Millions of river herring used to return to New England's fresh waterways to spawn, but at some collection spots today, populations have dropped into the dozens.
Blueback herring and alewife are the two species commonly called "river herring."
"I often characterize them as the field mice of the ocean," said Steve Gephard, a fisheries biologist with the Connecticut Department of Energy and Environmental Protection. "Everything eats them. The ospreys. The eagles. The striped bass. The river otter."
River herring live in the ocean, but return to freshwater to spawn. Populations spiked in the 1980s thanks to regional river restoration projects, but plummeted since then.
At one collection spot in Massachusetts, numbers peaked at 630,000 fish in 1985. But for each of the past 15 years, that number hasn't exceeded 2,000, and in some years, it's down to the dozens.
"Even though that habitat was still available. Even though there were more fish passage projects initiated," said Gephard. "And so that tells us that whatever is driving this population down is not due to the conditions in the river, it's something else."
The problem is, oceans are complex. There's probably no single smoking gun, but there are plenty of theories.
One, Gephard said, is that river herring hang out with Atlantic herring and are a frequent bycatch of offshore fisheries.
Another theory says food chains in the ocean have shifted.
And then there's the fact that river herring just move around a lot, migrating thousands of miles across fresh and salt water.
While the science continues, conservation efforts stay in place. Connecticut just reinitiated its ban on the taking of alewives and blueback herring from inland marine waters -- a ban that's been in place since 2002.
"If we let these forage species crash and go down to insignificant numbers, then all of the other species -- whether they be fish or birds or wildlife -- those species will also suffer," said Gephard.
Still, there are signs of hope. Gephard said river herring are doing better in the Gulf of Maine.