Long-term changes of summer larval fish community in relation to environmental trends in the NW Mediterranean

This work investigates the main changes undergone by the summer larval fish community over three decades along the Catalan coast, an area characterized by a wide array of environmental conditions. The study was based on nine ichthyoplankton surveys carried out in June, July and September in three decades, 1980s, 2000s and 2010s, covering the same area and applying the same sampling methodology. Throughout the study period, an increase in sea surface temperature, particularly marked in June, and a decrease in surface chlorophyll-a associated with a decline in runoff from the Ebro and Rhone rivers was observed. Marked changes in the composition and abundance of the larval fish community were detected between June and July in the 1980s and the following decades. These changes were mainly due to the presence for the first time in the area of warm-water species, such as Thalassoma pavo and Caranx rhonchus, or to the increase in their abundance, such as Sardinella aurita and Pomatomus saltatrix, in the 2000s in relation to the northward expansion of the adults‘ range. The presence of larvae of warm-water species in the 2000s and 2010s contributed to an increase in specific richness of the larval fish community compared to values obtained in the 1980s. Other species showed a decline in abundance over time, probably due to a decrease in surface chlorophyll-a, e.g. Engraulis encrasicolus, although overexploitation is also an important factor to consider. Larvae of other species, such as coastal and mesopelagic fishes, did not show changes in abundance over the three decades. In a future scenario of increasing sea water temperature and marine heatwaves events, enhanced stratification, and decreasing surface primary production, the changes that may occur in the fish larvae community will largely depend on the adaptive responses of individual species to the new environmental conditions.