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Scaled body-mass index shows how habitat quality influences the condition of four fish taxa in north-eastern Spain and provides a novel indicator of ecosystem health



  1. Natural and anthropogenic disturbances are key forces governing the structure and functioning of aquatic communities. Understanding how these factors shape organism performance can help to identify the most vulnerable species and develop effective management strategies. This is particularly important for ichthyofaunas with high endemicity and low diversity, such as those of the Iberian Peninsula.
  2. We explored the suitability of a novel and simple condition index, the scaled mass index (SMI), based on mass–length relationships, for analysis of the effects that abiotic and biotic pressures have on the body condition of four fish taxa widely distributed in Mediterranean rivers in north-eastern Spain: brown trout (Salmo trutta), Iberian redfin barbel (Barbus haasi), Ebro barbel (Luciobarbus graellsii) and minnows (Phoxinus spp.). The SMI performed better in explaining spatial variation in body condition than the Fulton Index, a traditional method for fish studies.
  3. For all taxa, anthropogenic stressors influencing water quality and physical habitat explained more variance in SMI than other factors. Variation partitioning and GLM approaches consistently showed that SMI increased with altitude, reduced concentrations of toxic nitrogenous compounds, and well-preserved riparian canopy and natural channel morphology, despite the fact that three of the study taxa are in expansion and generally considered ‘tolerant’. In addition, the application of SMI to an independent fish data set showed that SMI provides a novel indicator of ecosystem health, which performs better than the current index of biotic integrity developed in this region.
  4. We discuss the likely mechanisms behind the strong effects of habitat quality on SMI and the implications for our understanding of tolerance. Incorporating SMI into studies of fish monitoring is likely to improve the value of fish studies as indicators of river quality and ecological change. Further studies should compare the response of SMI to specific fish health indicators such as parasite load, haematological assays and pollutant bioaccumulation to improve our understanding of the value of SMI as a non-lethal diagnostic procedure.
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