Brad Robin says there’s no doubt the BP oil spill that began five years ago this month has caused lasting damage to the Gulf Coast.

The 60-year-old oysterman has watched his annual harvest fall by 75 percent in the past five years. It used to cost him about $200,000 a year in materials to build the oyster beds on which his living depends. Now he spends closer to $1 million but hasn’t seen it pay off.

In areas like this small fishing village on the shore of Lake Borgne in St. Bernard Parish, BP blames the decline in oyster production on Louisiana’s decision to flood the area with fresh water in a desperate attempt to keep the oil off its coast. Oysters cannot live in water that falls below a certain salinity, and historic flooding just a year after the spill dumped even more fresh water into the Lake Pontchartrain basin.

But Robin isn’t buying it. It wasn’t the first time fresh water inundated his oyster beds.

“I’m not a scientist. I’m not a biologist. I’m an oyster fisherman,” he said. “I can just tell you what I did for 40 years. It grew before, it’s not growing now, and we only had one other difference.”

That difference, according to Robin, was the millions of gallons of oil that washed up along the Gulf Coast for months after the spill, remnants of which still show up from time to time in the form of tar balls.

A measly oyster haul is not the only development that has raised questions about the Gulf’s health five years after the Deepwater Horizon rig exploded, killing 11 men and spewing oil into a delicate ecosystem unabated for 87 days.

Nearly 1,000 bottlenose dolphins have washed up dead in the northern Gulf in the past few years. Brown pelicans and laughing gulls have died off in unusual numbers as well, models show. Juvenile red snapper populations fell to their lowest level in almost two decades. And one study last year found that a 1,250-square-mile “bathtub ring” of oil particles still remained on the sea floor.

Yet many scientists tasked with measuring the spill’s aftermath share none of Robin’s certainty.

“This rebound has taken a longer time than usual,” said Thomas Soniat, an oyster biologist at the University of New Orleans, “and it could be related to extended, fairly low salinity until recently, or it could be something else involved.”

In an environment as large and complex as the Gulf, weighing the many factors at play and assigning blame has proven difficult and sometimes impossible.

That doesn’t mean that scientists or environmentalists share BP’s upbeat assessment.

Facing the possibility of enormous financial penalties, the company argues that Gulf ecosystems are returning to normal and in some cases are healthier than they have ever been.

No final answer soon

But even five years on, many scientists say it will take more time, perhaps even decades, to fully grasp the spill’s impact on the region.

“There are a lot of ‘possiblys’ and ‘probablys’ and ‘maybes’ in there,” David Muth, an official with the National Wildlife Federation, said about the research published so far. “That’s the proper way to talk about it. … That’s sometimes frustrating when you want a simple answer, and we’re probably not going to give you a simple answer.”

Two weeks ago, Muth led a group of reporters on a trip to Cat Island in Barataria Bay. The island was hit hard during the spill and became a familiar backdrop for ubiquitous images of oil-covered pelicans.

In some ways, Cat Island is also emblematic of the attempts to assess long-term damage. Until the spill, it was a lush, 4-acre rookery for brown pelicans and other species. Now the island has nearly disappeared. All that’s left is a pair of thin sandbars. The only vegetation is a few clusters of dead mangrove stumps scattered along the water’s edge.

Some blame all this on the spill, arguing that oil killed off the mangroves and other vegetation that used to keep the island’s soil in place. Erosion has completely erased another island that used to sit across Cat Bay to the west — another vibrant bird habitat that has now vanished.

But even here, conclusions will have to wait for more careful weighing of the evidence.

“It’s not exactly rocket science to predict that if you kill marsh grass, you’re going to have a faster rate of erosion,” Muth said. “But in terms of building a case in court, you actually have to go out and do the studies and prove that.”

In the meantime, BP has aggressively pursued its own side of the case, insisting in court filings and public statements that things are mostly back to normal in the Gulf, if not better than before.

“Despite initially dire predictions regarding the spill’s impact, more than four years of data show that the Gulf has made a significant recovery due in large part to the massive response and cleanup effort,” the company wrote in a court filing last year, pleading for leniency from U.S. District Judge Carl Barbier.

“Many Gulf beaches were left cleaner than they had been in years, seafood landings data show levels that are now consistent with those pre-spill, and tourism records have been broken.”

BP says it has removed “anywhere from 29 percent to 49 percent of the oil that was released — a rate roughly two to five times greater than in a typical spill response.” The company employed a range of techniques, using chemical dispersants at the source of the leak and then skimming and burning oil that reached the surface, deploying a fleet of more than 9,000 locally contracted vessels.

Hundreds of studies

Ultimately, of course, BP will not have the final say on the spill’s impact.

That will fall instead to a legal process known as the Natural Resource Damage Assessment. Outlined in the Oil Pollution Act of 1990, it puts a council of trustees who represent various federal agencies and the five Gulf Coast states affected in charge of measuring the disaster’s effect on natural resources.

Droves of scientists are involved, conducting hundreds of studies intended to measure the spill’s toll on organisms and habitats, from marine mammals and sea turtles to mud flats and coral reefs.

If BP doesn’t go along with the group’s final plan for what work remains to be done — and whatever it costs — another lawsuit could follow.

Already, BP has spent $1.3 billion to fund these studies and committed up to $1 billion for early restoration projects. But the trustees’ findings are expected to cost the company billions more.

To this point, BP has actively challenged initial findings of unhealthy conditions in the Gulf.

Take, for instance, the case of bird deaths, one of the most visible signs of havoc in the spill’s immediate aftermath.

Jeffrey Short, a marine chemist who was a key member of the National Oceanic and Atmospheric Administration team that studied Alaska’s Exxon Valdez oil spill in 1989, published a study last year estimating as many as 800,000 birds were killed by the Gulf spill.

While only 3,000 bird carcasses were collected, Short said that figure represents just a small fraction of the actual deaths, given how many oiled birds would inevitably have been lost to a combination of tides, beach sand, scavengers and decomposition.

BP immediately disputed Short’s findings. While Short and his team of researchers assumed that cleanup workers would have recovered only 42 percent of large birds and 7 percent of smaller species, the company insists that its own field research puts those figures at 97 percent and 78 percent, respectively.

The company also questioned the impartiality of Short and his co-author, Christopher Haney, pointing out their ties to environmental groups and trial attorneys involved in litigation against BP.

In an interview, Short dismissed the company’s attacks, arguing that, if anything, it’s BP that has the overwhelming incentive to reach certain predetermined conclusions. “Their reports are financed by corporations that are defendants,” he said. “OK, so what data do you have? What evidence do you have, is what it really comes down to. I will stand by our evidence to my dying breath.”

Dispute on dolphins

A similar back-and-forth has occurred over the unusual number of sick and dying bottlenose dolphins.

“I hate to speculate and point to one, singular dead dolphin being attributed to the oil spill, but there have been extensive studies in Barataria Bay where the dolphin deaths have been four times higher than they typically are,” said Alisha Renfro, a coastal scientist for the National Wildlife Federation.

In 2013, the National Oceanic and Atmospheric Administration reported that most dolphins examined in Barataria Bay in 2011 were in bad health, and the agency pointed to oil exposure as a potential cause. A study NOAA released in February stated more definitively that oil exposure was a contributing factor.

Again, BP has dismissed the reports. The company noted that an “unusual mortality event” for bottlenose dolphins had already been reported in the Gulf in February 2010, two months before the spill. And just as with oysters, the company says researchers have failed to establish a causal link between the oil spill and the deaths.

BP’s own five-year environmental report states flatly that there has been “no evidence to conclude that the Deepwater Horizon accident had an adverse impact on bottlenose dolphin populations in the Gulf of Mexico.”

In fact, the company’s assessment claims the existing data “do not indicate a significant long-term impact to the population of any Gulf species.”

Yet — as if to drive home how preliminary and incomplete any conclusions remain — thousands of pounds of oil mats surfaced in the sands of Grand Terre Island, about 5 miles from Grand Isle, just days after BP released its report.

The company’s contractors hauled away more than seven times as much oiled material from Grand Terre as was collected in all of Louisiana in the last year, giving ammunition to the critics who say BP is understating the spill’s impact and giving itself too much credit for its own cleanup work.

“Look, here we are five years later, and they’re still finding large oil mats that are out there,” said U.S. Rep. Garret Graves, a former chairman of the state’s Coastal Protection and Restoration Authority. “We called this. We predicted this years ago and said this was going to happen.”

Graves said the latest discovery shows that BP’s early cleanup effort was flawed, that the disaster “required a more proactive approach” than the company was willing to pay for.

“I just think that this has been totally mishandled,” he said of the four-year cleanup. “As a result of letting the responsible parties get off and not properly do a cleanup job, we’re paying for it, the fishermen are paying for it, the health of the Gulf is paying for it.”

Follow Richard Thompson on Twitter, @rthompsonMSY.