When asked to list the major hurdles facing Louisiana’s 50-year plan to rebuild its coast, state officials have pointed to a menu of truly massive challenges, from rapidly rising seas to finding the money for the $92 billion effort.

But its most complicated challenge may come from something much smaller, about six to eight feet long: The bottlenose dolphin, a much-loved native resident of Louisiana’s coast and bays.

The river diversions that are key to the state’s plan to rebuild wetlands could lower the salinity of the water to levels that would kill those dolphins. To prevent that, the Marine Mammal Protection Act of 1972 could require the National Marine Fisheries Service to block the diversions or force major modifications.

And the typical solutions to such critter-vs.-project conflicts are not available for this one.

Gulf-wide populations of bottlenose dolphins are not in trouble, but the ones that live off the coast of Louisiana can’t simply be relocated like some species living in the way of a project. Each coastal community of dolphins is wedded for life to its home waters. They don’t swim away as the habitat changes; instead they remain until they die of disease or starvation.

Nor can the state fall back on another common solution: “mitigating” the dolphins killed in Barataria Bay by funding the stocking of dolphins in other areas. The law protects the population of each geographic area.

[module align=”left” width=”half” type=”pull-quote”]Each coastal community of dolphins is wedded for life to its home waters. They don’t swim away as the habitat changes; instead they remain until they die of disease or starvation.[/module]So a project that makes the local habitat lethal for its dolphin population would not be approved for a permit, said Jolie Harrison, the chief of the protected species permits division at the National Oceanic and Atmospheric Administration.

She said this issue must be resolved before her agency can approve the $1.3 billion Mid-Barataria Diversion, the first such diversion in the state’s 50-year rebuilding plan. The state hopes to open the diversion in 2020.

“The law allows for unavoidable, incidental take [including killing], but it has to be insignificant – in other words, so low it wouldn’t endanger the health of that population,” she said.

Because this will be the first engineered river sediment diversion ever built, Harrison would not speculate about how much it could threaten dolphins.

“Actually, this is kind of novel for us,” she said. “Most of what we deal with are acoustic impacts to whales and dolphins.

The agency will work with applicants to overcome obstacles like this, she said, “but we need to hear what they propose to do, where and when.”

The state Coastal Protection and Restoration Authority hasn’t finalized when and how long the diversion would be open and how much water it would carry.

The state has known about this issue for some time. In 2013, the state asked various regulators for a heads-up about what concerns they may have with the diversions. The National Marine Fisheries Service listed a range of concerns about how estuarine species, including dolphins, would fare as water in bays freshened.

”We need to hear what they propose to do, where and when.”—Jolie Harrison, National Oceanic and Atmospheric Administration

The issue gained momentum recently when the Myrtle Grove project received fast-track status for the federal permitting process, cutting what was expected to be at least a five-year slog to something closer to two and a half years. It also came up last fall at a meeting of a committee advising the coastal authority on its restoration plan.

Coastal authority researchers already are working with colleagues at The Water Institute of the Gulf, a research arm of the agency, to find ways to prevent salinity near the diversion from dropping too much, especially in the lower half of the Barataria Basin.

The possibility of lower salinity has spurred strong opposition to the projects from some commercial and recreational fishers, who fear the change could relocate their target species, such as speckled trout, redfish, shrimp and oysters.

Erin Fougères, a scientist with the National Oceanic and Atmospheric Administration, said the healthiest salinity level for bottlenose dolphins is 20 to 25 parts per thousand — basically the same as the Gulf of Mexico — but they can live in levels as low as eight parts per thousand.

Just days in water less salty than that can lead to problems for this population, she said. Extended exposure to lower levels would result in skin lesions and ulcers, which often lead to infection and death, she said.

Computer modeling presented by the coastal authority in October showed that once the diversion is running, some parts of lower Barataria Bay could drop below eight parts per thousand for months at a time under certain scenarios.

Research conducted after the Deepwater Horizon oil spill showed the Barataria dolphins spent most of their time in water with a salinity of 14 parts per thousand.

“The freshwater changes their blood chemistry, too, which can lead to death,” Fougères said. “We’ve seen that these animals in low salinity for any length of time first become sick, then die.”

Harrison said the law allows her agency to give special permits for dolphin deaths, typically for military operations and subsistence hunting. Even those restrict the taking of the animals to a number that doesn’t threaten a population’s viability.

The effects of the Deepwater Horizon spill could make it difficult for the state to get a special permit, if one is required. Recently published research, conducted in part by the National Oceanic and Atmospheric Administration, concluded the oil would reduce the Barataria Bay dolphin population by half. The study estimated it could take 40 years for the group to regain its pre-spill population.

That time period would overlap with the time that the diversion would be open, pouring freshwater into nearby wetlands to rebuild the coast.

This story was updated after publication to add the projected salinity level in Barataria Bay when the diversion is running. (Feb. 24. 2017)