Improving assessments of coastal ecosystems – Adjusting coastal fish indicators to variation in ambient environmental factors

The application of ecological indictors for assessing the environmental status of ecosystems play an important role for effective management. However, natural variability may limit the indicators’ ability to provide relevant information about anthropogenic pressures and guide management action. Coastal fish species are not only a resource for commercial and recreational fisheries but also key ecosystem components in the Baltic Sea, and is therefore used as management objectives within the EU Marine Strategy Framework Directive and the HELCOM Baltic Sea Action Plan. A challenge, however, is that the distribution and abundance of coastal fish populations in Baltic Sea is also influenced by spatial and temporal variation in ambient environmental factors. Here, using 16 years of monitoring data, over a latitudinal range of 56 – 66°N along the Swedish Baltic Sea coast, we evaluated the effect of variability in water temperature and depth, and wave exposure for three indicators of environmental status assessment in the Baltic Sea: Abundance of perch, Abundance of Cyprinids, and Abundance of Piscivores. Generalized linear mixed models (GLMM) revealed an overall positive linear relationship between water temperature for all indicators, and overall negative linear relationships to depth and wave exposure. When adjusting indicator values using the parameter estimates from the GLMM models, the variability and 95 % confidence interval for all three indicators were reduced. The adjustment, however, did not have a strong impact on the assessment of the ecological state of the indicator. Our results suggest that adjusting coastal fish indicators to variation in local ambient environmental factors will increase their precision, and hence, the confidence in the assessment of environmental status.