Gulf Stream intrusions associated with extreme seasonal fluctuations among larval fishes

Abstract

Change in phenology is one of the hallmarks of global climate change. In marine fishes, warming is expected to cause the advancement of a spring peak in larval occurrence or the delay of a fall peak. However, empirical evidence has not consistently upheld this broad prediction, implying that more nuanced hypotheses are needed. Our study investigates oceanic impacts on fish phenology by examining patterns in larval occurrence on the Northeast US continental shelf, one of the most rapidly warming regions of the global ocean. We use data from NOAA’s Ecosystem Monitoring Program, which samples larval fish taxa across the shelf on a bimonthly basis. The sampling program began in 1999, thus documenting changes during a period of rapid warming. We calculated the central tendency of seasonal larval occurrence for 38 taxa and tested for temporal relationships with oceanic drivers thought to influence larval seasonality in other ecosystems. We did not find evidence for warming-related changes in larval phenology over the last 2 decades. Rather, we found high interannual variability in larval timing among many populations, especially those along the shelf break. Among examined factors, salinity maximum intrusions associated with Gulf Stream warm core rings showed the strongest explanatory power for variation in larval fish phenology. Additionally, the occurrence timing of highly variable larval populations overlapped with that of salinity maximum intrusions. Our results suggest that uniform phenological responses to warming are unlikely in this ecosystem, and that hydrodynamic processes connecting widely dispersed regions can strongly influence the phenology of fish.