Environmental correlates of elasmobranch and large fish distribution in a river-dominated estuary

Abstract

Coastal areas serve as vital habitat for many marine fishes and often host broad gradients of environmental conditions that result in diverse species assemblages. Understanding the influence of environmental variables on the distribution and abundance of fishes is especially important for devising appropriate management strategies. We documented large fish community structure in the Apalachicola Bay system from 2018 to 2021 using fishery-independent longline and gillnet surveys. Spatiotemporal variation in fish assemblages was evaluated with permutational multivariate ANOVA, while community- and species-level habitat relationships were described across gradients of environmental variables with non-metric multidimensional scaling and generalized additive models. We captured 5085 fishes from 17 elasmobranch and 33 bony fish species, with gafftopsail catfish Bagre marinus, blacktip shark Carcharhinus limbatus, Atlantic sharpnose shark Rhizoprionodon terraenovae, and hardhead catfish Ariopsis felis comprising 82.1% of the total catch. Fish communities varied spatially throughout the system, and changes in these communities were most strongly correlated with gradients in salinity, clarity, and depth. We documented higher species richness and density of young elasmobranchs in our surveys compared to adjacent systems, supporting past suggestions that this area serves as important juvenile shark habitat. We suggest that flow from the Apalachicola River, transporting nutrients to the system and supporting numerous environmental niches across spatiotemporal scales, plays an influential role in maintaining the elevated diversity and density of elasmobranchs observed in this study.