WASHINGTON – Last year's BP oil spill probably won't push the troubled bluefin tuna population in the Gulf of Mexico over the edge as some scientists had worried, a federal analysis shows.
Of all the potential damage from the 172-million-gallon (651-million-liter) spill in April 2010, scientists had been most concerned about how the oil spill would harm an already overfished species of large tuna. That's because about one-fifth of the spawning habitat where the Gulf's baby tuna were living was coated with oil, according to satellite records. Tuna less than a year old are most vulnerable to pollution.
An analysis by the National Oceanic and Atmospheric Administration, using two different projections from computer models, says that at most, such a spill probably would result in a 4 percent reduction in future spawning of the fish, but probably far less.
Bluefish tuna is considered one of the Gulf's signature species. A summit that begins Monday in Houston will examine the Gulf's health, including the government's restoration plans and the tuna's fate.
"It appears so far that the impact on the larval population is relatively small," said Clay Porch, director of sustainable fisheries for NOAA's Southeast Fisheries Science Center in Miami.
The agency's analysis, which was mentioned in two pages of a 114-page government update on overall tuna health released in May, is based on an assumption that 1 in 5 baby tuna was killed or unable to reproduce in the future because that's the size of the spill in the spawning area.
That 20 percent potential loss of year-old tuna translates to 4 percent of the overall tuna population in the future. Overall population figures also have to factor in the fact that in general many baby tuna at that age die naturally.
But that is probably way too high a figure, Porch said in an interview.
Instead of 20 percent of baby tuna being harmed, more recent analysis yet to be published said it should be 11 percent or maybe even 5 percent, he said. Those figures should be reduced even more for the overall future population of tuna, down nearer to 2 percent.
At most that number should be 1 in 9 or even in 1 in 20 deaths of baby tuna, and that's only the effect on one year for the long-lived tuna.
Those smaller figures are based on larval surveys that have not been released publicly because of a potential court case with BP over damages from the spill, and more simulations "that are conditioned on real data," Porch said.
Porch said it's unlikely that the effect on tuna stock would hit 4 percent and "it is not an additional major source of stress" on the overall population of the bluefin tuna in the Gulf. Other work on baby tuna health will be published in peer reviewed science journals.
But that's only the young. So far NOAA doesn't know how the spill affected adults and whether adults of all ages were killed or made infertile in massive numbers that could have a bigger effect on the overall population than the oiling of one year's worth of young, Porch said.
Boris Worm, a fisheries professor at Dalhousie University in Canada who has warned of problems with tuna populations in the past, said the NOAA figures are within the yearly variations of mortality for tuna.
"So it will be a bad year, but not a catastrophic year," Worm said. "This wouldn't push them over the brink."
Former NOAA chief scientist Sylvia Earle, a renowned ocean explorer who has campaigned against overfishing of tuna, isn't convinced that bluefin tuna weathered the oil slick.
"I think it's too early to celebrate a possible greater survival than had been predicted. These are, after all, models," Earle said. "The truth is we don't have enough information to be able to clearly say one way or another what happened to the 2010 class of baby tuna."
Gulf scientists have wondered for months about the health of the bluefin tuna, said Larry McKinney, executive director of the Harte Research Institute for Gulf of Mexico Studies at Texas A&M University in Corpus Christi.
"They are sentinel species that gives us an idea of the health of the open ocean, where we don't know a lot," McKinney said.
NOAA's status of the bluefin tuna: http://tinyurl.com/bmwoauf
NOAA's Southeast Fisheries Science Center: http://www.sefsc.noaa.gov
Gulf summit: http://www.sgmsummit.org
Harte Research Institute for the Gulf of Mexico Studies: http://www.harteresearchinstitute.org