Larval fish assemblages in the eastern and western Gulf of Alaska: Patterns, drivers, and implications for connectivity

Abstract

As stewardship goals in marine environments expand to encompass ecosystem dynamics, our comprehension of ecological structure must incorporate complex interactions among ecosystem components to define linkages and spatial boundaries for management. For example, the Gulf of Alaska (GOA) is currently considered a single Large Marine Ecosystem (LME), yet the GOA spans a large geographic domain, suggesting that ecosystem linkages may vary depending upon population, community, or process-based assessments. We used six years of larval fish assemblage data (2010–2015), co-collected oceanographic data, satellite-tracked drifters, adult fish trawl surveys, and an individual-based biophysical model to determine the processes that structure larval fish assemblages in the GOA focusing on 1) the local physical environment, 2) seasonality, 3) oceanographically-driven connectivity, and 4) adult fish distributions. Larval assemblages occurred along an eastern to western longitudinal gradient with regionally distinct assemblages in the spring and to a lesser degree in the summer. Assemblage patterns were largely driven by regional spring spawning events and only minimally influenced by temperature, salinity, and bottom depth over the continental shelf. Assemblage analyses and modeled particle trajectories suggest that longitudinal gradients in larval assemblages across the LME are influenced by the prevailing currents that promote connectivity despite variability in species-specific life-history characteristics such as spawning location and pelagic larval duration. Divergences in eastern and western GOA assemblages during the spring, and similarities in the summer, were consistent across most study years, but there were shifts in larval assemblages in the western GOA and not the eastern GOA during 2013 and 2015. Such geographic decoupling of assemblages in recent years highlights the potential for differential regional responses to environmental change across the GOA. Patterns in larval fish assemblages underscore the complex processes that structure pelagic ecosystems and suggest that assemblages diverged along a longitudinal gradient resulting in distinct communities in the eastern and western GOA. We propose that the influence of environmental or climatic variability on regional larval assemblages and life-history traits such as pelagic larval duration and phenology may alter future ecosystem linkages in rapidly changing environments.