Abundance of common bottlenose dolphins in Breton and Chandeleur Sounds from aerial surveys

As part of Louisiana’s Coastal Master Plan (CPRA 2023), the state is proposing the MidBreton Sediment Diversion Project which, when complete, will divert water and sediments from the Mississippi River into Breton Sound (Figure 1) to facilitate the preservation and restoration of wetlands (CPRA 2017). There is natural flow of freshwater from the Mississippi River into these estuarine systems which drives deposition of sediments along with regulation of salinity, temperature, and nutrient concentrations. The input of additional freshwater into estuarine systems associated with artificial sediment diversion projects has the potential to negatively impact the biota inhabiting these environments. Common bottlenose dolphins (Tursiops truncatus) inhabiting estuarine environments may be particularly sensitive to exposure to low salinity water. A summary of available data on the morbidity and mortality in bottlenose dolphins exposed to extended periods of low salinity (< 5 p.s.u.; e.g., McClain et al. 2020; Duignan et al. 2020) informed an expert elicitation that developed a dose-response function relating the time of exposure to reduced survival rates for bottlenose dolphins. AUTHOR’S NOTE: In October 2025, the state of Louisiana Coastal Protection and Restoration Authority announced the cancellation of plans to conduct the Mid-Breton Sediment Diversion project. This report reflects the Louisiana Coastal Master Plan at the time of drafting.