Decadal regime shifts in southern California’s ichthyoplankton assemblage

Marine ecosystems worldwide experience rapid, persistent reorganization of biological and physical characteristics, known as ‘regime shifts.’ Here we provide a multivariate analysis of the ichthyoplankton assemblage in southern California (USA) from 1951 to 2017 in both spring and summer. We found unequivocal support for compositional regime shifts in 1965 and 1976 across both seasons, and additional strong evidence for shifts in 1983 and 1990 in the spring assemblage. The 1965 regime shift was associated with increases from low to moderate abundance of several offshore taxa, as well as peaks in the abundance of Sebastes spp. The impacts of the 1976 regime shift have been widely documented elsewhere across the North Pacific; here we demonstrate that the 1976 regime shift in the southern Californian ichthyoplankton assemblage coincided with increases in a large group of taxa characteristic of southern and offshore environments. By contrast, shifts in 1983 and 1989 in the spring assemblage appear to have been driven by variations in just a few highly abundant taxa. Finally, despite speculation that there was a climate regime shift from a ‘warm’ to a ‘cool’ period in the late 1990s, we found no support for a shift in the larval fish assemblage during these years. Our analyses provide context for understanding the countervailing effects on fish of natural environmental variability from annual through multidecadal scales, with important implications for how marine assemblages will respond to secular, global changes in climate.