Other creatures, like sea otters and intricately patterned harlequin ducks, are only now approaching recovery. Otter and duck biologists had braced themselves for large death tolls, because animals that rely on fur or feathers for warmth are susceptible to hypothermia after oil exposure. They can also suffer immune suppression and organ dysfunction from ingesting oil via grooming. But when populations failed to rebound after 12 years, Esler and Bodkin were baffled. They began scrutinizing the animals' underwater routines, eventually learning that the invertebrates they eat in the intertidal zone, like clams and mussels, were full of oil. Invertebrates don't metabolize oil as well as vertebrates, so otters and harlequins were ingesting it long after the spill had been cleaned up.

Though the mechanism of injury for some species has become clear, the collapse of Prince William Sound's herring fishery remains a mystery. Today, many researchers say herring – tiny silver fish eaten by nearly everything – are the species most impacted by Exxon Valdez. Unfortunately, the fishery collapsed during a lull in funding "when no one was looking," laments Scott Pegau, coordinator of the Herring Research and Monitoring program, run by the Prince William Sound Science Center with funding from the Oil Spill Trustee Council. Industry-funded scientists therefore argue that it can't be linked directly to the spill.

Pegau suspects that oil exposure suppressed herring's immune systems, leading to a disease outbreak, or that a dearth of zooplankton after the spill left the fish malnourished. Whatever the cause, the economic impact still stings in fishing villages like Cordova, where herring's boom-and-bust population cycles have historically mirrored those of Sitka, Alaska. Yet for the past 20 years, Cordova fishermen have been sitting on their hands while the Sitka fishery remains productive. "Herring used to be what woke Cordova up from its winter slumber," Pegau says. "Now we don't get going until May."

This year, though, there's cause for hope: Pegau counted 2,100 schools of juvenile herring in 2013, compared to 150 three years ago. Fishermen who've held herring permits for generations stop him on the street to ask when they can fish. His answer is cautious: "We won't know for sure for a couple more years."

Exxon Valdez also left some positive legacies. The Oil Pollution Act of 1990 requires single-hull tankers to be fully phased out by 2015, a goal that's nearly achieved, says Betty Schorr of the Alaska Department of Environmental Conservation. And regulations "aren't hypothetical anymore," she adds: Alaska's standard response plan now spans three days (the window of good weather after Exxon Valdez) and crude tankers must be capable of dealing with an Exxon-sized spill to operate in state waters.

But Alaskan jurisdiction extends just three miles offshore. After that, responsibility falls to the Coast Guard, which allows ships in western Alaska to circumvent certain regulations because the emergency response infrastructure needed to comply with them simply doesn't exist there. That's exactly why environmental watchdogs think extra precaution is warranted: As the ice-free season grows longer, more international tankers are passing through Arctic and Aleutian waters without stopping at Alaskan ports, where they'd be subject to state laws. Plus, given the remoteness and extreme weather, a slip-up – either from a shipping vessel or the drilling proposed by Shell – could be disastrous. "The Arctic is a grave concern of ours," Schorr says.

Prince William Sound communities may be the nation's best prepared to deal with environmental disaster, with extra precautions like satellite monitoring of fully laden tankers and "escort ships" to flank them, as well as programs like Gulf Watch that provide a solid understanding of the marine environment. Yet many of the lessons learned there aren't being applied elsewhere. "Deepwater Horizon was a great example," Bodkin says, referring to the 2010 Gulf of Mexico disaster. "A lot of us that worked on Exxon Valdez were frustrated because we felt the mistakes we made were simply repeated 20 years later."

The most significant, Bodkin says, was the lack of baseline data. "That was our primary recommendation: Where oil and gas is developed, extracted or transported, we need to have a good understanding of the ecosystem prior to a spill." Monitoring was almost as inadequate in the Gulf of Mexico in 2010 as it was in Prince William Sound in the '80s, Bodkin adds, making it harder to assess damage. "The yardstick we have to measure recovery is really hampered by a lack of knowledge of what the system was before."

In Alaska, at least, the message has partly sunk in: Oil exploration in the Chukchi and Bering seas has prompted more robust research. "We know far more about how (those places) operate, from both the physical and biological side, than we knew about Prince William Sound," says Tom Weingartner, an oceanographer at the University of Alaska Fairbanks who works in both locations.

That's reassuring for Dave Janka, who's constantly reminded of the Exxon Valdez as he drives researchers to field sites in his charter boat. Every year, Janka revisits some of Prince William Sound's remote beaches to turn over rocks and photograph residual oil. And every year, he finds not only the gooey substrate left over from Exxon Valdez, but also tiny, hardened globs of oil from the 9.2-magnitude earthquake that ruptured onshore oil storage tanks and killed 115 Alaskans in 1964.

This month, on the 25th anniversary of the spill and the 50th anniversary of the earthquake, Janka will be prepared: "I'm going to go camp out on top of a mountain," he jokes, "and wear my lifejacket and hardhat to bed."

Krista Langlois is an editorial fellow at High Country News.