The Chesapeake Bay is only about a quarter as healthy today as it was when Capt. John Smith and his colleagues settled here nearly four centuries years ago — but it is in better shape than it was only a few years ago, according to a new analysis.

On a scale of 100 — 100 being what John Smith saw — the Bay rates 27, according to the Chesapeake Bay Foundation.

The number, which the environmental group plans to update annually, is the average of 12 indicators that reflect the health of fish stocks, levels of pollution and conditions of habitats in the Bay.

“We don’t expect the Bay to be, ever again, 100,” said Mike Hirshfield, CBF vice president, “but we think it can be more than it is. Somewhere in the 70 range would seem like an ambitious, but doable, goal.”

Hirshfield said while the index indicates the Bay is severely degraded, the Chesapeake nonetheless has improved over the last 15 years as state and federal governments began cleanup efforts. At that time, beds of Bay grasses and striped bass populations — two of the indicators — were at record lows. In the early 1980s, Hirshfield said, the index would have been in the low 20s.

The CBF’s analysis may be the first time anyone has attempted to use a single number to rate the Bay’s overall health. The state-federal Chesapeake Bay Program uses a variety of indicators to gauge different aspects of the Bay ecosystem, but does not attempt to rate its overall condition.

Some of the CBF’s scores are straightforward: Bay grasses get a 12 because they cover only about 12 percent of the area they once occupied in the Chesapeake, and oysters are a 1 because they are at about 1 percent of their historic level.

Others are more subjective. Toxics, for example get a 30 out of a possible 100, though scientists and managers for years have debated the degree to which the Bay has a toxics problem. Monitoring efforts have found low levels of toxic chemicals throughout the Bay, but only three areas — the Anacostia and Elizabeth rivers and Baltimore Harbor — are considered to have severe problems.

“We don’t have a Bay that is entirely Baltimore Harbor, by any means,” Hirshfield said, “but we would argue that if you look at just the status of trace elements everywhere, toxicity everywhere, a lot of real hot spots — this is a Bay that has come a hell of a long way in the wrong direction from where it was in John Smith’s time.”

Likewise, while striped bass are at near record levels in the Bay — at least for recent decades — they only score a 70. That is because of concerns that many fish are not healthy, perhaps because of problems in the Bay’s food chain, and that the population as a whole does not yet have as many “old” fish as fisheries biologists say a healthy population should contain.

Because the CBF’s index is based on 12 indicators, simply improving one factor would not significantly increase the Bay’s overall score. “Some people complained that the index is too hard to move,” Hirshfield said, “but that reflects the reality that the system that we’ve got is pretty hard to move, and that if you really want to move it as a system, it’s going to take time.”

In fact, Hirshfield said, restoring the Bay to an index of 70 may take decades. Even that, he said, probably depends on a helping hand from Mother Nature — not just restoration efforts. Some scientists have suggested that as the Bay is cleaned up, it may eventually help to restore itself: Increased populations of oysters or expanded grass beds, for example, would filter the water on their own.

But no matter how much is done, the index will never hit 100, because a pre-settlement Bay, with a largely forested watershed and a small human population, will forever be out of reach for a watershed that today supports more than 13 million people and has had much of its land converted to other uses. The 100 mark, Hirshfield said, is to help people “keep in mind what we we’ve lost and what we’ve given up for civilization.”

“We’re not going to diminish efforts to move forward to a Bay that is significantly better than it is today,” he said, “but we have to keep in mind that it is always going to be a compromised system.”

State of Bay Health Index

Habitat

Wetlands 43

Forested Buffers 53

Underwater Grasses 12

Dissolved Oxygen 15

Pollution

Toxics 30

Water Clarity 15

Phosphorus 15

Nitrogen 15

Fisheries

Crabs 50

Rockfish 70

Oysters 1

Shad 2

Average 26.75