Bottlenose dolphins might go 'functionally extinct' here due to Mid-Barataria diversion

Bottlenose dolphins would become "functionally extinct" in two of four areas of Barataria Bay, and the number of dolphins will drop dramatically in the rest of the bay, within 10 years of the start of operations of the proposed $2 billion Mid-Barataria Sediment Diversion project, according to a new study modeling the effects of the diversion's freshwater flow on the marine mammals.

By the end of the diversion's first 50 years of operation, only the southernmost bay area containing Grand Isle, Grand Terre and other barrier islands will continue to be home to any dolphins, and the number there will have dropped by 85 percent, the study said.

Without the diversion, the study concludes, the number of dolphins will see a steady increase of about 3% a year over the same 50-year period, continuing a slow recovery of the basin's population from the years immediately after the 2010 BP Deepwater Horizon oil spill, which reduced the bay's dolphin population by half. 

Freshwater lesions on dolphins

This slide presented during a March meeting of the federal Marine Mammal Commission shows skin lesions caused by exposure to freshwater coveri…

The new model results quantify the potential damage to dolphins on a year-by-year basis during its operation. Studies conducted for an environmental impact statement for the project being reviewed by the Army Corps of Engineers estimate that dolphin losses would total 36 percent of the approximately 2,000 individual dolphins that called the bay home in 2019, but that estimate was based only on looking at the diversion's effects over a one-year period after being in operation for 10 years. 

The new study, which has not yet been peer-reviewed, was prepared by researchers with the University of St. Andrews in Scotland, SMRU Consulting and the National Marine Mammal Foundation at the request of the federal Marine Mammal Commission. The commission will submit the study along with its comments on the diversion project by the June 3 deadline for public comments on the impact statement.

The researchers who developed the new report were also involved in two major studies of the diversion's freshwater effects on dolphins that were prepared in support of the environmental impact statement. The new study basically reruns the results of a NOAA modeling effort using state-developed models to better define the salinity effects from year to year. 

The state Coastal Protection and Restoration Authority warned Monday that the survivorship results developed by NOAA were themselves a combination of modeling results that "contain significant uncertainty." 

The state also warned that the dolphin-loss estimates were only based on salinity changes and fail to compare its predicted losses to a no-diversion scenario that would measure "the permanent, adverse impacts to the dolphin's foraging habitat, prey resources, overall estuarian degradation and other variables that will impact dolphins without the project."  

Mid-Barataria diversion map

Map of the Mid-Barataria sediment diversion project.

Lori Schwacke, a marine biologist with the National Marine Mammal Foundation and a co-author of the new report, agreed that there are significant uncertainties in the modeling results. Researchers have pointed out that there remains a need for long-term studies of the survival of dolphins in Barataria Bay, as well as in many other locations along the Gulf Coast. 

But she said the modeling results were pretty clear in predicting a major reduction in the total number of dolphins surviving even after the diversion's first 10 years of operation. 

However, Schwacke said she doesn't think this single new study will itself block approval of the diversion, since so many other issues are involved in determining whether it will be approved.

Where bottlenose dolphins gather

This map shows how many bottlenose dolphins were located in various parts of Barataria Bay during a 2019 survey by biologists with the Nationa…

The effects on dolphins will be weighed against the project's positive effects, including the 21 square miles of new wetlands it is supposed to create over 50 years and the hurricane storm-surge reduction effects of that new land. 

"What I'm hoping is that this science can be used to do better management, to look at mitigation and minimization of the impacts," she said. "To do that, you really have got to understand where the impacts on the dolphins is going to occur. 

"Certainly there are things that can be done. That's something that Louisiana and NOAA are going to have to talk about. What do you do when you start to see a lot of animals showing up with skin lesions? What do you when those animals start to die?" she said. "That's what they need to start looking at."

If built, the diversion would funnel as much as 75,000 cubic feet per second of freshwater into the bay for an average 177 days a year through a channel to be built at Mile Marker 60.7 on the east bank of the Mississippi River in Plaquemines Parish.

That's expected to result in multi-day periods when the salinity level in many parts of the bay are reduced to only 5 parts per thousand or less. At a salinity level that low, dolphins experience skin lesions that can cover much of their body, causing sickness or death. 

Researchers attributed low-salinity levels in Lake Pontchartrain, Mississippi Sound, along the Alabama coast and in other Louisiana locations for the death of more than 300 dolphins in 2019, when Mississippi River water flowed through the Bonnet Carre Spillway for more than 123 days.

The state's mitigation plan for the diversion includes a $20 million plan for monitoring and adaptive management to minimize the impact on the marine mammals where practical. It also funds an enhanced statewide stranding network aimed at identifying dolphins in trouble and treating them. The network will be run by the Audubon Nature Institute.

The state will spend another $20 millions on "stewardship" strategies, including funding efforts aimed at reducing other stressors on the dolphin population, including fishing gear changes, illegal feeding, addressing noise issues and illegal shooting of the animals. 

The state also will use part of those funds to establish a contingency fund to address future dolphin "unusual mortality event" declarations in the basin. 

Schwacke pointed out that while the individual groupings of Barataria dolphins are at risk, the common bottlenose dolphin species itself is not categorized as endangered or threatened. But she said dolphins are under threat in several locations along the Gulf Coast and elsewhere, in part because of increased freshwater invading their home territories because of more frequent and more intense rainfall events driven by global warming.