Impacts of the Bonnet Carré Spillway on common bottlenose dolphins in the Mississippi Sound from 2010 to 2021.

In the last decade, the frequency of the use of the Bonnet Carré Spillway (BCS) to divert water from the Mississippi River by the United States Army Corps of Engineers (USACE) has dramatically increased. The BCS is designed to protect the city of New Orleans, Louisiana from levee breaches and devastating floods. In 2011 and 2019, during extreme flood events, the USACE diverted 6.4 trillion gallons and 10.07 trillion gallons of Mississippi River water into Lake Borgne and Biloxi Marsh, Louisiana, and the Mississippi Sound, Mississippi. The 2011 and 2019 diversions and lower-discharge diversions in other years have coincided with high freshwater discharges of coastal Mississippi streams, the appearance of common bottlenose dolphins (Tursiops truncatus, dolphins) with skin lesions, and large numbers of stranded dolphins. To determine what might be causing dolphin skin lesions and strandings, we investigated BCS and coastal stream discharges between 2010 and 2021 as possible drivers. Generalized additive, kriging, and seasonal-trend decomposition using loess models showed that the prevalence of skin lesions in the western Mississippi Sound and Biloxi Marsh was significantly related to BCS discharge and not to stream discharge. From 2010 to 2021, the frequency of stranded dolphins across the Mississippi Sound was significantly related to BCS discharges, while coastal stream discharges had localized effects. Between 2018 and 2019, the relationship between the frequency of dolphin strandings and BCS discharge was highly significant. In this model, the relationship between dolphin strandings and stream discharge was not significant. This research provides evidence that freshwater diversions through the BCS likely increase the prevalence of skin lesions on live dolphins, strandings, and mortality in the Mississippi Sound.