiple natural and anthropogenic stressors. Mortality during migration is typically high, and may be exacerbated by human-induced pressures. Understanding individual responses to these selection pressures is rarely attempted, because of the challenges of relating individual phenotypic and genetic data to migration success. Here we show distinct Single Nucleotide Polymorphism (SNP) sets significantly differentiated between Atlantic salmon smolts making successful migrations to sea and those that failed to migrate, in two different rivers. In contrast, morphological variation was not diagnostic of migration success. Populations from each river were genetically distinct, and while different genes were possibly implicated in migration success in each river, they related to common biological processes (for example osmoregulation and immune and stress response). Given that migration failure should quickly purge polymorphism at selected SNPs from a population, the question of how genetic diversity in these populations is maintained is an important one. Standing genetic variation could be maintained by different life history strategies and/or environmentally driven balancing selection. Our work highlights the importance of preserving genetic diversity to ensure evolutionary resilience at the population level, and has practical implications for management.

evolutionary biology↗

bioRxiv2022

Is shape in the eye of the beholder? The reproducibility of geometric morphometric analyses on live animals

Geometric morphometrics is widely used to quantify morphological variation between biological specimens, but the fundamental influence of operator bias on data reproducibility is rarely considered, particularly in studies using photographs of live animals taken under field conditions. We examined this using four independent operators that applied an identical landmarking scheme to replicate photographs of 291 live Atlantic salmon (Salmo salar L.) from two rivers. Using repeated measures tests, we found significant inter-operator differences in mean body shape, suggesting that the operators introduced a systematic error despite following the same landmarking scheme. No significant differences were detected when the landmarking process was repeated by the same operator on a random subset of photographs. Importantly, in spite of significant operator bias, small but statistically significant morphological differences between fish from the two rivers were found consistently by all operators. Pairwise tests of angles of vectors of shape change showed that these between-river differences in body shape were analogous across operator datasets, suggesting a general reproducibility of findings obtained by geometric morphometric studies. In contrast, merging landmark data when fish from each river are digitised by different operators had a significant impact on downstream analyses, highlighting an intrinsic risk of bias. Overall, we show that, even when significant inter-operator error is introduced during digitisation, following an identical landmarking scheme can identify morphological differences between populations. This study indicates that operators digitising at least a sub-set of all data groups of interest may be an effective way of mitigating inter-operator error and potentially enabling data sharing.

zoology↗