Four decades of climatic fluctuations and fish recruitment stability across a marine-freshwater gradient

Investigating the effects of climatic variability on biological diversity,
productivity, and stability is key to understanding possible futures for
ecosystems under accelerating climate change. A critical question is, how
does climatic variability influence juvenile recruitment of different fish
species and life histories that use estuaries as nursery habitats? Here we
examined spatiotemporal abundance trends and environmental responses of 18
fish species that frequently spend the juvenile stage rearing in the San
Francisco Estuary, CA, USA. First, we constructed multivariate
autoregressive state-space models using age-0 fish abundance, freshwater
flow (Flow), and sea surface temperature (SST) data collected over four
decades. Next, we calculated coefficients of variation (CV) to assess
portfolio effects (1) within and among species, life histories
(anadromous, marine opportunist, estuarine dependent species), and the
whole community, and (2) within and among regions of the estuary. We found
that species abundances varied over space, across time (increasing,
decreasing, dynamically stable), and in 83% of cases, in response to
environmental conditions (wet/dry, cool/warm). Anadromous species
responded strongly to flow in the upper estuary; marine opportunist
species responded to flow and/or SST in the lower estuary; and estuarine
dependent species had diverse responses across the estuary. Overall, the
whole community when considered across the entire estuary had the lowest
CV, and aggregate life histories and species provided strong biological
insurance to the portfolio (2.4 to 3.5-fold increases in stability,
respectively). Spatial insurance also increased stability, although to a
lesser extent (up to 1.6-fold increases). Our study advances the notion
that fish recruitment stability in estuaries is controlled by
biocomplexity – life history diversity and spatiotemporal variation in the
environment. However, under climate change, intensified drought and marine
heatwaves may increase the risk of multiple consecutive recruitment
failures by synchronizing species dynamics and trajectories via Moran
effects and ultimately erode estuarine nursery function.