Investigating Nekton Response to Changing Salinities in the Mississippi Sound: An Experimental and Statistical Approach

The Mississippi Sound provides nursery habitats for many coastal species and is recognized for its commercial fisheries. Previous freshening events linked to the Bonnet Carré Spillway, a Mississippi River flood diversion structure, proved catastrophic for oyster populations in the Mississippi Sound, but effects on mobile fish and shellfish species are not well-defined. The planned Mid-Breton Sediment Diversion (MBSD), an initiative to combat wetland loss, is also forecasted to lower salinities in the region, increasing the need to better understand responses of commercially and ecologically important species to freshening events. The objective of this study was to characterize effects of salinity and temperature on nekton under average conditions and during freshening events. Laboratory-based experiments were used to quantify survival, biomass change, and caloric content of grass shrimp (Palaemon spp.), an important prey species, under different combinations of salinity and temperature. Salinities between 2-20 and a temperature of 26°C were optimal for grass shrimp, and survival results were used to construct salinity response curves in an ecosystem model. Secondly, field survey data were analyzed using PCA, NMDS, and a negative binomial GLM to investigate how salinity and temperature influence the abundance of blue crab, Atlantic croaker, bay anchovy, and brown and white shrimp. Results varied by species, with some likely benefiting from prolonged freshenings and others exhibiting more complex relationships with salinity and temperature, indicating a need to better understand these interactions. Ultimately, this provides insight on the potential responses of multiple mobile species to future freshening events in the Mississippi Sound.