Ocean acidification will likely change the structure and function of coastal marine ecosystems over coming decades. Volcanic carbon dioxide seeps generate dissolved CO2 and pH gradients that provide realistic insights into the direction and magnitude of these changes. Here, we used fish and benthic community surveys to assess the spatio-temporal dynamics of fish community properties off CO2 seeps in Japan. Adding to previous evidence from ocean acidification ecosystem studies conducted elsewhere, our findings documented shifts from calcified to non-calcified habitats with reduced benthic complexity.In addition, we found that such habitat transition led to decreased diversity of associated fish and to selection of those fish species better adapted to simplified ecosystems dominated by algae. Our data suggest that near-future projected ocean acidification levels will oppose the ongoing range expansion of coral reef-associated fish due to global warming.

In the area surrounding the Ebro Delta, similar to the rest of the northwestern Mediterranean Sea, the sardine (Sardina pilchardus), one of the most exploited small pelagic fishes, has suffered a decreasing trend in abundance and biomass in the last decade, with low values in evidence since 2007. The dependence of this species on environmental factors makes it vulnerable to environmental changes; consequently, the abundance of the species is highly variable. Using segmented regression, we evaluated the presence of discontinuities in the temporal pattern of the seasonally adjusted landings per unit effort (LPUE), which was used as a proxy of abundance, between 2000 and 2013. The results suggested a sudden increase in mid-2005, followed by a sharp decrease starting in 2006. A generalized additive mixed model (GAMM), incorporating the linear correlation structure, was used to identify relationships between the seasonally adjusted LPUE and trends of the Western Mediterranean Oscillation index (WeMOI), Sea Surface Temperature (SST), Salinity (SAL) and the Zonal and Meridional Currents (ZC and MC, respectively). The variance inflation factors (VIFs) were calculated between all environmental variables to avoid high-dimensional collinearities. The final GAMM, selected using the Akaike information criterion, indicated that positive WeMOI values, which favour the productivity of the area, along with SAL (at ca.38) and a northward-flowing MC, favoured LPUE. Our results, obtained by applying a method in which variation due to season, non-linearity, autocorrelation and collinearity of the covariates was taken into account, provided further evidence of the dependence of the sardine population upon specific hydrographic variables.

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