Rainbow smelt were once found as far south as Chesapeake Bay, and were particularly abundant along the Gulf of Maine. Now, the small, tasty fish has disappeared from Connecticut south, in a depopulation so rapid that researchers were too late to pinpoint an exact cause.

A similar trend is occurring in Maine, where the smelt numbers are dropping around Kennebec County and southern Maine. Their decline could be a sign that something is wrong with the water, and the commercial, recreational and ecological impact of their loss would be significant.

The state should continue its work surveying the smelt population, looking for factors in common with those in the areas where the fish are no longer found. And communities throughout the state need to address the changes resulting from development that may be affecting the ecosystem smelt inhabit.

Smelt have a long history along the East Coast, but for the last three centuries, their southern edge has been moving steadily north.

First, smelt disappeared from the Mid-Atlantic states, then, by the 1940s, from New Jersey. The 1980s and ’90s marked the end for the fish in Connecticut and New York, both of which were once the sight of robust commercial fisheries.

Smelt runs are now quiet in Massachusetts, as well, and there has been a noticeable dropoff over the last 40 years in Maine, except Down East, where populations appear more stable.

There are many possible reasons for the decline. In the 19th century, overfishing and industrial pollution played a role. More recently, dams and poorly made culverts have been blamed for blocking spawning areas. Coastal development has increased runoff and harmed water quality in a number of different ways. Rising water temperatures also may play a role.

In any case, the disappearance has implications for Maine, where smelt play an important role.

Smelt are an anadromous fish, which means they live in the ocean but migrate up rivers and streams to spawn. For generations, Mainers along these waterways have gathered to catch smelt, for fun, for bait, for their own food and for profit.

At the same time, smelt are important as a food source for many of the state’s cherished game fish, which will suffer as the smelt population drops.

Finally, the decline of the smelt could be a sign that water quality along the rivers and streams they use is reaching a dangerous point, when it will start to harm the ecosystem as a whole.

Maine has to figure out how to reverse the decline of the smelt, or they’ll go away, and take years of tradition — and probably much more — with them.