I don’t want to upset the good people of the quiet Howard County hamlet of Elkridge, but a blood-sucking creature got into your community the other day.

I saw it with my own eyes, a sea lamprey — the eel-like parasitic fish with a round mouth and bristling teeth that attaches itself to other species and sucks the life out of them.

The one I saw had not selected a victim yet. It was unattached, a black ribbon of slithery life in the middle of the Patapsco River, downstream of the Elkridge Furnace Inn.

I only knew what it was because of the state biologist in brown chest waders standing to my left on the high bank of the river.

“There goes a sea lamprey,” William Harbold said.

“Sea lamprey?” I said. “What’s he doing here?”

“Spawning.”

Ponder that: Sometime in March or April, a lamprey in the vast Atlantic Ocean suddenly felt compelled to swim hundreds or even thousands of miles into the Chesapeake Bay and then all the way up the bay, past the fallen Key Bridge, into Baltimore waters, then up the Patapsco, under highways, roads and railroad bridges to Elkridge.

And all that to procreate, then to die.

You’ll have to excuse me. I did not know sea lampreys were migratory. I did not know they made the long journey to the freshwater of the Patapsco. I would never have put those two things together.

I am still amazed at the spring migration of fish — even the ugly, blood-sucking kind — from sea to streams in urban and suburban areas.

I have an interest in matters piscatorial, having fished from the deep sea off New England to the trickly brooks of the Appalachians. I have spoken with fisheries biologists numerous times, and I remain curious about the Patapsco now that three of its dams have been removed, allowing more fish to migrate and spawn in waters that their ancestors were blocked from reaching. I have shared that good story with readers, and I keep coming back to it.

Herring and shad have returned to the Patapsco, and, as The Sun’s Christine Condon reported last year, American eels have been swimming again in big numbers all the way from the Atlantic’s Sargasso Sea to the last remaining dam at Daniels, west of the Beltway.

But I did not expect to see a sea lamprey.

Here’s the thing: Decades ago, long before I landed in the Queen City of the Patapsco Drainage Basin — before the dams were constructed to create power for industry, back when the world was young and the waters not yet debased with human waste, two centuries before anyone put the words “climate” and “change” together — people who inhabited these parts were quite familiar with the spring migrations of fish. They saw them, caught them, ate them, salted them and used them for fertilizer.

Herring came into the tributaries of the Chesapeake in huge numbers. Same for the nation’s “founding fish,” the hickory and American shad. The water and air around here were cleaner and cooler in those days.

Then came the Industrial Revolution and dams to power mills that produced cornmeal and flour, lumber, paper and cotton. The dams powered foundries and forges, too. Communities sprang up around them. The Patapsco Valley was a busy place ; the river made it so. The owners of mills in places like Ilchester, Oella and Ellicott City prospered. The region grew.

And the migratory fish disappeared from where they had been common; they could not reach their ancestral spawning grounds because of the dams.

But the decades passed and, in time, the dams went away.

The Avalon Dam that once powered an ironworks blew out with Agnes, the deadly tropical storm that flooded the valley in 1972. Four decades later, the obsolete Simkins, Union and Bloede dams were removed, and for the last 10 years or so, biologists with the Maryland Department of Natural Resources have been watching to see what creatures show up in the liberated waters.

Three springs ago, Harbold and another DNR biologist, Jim Thompson, recorded an alewife, a cousin to the Atlantic blue-back herring, about three miles from Ellicott City and just upstream from where the Bloede Dam had been. It was a big deal, finding that silvery fish.

I declared it “Obama’s herring,” because the federal funds to start the dam-removal project came from the American Recovery and Reinvestment Act, the $787 billion stimulus package that President Barack Obama and fellow Democrats pushed in 2009 to help pull the country out of the Great Recession. (Not a single Republican in the House of Representatives voted for it, and only three Republicans in the Senate joined the majority.)

Since then, state biologists have taken to their survey raft each year to keep a record of fish at different points on the Patapsco. This spring the DNR surveyors found hickory shad and plenty of herring but none, so far, upstream of the old Bloede Dam site, as in the past three years.

Climate conditions have certainly affected when, where and how far herring, shad and other fish migrate.

Maybe it takes time for the word to spread among the herring that the dams are gone and the Patapsco looks more like it did when their ancient ancestors migrated throughout the river. Maybe the silvery fish are just waiting for the right conditions, a little rain or certain air temperature. Maybe they’re worried about running into that sea lamprey. We don’t know yet.