ï»¿

GULF OF MEXICO -- Scientists are starting to believe the most powerful environmental effect stemming from the BP oil spill may have nothing to do with the millions of gallons of petroleum loosed in the Gulf of Mexico.

Instead, ongoing research suggests the federal closure of the richest portion of the Gulf to all fishing through the spring and summer months resulted in dramatic increases in the abundance of numerous marine creatures, from shrimp to sharks.

Scientists said the forced closure illustrates the profound influence fishing pressure has on the marine world, though a federal fisheries regulator said many other factors might be at play.

In the end, a group of Gulf Coast researchers believe the positive impact of the fishing closure will likely make it difficult to detect the full suite of negative effects caused by the oil.

“It is possible that the federal management activities had more of an effect on the Gulf than anything BP did with their oil,” said John Valentine, senior marine scientist at the Dauphin Island Sea Lab, discussing data collected before, during and after the spill.

“It’s not what you would have guessed, given all this stuff said about low oxygen levels and Corexit (dispersant). You would have expected something horrible, but that’s not what we’re seeing.”

Scientists from the Sea Lab have been surveying the coastal waters of Alabama and Mississippi for years, sampling everywhere from the brackish estuaries along shore to the edge of the continental shelf 60 miles out.

Data collected this year shows a marked departure from previous years.

Valentine’s research, which consists of trawl surveys in Mobile Bay, Mississippi Sound and around the barrier islands shows a roughly threefold increase in what the nets captured after the spill compared to before, in terms of both the weight of the catch and the number of animals caught. Valentine said it was possible seasonal factors played a role in the changes in the data, though he believed the lack of fishing was the key.

“There has been an awful lot of debate about longlining, gill netting, commercial fishing, recreational fishing, about how the ocean has been restructured by man,” Valentine said. “This was the first time we’ve ever seen such a large scale cessation of fishing.”

He said that the fishing closure appears to have demonstrated for the first time “how resilient the ocean really is if widespread management measures are applied.”

Two species in particular may have benefited from the closure: menhaden and shrimp. They are the first and second largest fisheries in the Gulf of Mexico by weight, according to federal statistics, and represent two of the key prey items eaten by fish.

Roy Crabtree, who supervises Gulf fisheries for the National Marine Fisheries Service, said the spill caused one of the most widespread and long-term fishing closures ever seen in U.S. waters. But, he said, there are many variables that might explain the increased fish numbers seen off Alabama and Mississippi. Chief among them, he said, is the possibility that some fish were displaced by oil

“I don’t know how to weigh the impacts of the spill versus the closure. I can’t comment on what effect it may have had on fish stocks because we don’t have any numbers yet,” said Crabtree. “There could be several reasons there are more fish in the areas they are sampling.”

Sean Powers, a University of South Alabama marine biologist, studies sharks and reef fish, such as snapper and grouper. He said that longline sampling during and after the closure suggested that some shark populations had tripled around coastal Alabama.

“It’s just been amazing how many more sharks we are seeing this year. I didn’t believe it at first. Compared to 2009, tiger sharks are up 300 percent,” Powers said, referring to one of the Gulf’s largest shark species. “They ranged in size, and a lot of them were young of the year.”

Powers discounted the notion that the increased fish populations along the northern Gulf Coast resulted from animals fleeing the pollution offshore, saying the population swing was too large to be the result of seasonal variation.

“I think the explanation I’m most comfortable with is the fact that it was an incredible reduction in fishing pressure. What’s interesting to me, we are seeing it across the whole range, from the shrimp and small croaker all the way up to the large sharks.”

Powers said questions still linger regarding the fate of fish and other creatures born during the spill. Those larval animals would have been the most vulnerable to oil.

Ken Heck, also of the Dauphin Island Sea Lab, has been surveying juvenile fish populations in grass beds from the Florida Panhandle to Louisiana’s Chandeleur Islands for the last five years. He said populations of snapper and other species whose young live in seagrass meadows appeared to be robust.

“The data really don’t show any large-scale effects that we can detect. It’s quite amazing. Everyone speculated that an entire year class of larvae might have been lost, smothered in oil,” Heck said. “The fear was that their young — the juveniles — simply wouldn’t show up. That hasn’t happened.”

Heck, who pushed for an intensive study effort, said there were still concerns that some losses might not become visible for a year or more.

Michael Carron, head of the Northern Gulf Institute, is responsible for doling out research money to facilities all along the Gulf Coast, including the funding provided to the Sea Lab scientists. Carron agreed that any problems would be revealed during the next several years.

“The research to track this stuff is really import. It’s probably going to be three or four years before we know anything for sure,” Carron said.

All of the scientists expressed concern that the robust fish populations near the coast would obscure the damages caused by the spill.

“The problem with the fishing closure, that impact is so large it is probably going to swamp any impact of the oil spill,” Powers said. “We’re not saying we didn’t lose any fish to the spill or the contaminants. We’re saying it is going to be harder to detect any smaller changes due to oil spill contamination. We’ll have to look carefully.”