A New Approach to Functionally Assess Estuarine Fish Communities in Response to Hydrologic Change

Abstract

Functional assessment approaches can identify indicators of environmental change and ecological response in estuaries. This study used a 24-year coastal fisheries dataset (1993–2016) and environmental data to evaluate fish assemblage changes in Galveston Bay, Texas (USA) in response to environmental parameters and freshwater inflows, drought, and flood. Functional classification categorized 107 fish species into eleven functional groups to determine if estuarine species serve as suitable indicators of environmental condition. Nonmetric multidimensional scaling (nMDS), distance-based linear models, and graphing techniques using annual departures from period averages determined relationships between functional group abundance, freshwater inflows, and environmental parameters. Results show that estuarine-resident carnivore, estuarine-resident omnivore, marine migrant carnivore, and marine straggler carnivore functional groups exhibit highest species richness (mean = 21 species) and abundance (mean = 48,411 individuals) compared with other functional groups (mean species richness = 3; mean abundance = 1143 individuals). Declines in abundance of functional groups occurred during a 2011–2014 drought, with rebounds in 2015–2016. Relationships between inflows, salinity, and functional group abundance were evident in upper Galveston Bay, diminishing with increasing distance from major freshwater sources. Functional abundance in lower parts of the bay was decoupled from inflows and salinity changes. Findings support the conclusion that estuarine-resident and marine functional groups have limited potential to serve as bioindicators, while freshwater migrant carnivores are potential bioindicators for upper portions of the estuary. Functional methodologies provide insight into effects of environmental change on estuarine ecosystems and serve as management tools to assess changes in fish assemblages.