AS DAMAGED as the Chesapeake Bay is from centuries of pollution and exploitation, the only species that called these waters home in Colonial days has declined to perhaps the point of no return.
That would be the sturgeon, both shortnose and Atlantic.
The larger, more numerous Atlantics were the dreadnoughts of the Chesapeake in the spring as they lumbered up every significant tributary to spawn. They reached as far upstream as Washington on the Potomac, and well into Pennsylvania on the Susquehanna.
Living the better part of a century and reaching nearly 15 feet and 800 pounds, with long, whiskered snouts and five rows of armored plates down each side, they're the closest candidates for the proverbial sea monster, "Chessie."
On the Potomac and the James, they were abundant enough to support a caviar industry. Their air bladders made isinglass for windows in early automobiles. During the American Revolution, a leaping sturgeon landed on a soldier rowing near Georgetown. He died of complications from a broken thigh.
The bay never had millions of big sturgeon, which don't reproduce every year and take up to 14 years to reach spawning age. Peak spawning populations were more likely in the tens of thousands, scientists think.
A century has passed since they were plentiful in the bay, and there's little sign of continuing reproduction, except sporadically in a few Virginia rivers. Genetic evidence indicates they are wanderers from the Delaware or Hudson rivers, both of which have viable populations.
But in recent years, mainly through the work of University of Maryland scientist David Secor, the sturgeon has emerged from obscurity to influence how high we set the bar for restoring the modern day Chesapeake.
The sturgeon has become the bay's equivalent of the coal miner's canary - the little bird once taken into mines as an exquisitely sensitive, early warning against leakage of deadly gases.
Big as a log and once compared by Secor to the dodo bird, the ancient sturgeon doesn't immediately seem a sensitive soul.
But in fact, Secor has found the fish is quite "oxyphilic," meaning sensitive to low levels of oxygen in the water. In its need for oxygen, the sturgeon is less like other Chesapeake fishes and more like the rainbow trout that inhabit oxygen-rich, rushing, cold water streams.
Meantime, declining oxygen has become one of the bay's biggest pollution problems. Nutrients running into the estuary from sewage, farms, polluted air and urban pavements all promote too much algae, which use up oxygen as they die.
Some summers, as much as half the bay's volume doesn't have enough oxygen for healthy marine life - and that's using the long-accepted standard of 2 parts oxygen per million parts of water as the minimum.
For sturgeon, 2 ppm can be lethal. They need at least 50 percent more oxygen, and in some cases, more than double that proportion. Worse, they are highly adapted to swim and feed along the bay's bottom and deep channels, which is precisely where the oxygen level is consistently lowest.
They also need cooler water than many other bay species. So in hot summers they're "squeezed," Secor says, between well-oxygenated surface waters that are too warm, and cooler bottom waters with too little oxygen.
The squeeze is even worse for young sturgeon, which can't tolerate the bay's saltier regions, shrinking even further the number of locations where the fish can survive. In some years, Secor says, the combination of heat, low oxygen and salt can diminish prime sturgeon habitat to virtually nothing.
Currently the state-federal restoration program for the Chesapeake is setting new water-quality standards for oxygen and other factors, trying to decide how protective these must be to bring the bay back to something like the health it enjoyed in the 1950s.
The stakes are large. Cities and farms in the bay's six-state, 64,000-square-mile watershed will have to invest billions to stop overfertilizing bay waters, and the degree of cleanup depends on what oxygen levels are required for fish health.
With the sensitive sturgeon in mind, bay managers have tentatively bumped up minimum oxygen levels significantly, and arguments continue as to whether they need boosting even more.
Ironically, while all bay species will benefit from higher oxygen levels, Secor says he is "more pessimistic than I used to be" about a sturgeon renaissance.
The fact that there's been no real comeback in more than half a century, despite minimal fishing, makes him wonder whether even the 1950s bay was too habitat-limited for sturgeon, he says.
Besides cool, well-oxygenated water, sturgeon need clean, rocky bottoms for successful egg attachment when they spawn. "The Chesapeake probably always had very little of it, and a lot of that is silted over," he says.
But Secor also knows enough about sturgeon to know how much more there is to learn: "I could be wrong," he adds.