Impacts of marine protected areas and the environment on larval rockfish species richness and assemblage structure in the Southern California Bight

Although it is well established that marine protected areas (MPAs) are effective tools for augmenting the abundance of fished species, effects on biodiversity and assemblage structure are less clear. Many rockfish (genus Sebastes) species were overfished in the California Current Ecosystem at the turn of the 21st century, leading to establishment of 2 large MPAs in the Southern California Bight, the Cowcod Conservation Areas (CCAs). The CCAs, alongside favorable environmental conditions, helped rebuild overfished rockfish stocks by 2019. Here we assessed the impacts of the CCAs and environment (temperature, oxygen, salinity, chlorophyll a) on rockfish diversity and assemblage structure using a 16 yr time series (1998-2013) of genetically identified rockfish larvae. Larval rockfish richness generally increased across the time series, with the greatest increases inside CCAs when temperature was low and dissolved oxygen was high. We also found a clear species assemblage shift after the implementations of the CCAs, driven largely by rockfishes targeted by fisheries. This shift was most likely initially driven by maturation of a strong 1999 year class, and sustained by protection from fishing and favorable environmental conditions. Our results suggest that the large and long-term MPAs along the southern California coast have induced increases in the diversity of larval rockfish, driven largely by the direct effects of fishing cessation on targeted species.