Overview
In 2019, an unusual mortality event (UME) occurred in the Gulf of Mexico when 337 bottlenose dolphins were found stranded between Louisiana, Mississippi, Alabama, and Florida. NOAA determined the cause to be prolonged exposure to low-salinity waters, evidenced by skin lesions and other pathological findings. Because dolphin carcasses are often discovered days after death, the original location of each dolphin’s death was unknown.
This study applied an existing 2D hydrodynamic model of the Mississippi Sound — built with EFDC+ 11.2 — combined with Lagrangian particle tracking (LPT) to hindcast the most probable death locations and simulate the salinity conditions each dolphin encountered.
Model Setup
The 2D EFDC+ model of the Mississippi Sound, previously developed and calibrated, was used to simulate hydrodynamic conditions during the UME period. A large number of virtual particles representing dolphin carcasses were seeded throughout the model domain and tracked backward in time over a five-day window — consistent with the typical interval between death and stranding.
The simulation period coincided with the opening of the Bonnet Carré Spillway, which significantly reduced salinity across the Sound.
Key Findings
Results indicated that 12 of the dolphins stranded on Mississippi beaches, along with two found on Ship and Horn Islands, most likely originated west of their stranding locations. The two dolphins found on Dauphin Island most probably died in the Mobile Bay area north of their stranding sites.
The average simulated salinity at each dolphin’s most probable place of death was below five PSU in all but two cases — well within the range associated with the lethal low-salinity conditions identified by NOAA. The study highlights the significant environmental impact of the Bonnet Carré Spillway opening on salinity reduction across the Mississippi Sound.