An estuary stress index based on nekton relationships with thematic watershed stressors

Abstract

Estuaries provide essential ecosystem functions, including serving as nurseries for fish and aquatic invertebrates (i.e., nekton). Understanding how anthropogenic stressors affect the habitat quality of estuaries is a crucial component of fisheries conservation. A meaningful composite indicator for estuary stress on the nekton community at regional scales should encapsulate major anthropogenic effects over an assemblage of important species. This study introduces a composite stressor index, using meta-analysis to synthesize trawl and seine-based sampling data on 39 nekton species from 25 monitoring programs covering 38 Pacific Coast estuaries spanning four ecoregions. Candidate stressor variables include pollution sources, urban and agricultural land use, and stream impairments, considered at different spatial scales. For each of the 39 focal species, we screened these stressors using hierarchical statistical models that account for environmental variability (e.g., salinity) across sampling events and systematic biases due to monitoring program protocols and species-specific biogeographic ranges. Based on this screening, we identified a subset of stressors most strongly related to nekton presence/absence. Then we used principal component analysis (PCA) to identify major thematic axes of variation across these stressors and the 38 estuaries. Hierarchical nekton models fit to these axes (i.e., thematic stressors) indicate that urban development is negatively related to most nekton species, while agriculture is often positively related to species presence, though with smaller estimated effects. These models were also used to create the estuary-level composite index of anthropogenic stress. Urban development is the major contributor to the index scores of most estuaries, except in the Oregon-Washington Coast ecoregion, where agriculture is dominant. The composite index indicates that estuaries in the Southern California Bight ecoregion are the most impacted, along with South San Francisco Bay.