Enhanced fish diversity suggests healthy fish assemblages in southern California Current despite increased "tropicalization" of coastal waters

Changes in large-scale ocean circulation due to climate change appear to be underway throughout the world ocean and stand to significantly affect marine ecosystems and resources. This new study by McClatchie et al. investigated whether changes in the character of coastal waters over the last 30 years have affected the composition of fish communities in the southern California Current ecosystem.  The California Undercurrent transports Pacific Equatorial Water into the Southern California Bight from the eastern tropical Pacific Ocean. Pacific Equatorial Water is characterized by higher temperatures and salinities and lower pH and aragonite saturation state values than those in the California Current. Thus, the increased prevalence of Pacific Equatorial Water in the Southern California Bight exposes marine organisms in the region to different environmental conditions than those typical of the recent past. Ichthyoplankton (fish eggs and larvae) assemblages from characteristic California Current and California Undercurrent locations were compared through time to determine whether this recent change in the marine environment had influenced regional fish diversity—the number and relative abundance of fish species—between 1984 and 2014.

Measurements of temperature, salinity, and oxygen made on quarterly CalCOFI (California Cooperative Oceanic Fisheries Investigations) cruises were used as proxies to estimate interannual and seasonal variability of pH and aragonite saturation at depths up to 500 meters. Substantial seasonal and interannual variability in seawater conditions results in large fluctuations in the depth of waters with low pH (more acidic) and aragonite saturation (more corrosive) values that may have physiological effects on fish and other organisms. A notable and long-lasting shift in the influence of Pacific Equatorial Water was seen before and after the 1997−1999 El Niño–Southern Oscillation (ENSO, or El Niño/La Niña) event, with the average depth to corrosive waters (aragonite saturation values below 1) shoaling by 18−26 meters at the study sites between the periods 1985−1996 and 1999−2011. Despite this decadal shift in average environmental conditions, fish diversity was stable during these periods. However, during the 1997–1999 ENSO perturbation, both the number and relative abundance of species changed significantly. Lack of significant differences in the ichthyoplankton communities in the decades before and after the 1997–1999 ENSO event showed that the ichthyoplankton assemblage had resilience and was very similar before and after the ENSO perturbation. Interestingly, while overall fish diversity remained stable, the number of warm-water species increased between the decades, consistent with increased prevalence of tropical water associated with more diverse fish assemblages into the southern California region. The resilience of fish diversity in the southern California Current System after major disturbances, such as ENSO events, suggests that these fish assemblages are still healthy, although there is evidence that both the waters and the fish assemblages are becoming somewhat “tropicalized.”