Trophodynamics and density-dependence modulate impacts of severe weather conditions to drive abundances of estuarine fishery species

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Abstract
Climate change is exacerbating the frequency and intensity of severe weather conditions. The ecosystem approach to fisheries management (EAFM) may benefit from considering how severe weather conditions interplay with biotic relationships, including predator-prey relationships and density dependence, to predict spatiotemporal trends of relative abundance of fishery species. Dynamic structural equation modelling (DSEM) is used to assess the impact of severe drought, salinity, and biotic relationships in predicting abundances of select predator and prey species in two Gulf of Mexico estuaries: the Mission-Aransas Estuary and Trinity-San Jacinto Estuary. In the DSEM framework, models include lagged and non-lagged effects, as well as direct and indirect effects in two trophic systems: a sciaenid trophic system of Red Drum (predator), Spotted Seatrout (predator), Atlantic Croaker (prey), and Blue Crab (prey); and a keystone predator system of Bull Shark (predator), Alligator Gar (predator), Gulf Menhaden (prey), and Striped Mullet (prey). The most parsimonious models retained trophic (manifested through bottom-up and top-down effects) and density-dependent relationships in both trophic systems in the Trinity-San Jacinto Estuary, but the sciaenid trophic system in the Mission-Aransas Estuary did not retain trophic relationships. Tight predator-prey relationships (Red Drum-Atlantic Croaker and Bull Shark-Striped Mullet) helped produce high prediction skill for predators (e.g. >0.83 Spearman’s rho correlations for these predators). Drought impacts were often negative, but varied between and within estuaries. For example, Spotted Seatrout abundances were up to 55% lower in severely dry years in the most saline bay within the Mission-Aransas Estuary, but were up to 45% higher in severely dry years in fresher bays within the Trinity-San Jacinto Estuary. The DSEM approach allowed us to identify that drought can have lagged and indirect effects on predators through bottom-up and density-dependent effects, and that biotic lagged and indirect effects can offset immediate direct impacts of drought. Overall, the results indicate that accounting for trophodynamics, density dependence, and drought can lead to robust predictions of estuarine predators via DSEM, an EAFM tool that can enhance resource management. Additionally, management approaches should be locale-specific because baseline salinity conditions affect how different estuaries and bays respond to severe weather conditions.
Funding Information
  • National Oceanic and Atmospheric Administration (NA22NOS4510203)
  • National Center for Ecological Analysis and Synthesis
  • University of California

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