Population Dynamics of an Estuarine Forage Fish: Disaggregating Forces Driving Long‐Term Decline of Longfin Smelt in California's San Francisco Estuary

Abstract

AbstractForage fish production has become a central concern of fisheries and ecosystem managers because populations of small fish are a critical energetic pathway between primary producers and predator populations. Management of forage fish often focuses on controlling exploitation rates, but it is also possible to manage productivity of these species in coastal ecosystems, particularly estuaries. Like several forage fish species that are native to the San Francisco Estuary (SFE) in California, the Longfin Smelt Spirinchus thaleichthys has experienced dramatic population declines over the past few decades. This population is not fished commercially or recreationally; trends in its relative abundance have been described statistically, but the mechanisms that drive population dynamics are still poorly understood. Our objective was to evaluate alternative conceptual models of Longfin Smelt population dynamics to better understand the forces that may constrain the species' productivity during different phases of its life cycle. We created contrasting variants of a generalizable population model (the Ricker model) and parameterized those variants using empirical data from a long‐term sampling program in the SFE. Predictions from alternative models were compared with empirical results from a second (independent) data series of relative abundance to identify the model variants that best captured the empirical trend. The results indicated that (1) freshwater flow had a positive association with recruits per spawner and (2) both recruits per spawner and spawners per recruit appeared to be density‐dependent life stage transitions. Juvenile survival may have declined to some extent, but we could not conclusively demonstrate this. By constraining the possible timing and location of mechanisms that modulate productivity at different life stages, the present results improve our understanding of production for a key native forage fish in the SFE.