The oil tanker Exxon Valdez on the rocks/Wikimedia Commons

Almost 30 years after the oil tanker Exxon Valdez hit Bligh Reef and smeared Prince William Sound with more than 11 million gallons of Alaska crude oil, a team of state and federal scientists have concluded the spill – as bad as it looked and as much impact as it had on marine mammals and birds – appears to have done no real damage to fisheries.

“We found no evidence supporting a negative EVOS  (Exxon Valdez Oil Spill) impact on herring, sockeye salmon, or pink salmon productivity, and weak evidence of a slightly positive EVOS signal on Copper River Chinook (king) salmon productivity,” the study says. “It is unclear how EVOS may have impacted Chinook salmon positively.”

Somewhat surprisingly, however, the study found two non-oil spill events – one natural and one manmade – that appear to have caused significant changes in Sound fisheries. And one of them, a naturally occurring spill of fresh water, appears to be what crippled herring stocks there.

Exxon has long been blamed for the collapse of herring, which once supported a fishery worth $6 to $11 million per year, but the new study funded by the Exxon Valdez Oil Spill Trustee Council, a government entity established to track the Sound’s recovery in the wake of the spill, fingers fresh water as the real culprit.

“Herring productivity was most strongly affected by changing environmental conditions,” the study says, “specifically, freshwater discharge into the Gulf of Alaska was linked to a series of recruitment failures—before, during, and after EVOS (Exxon Valdez Oil Spill).”

And the other big factor driving significant environmental changes in the Sound?

The annual spills of hundreds of millions of pink salmon fry.

When it came to salmon stocks, the study said, the search for the fingerprints of oil found “little overall support for an oil spill effect. Of the salmon species, the largest driver was the negative impact of adult pink salmon returns on sockeye salmon productivity…These results highlight the need to better understand long terms impacts of pink salmon on food webs, as well as the interactions between nearshore species and freshwater inputs, particularly as they relate to climate change and increasing water temperatures.”

An invisible study

The peer-reviewed study was published by the Public Library of Science (PLOS) in March. PLOS was established in 2000 to ensure free and open public access to science that has undergone thorough professional review.

The study – “Evaluating signals of oil spill impacts, climate and specific interactions in Pacific herring and Pacific salmon populations in Prince William Sound and and Copper River, Alaska” – has gone unreported in popular media.

Some of the authors when contacted said they have no idea why, although one suggested it might be because the work too closely parallels the findings of a study funded by Exxon almost 20 years ago.

“A study commissioned by the Exxon Corporation has found that the Exxon Valdez oil spill is unlikely to cause long-term harm to fish and other organisms in Prince William Sound and has drawn immediate fire from Alaskan officials,” the New York Times reported in April 1990.

That study by Jerry Neff of the Battelle Memorial Institute was as the time widely “pooh-poohed,” as one of the scientists who worked on the latest study observed. Part of the reason the Trustees Council ordered another look at fish was to determine the accuracy of Neff’s findings.

In the immediate wake of his work, National Oceanic and Atmospheric Administration scientists conducted what they described as “a painstaking experiment” to show that even very low concentrations of oil in water could cause deformities in pink salmon embryos that would damage their chances of growing into adult fish and returning to spawn. 

Thus there was cause to believe Neff’s examination might have missed subtle impacts from the 1989 oil spill.

What the new study concluded is that while those probably existed, whatever damage they did failed to show a discernible influence at the population level. Part of the reason might be that nature is in large part a dance of death. University of Washington scientists studying pink salmon found that more than 87 percent of the fishes’ eggs died before becoming fry, 65 percent of those that made it to the fry stage died within 40 days, and 85 percent of the fry died before becoming adults able to return and spawn.

The same scientists found that if a wild Chinook (king) salmon spawned 5,122 eggs, 3,345 would become fry – a much better survival rate than for pinks. But only 336 would make it from there to the smolt stage, and of those smolt only eight would live to return as adults. Less than two-tenths of a percent of the eggs survived to become adult fish.

Given this, Rich Brenner, an Alaska Department of Fish and Game biologist involved in the Exxon study, said it becomes easier to understand how whatever damage the oil spill might have done to immature fish gets lost in the statistical noise and becomes biologically invisible.

He and the other government scientists, he added,  looked long and hard for any evidence that would enable them to say the oil spill had done longterm damage to Prince William Sound and Copper River salmon stocks.

“It just wasn’t there,” he said.

The only salmon change scientists could tie to the spill was the positive influence on Chinook numbers, and they simply can’t explain that

“It could be a spurious effect,” Brenner said. “It could be an anomaly…Maybe there was a marine mammal effect there. It’s like everything else in the paper, it’s not mechanistic.”

In other words, scientists know the results, but they don’t know how the results came about. It is possible, for instance, that when the spill reduced populations of killer whales, sea lions and seals – which the spill did – this  in turn lowered predation rates on Chinook and thus Chinook salmon survival increased.

It’s complicated

Marine ecosystems are complex almost beyond belief. The clear interactions of predator and prey that exist in terrestrial systems – where, for example, wolves and bears eat moose, but moose never eat wolves or bears – are muddled by the predatory nature of all fishes.

In the marine environment, predation is both very simple – big fish eat little fish – and incredibly complicated – big pink salmon will not only eat what are classically known as “bait fish” – herring, sandlance, euchaleon – they will also eat the young of their own species, immature salmon of other species, and plankton.

To complicate things even further, the young of all salmon often compete for similar foods, something which has been noted by scientists as pink salmon populations have exploded in the North Pacific in recent years.

“In years of high abundance, (pinks) now constitute a pelagic consumer front as they return to their spawning rivers, exert top-down control over the open-ocean ecosystem by out competing other species for shared prey resources, and drive major ecological shifts between years of high and low abundance,” a 2014 study concluded. “Their effect on competing species must be considered in international conservation policies and when developing informed ecosystem-based management strategies.”

And the oceanic complexity doesn’t end with fish. Toss in predation and food competition from seabirds and marine mammals, sprinkle on climate change, add a dash of human tampering both obvious – the Exxon Valdez oil spill – and less so – Alaska’s annual dump of what is now approaching 2 billion hatchery fish – and scientists are left with a Gordian knot to untangle.

That the consequences of a one-time event, such as the Exxon oil spill, might get as lost as a sunflower seed in a giant, mixed salad is probably to be expected. What might actually be surprising is that other events have enough of an environmental footprint to jump out as significant.

The herring crash

“The strongest relationship between the environmental covariates (possible statistical predictors) we examined and productivity was the estimated effect of freshwater discharge on herring,” the study concluded. “The estimated (herring) productivity was lower than average in years of high discharge. Discharge into the Gulf of Alaska was episodic both before and after the EVOS event, and periods of high discharge generally coincided with three multi-year herring productivity failures: 1985–1987, 1991–1992, and 1996–1998.”

The freshwater comes from glacial melt variations. It is possible, Brenner said, that the accelerated melt is tied to climate change.  It is certain it is tied to climate variability in Alaska. The state has been going through a warm phase since the 1980s as part of a cycle tied to the Pacific Decadal Oscillation (PDO).

“Though we found no evidence relating herring productivity to EVOS,” the study says, “or most climate drivers, we did find evidence of a strong negative correlation between herring productivity and freshwater discharge into the Gulf of Alaska.

“This finding suggests that herring survival may be vulnerable to changing climate conditions which may be affecting herring survival via multiple pathways. Over the past 40 years, the northern Gulf of Alaska has undergone a general warming and freshening in the upper 100 meters (300 feet) of the water column, and an increase in salinity in depths between 100–200 meters (300 and 600 feet).

“This suggests that vertical stratification of the upper water column in the Gulf of Alaska has increased substantially over this time frame. A second effect of warming conditions may be changing amounts of rain and snowpack melted, as well as the timing of the spring discharge. Other studies have found support for increased freshwater discharge suppressing phytoplankton and favoring microbial production.”

The phytoplankton are food for herring. Less food means the Sound supports fewer herring.

How much phytoplankton biomass was lost to changing freshwater discharges is unknown, but the study concluded, “the shift in timing and/or decreased primary production related to increasing water temperatures and water column stability, or increased freshwater inputs may be one of many factors that have kept herring abundances in the north-central Gulf of Alaska low over the past 25 years.”

Hatchery complications

And then there are those hatchery fish. Hatcheries have been a godsend for Prince William Sound.

The Prince William Sound Aquaculture Association, a commercial fishermen’s group, has turned the region – which once supported only a marginal commercial fishery – into an Alaska salmon powerhouse.

When Alaska set a record harvest of 272 million salmon in 2013, about one in every three of the fish was a Prince William Sound pink or “humpie” as Alaskans often call them. The catch of 91 million pinks was more than 30 times bigger than the annual average harvest of 3 million fish a year before hatchery production started in the mid-70s.

The Exxon study, however, raises questions about what the hatchery production of low-value pinks does to the natural production of more valuable sockeye salmon – not only in the Sound but possibly beyond the Sound.

“We do not know if possible deleterious interactions between hatchery pink salmon and wild sockeye salmon in this study are from predation or competition, or whether they occur in nearshore or offshore areas,” the study says. “Pink salmon feeding may cause a general depletion of prey availability that could impact sockeye salmon without tight spatial overlap of these two species. In this regard, the apparent impact to sockeye productivity may reflect a general increase in pink salmon abundance across the northeast Pacific rather than increased abundance of hatchery pink salmon to PWS (Prince William Sound) in particular.”

What the study did clearly find, however, was that when it comes to sockeye salmon in the Sound, a big dump of hatchery pinks resulting in a big return does more damage than a big spill of oil.

“All sockeye salmon stocks examined exhibited a downward trend in productivity with increasing PWS hatchery pink salmon returns,” the study says. “While there was considerable variation in sockeye salmon productivity across the low- and mid-range of hatchery returns (0–30 million), productivity was particularly impacted at higher levels of hatchery returns.

“Pink salmon have been found to negatively affect sockeye salmon productivity and growth from British Columbia and Southeast Alaska, Bristol Bay, Kodiak , and Russia. Pink and sockeye salmon compete in the marine environment due to a high degree of similarity in diets, including similarities in diets of adult pink salmon and juvenile sockeye salmon.”

Brenner described pink salmon as the vacuum cleaners of the sea. Not only do they eat bait fish, he said, adults will continue to eat plankton that could prove vital to the young of other species of salmon.

“Pink salmon are amazing species,” he said. “They need to eat almost anything that swims.”

Because of that, he added, “they have the potential to compete against a lot of other species.”

“Competitive interactions in nearshore and offshore environments deserve greater attention in future research in the face of general increase in the abundance of pink salmon in the North Pacific,” he and the other authors of the Exxon paper wrote. Other scientists before have made similar statements. None of their comments have slowed a steady push for increased hatchery production around the rim of the North Pacific.

It might all be as simple as human perceptions: a huge spill of baby fish to most people looks good while a big spill of oil to everyone looks bad.