Efficacy of population size structure as a bioassessment tool in freshwaters
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Bioassessments are used to measure system health and assess disturbance. While fish-based freshwater
bioassessments are cost-effective and perform well in speciose systems, such bioassessments remain
difficult to implement in species-poor Mediterranean regions. Population size structure metrics may
provide meaningful biological information where depauperate communities preclude the richness and
composition measures generally used. We focus our assessments of population size structure responses
to anthropogenic perturbation on one of the most widespread native stream fish (Squalius laietanus).
We explore a number of population size statistics as metrics for a Mediterranean region, where current
bioassessments perform poorly. Our sampling encompassed 311 sites across Catalonia (NE Spain) where
we characterized anthropogenic perturbation using a summary of impacts, including local data on stream
condition and landscape indicators of degradation, via a principal component analysis. Anthropogenic
perturbation in streams was collinear with altitudinal gradients and highlights the importance of appropriate
statistical techniques. Of the population size structure metrics explored, average length was the
most sensitive to anthropogenic perturbation and generally increased along the disturbance gradient.
Although we expected to find consistent changes in variance, kurtosis, and skewness, the observed relationships
were weak. River basin mediated responses suggest the importance of environmental landscape
factors. The unexpected increases of mean S. laietanus body size with anthropogenic perturbation, strong
effects of river basin, collinearity with spatial gradients and the species-specific nature of responses preclude
the direct application of size structure in freshwater bioassessments. Although its application in
fish-based freshwater bioassessments appears difficult, population size structure can provide insights in
species-specific applications and management
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