Assessing the effects of sewage effluents in a Mediterranean creek: fish population features and biotic indices

Abstract

Sewage effluents are one of the main anthropogenic stressors in Mediterranean rivers. The establishment of a cause–effect relationship is hindered in natural systems by the existence of confounding factors (i.e. biotic interactions). Here we analysed the effects that anthropogenic stressors have on a mono-specific fish community (Iberian redfin barbel population, Barbus haasi) inhabiting the northern edge of its distribution range. In Spring 2004, a total of 40 consecutive sampling sites were surveyed in Vallvidrera creek, and 1,331 specimens were measured and weighed. A principal component analysis was performed to synthesize the information provided by 22 environmental variables. Analysis of variance, bivariate correlation analyses and multiple linear regressions were then used to determine the influence of the environmental gradients built (water quality, hydromorphology, woods and macrophytes, and degree of silting) on fish population features (fish size, body condition status, density and biomass). The findings revealed that water quality was the most significant environmental gradient for this fish population. In particular, fish density decreases and fish length increases in those sites exposed to sewage. Additionally, our results showed the best body condition of those specimens inhabiting fast flow reaches which confirms the rheophilous condition of B. haasi. However, these findings were unnoticed for the current version of the index of biotic integrity using fish as bioindicators in Catalonia. Resource managers need to refine diagnostic tools in order to detect subtle deleterious changes on fish communities before they become evident at population scale. Conservation measures should be focused in these small streams in where the best preserved native fish populations usually inhabit. This study suggests the need to change water management policies in Mediterranean rivers to improve the water quality of sewage effluents and increase the dilution power of these rivers.