River herring and eels, at first glance, don’t seem to have much in common. Eels spawn far out in the ocean but swim to freshwater rivers to live out most of their lives. River herring spawn in freshwater rivers, but spend most of their lives in the ocean.

And river herring — alewife and blueback herring — look like fish, while eels look like snakes, even though they are fish.

What they have in common isn’t good.

Both were once amazingly abundant, both in the Bay watershed and along the East Coast. River herring migrated upstream by the tens — sometimes hundreds — of millions. Eels once accounted for a quarter of the fish biomass in the headwaters of many streams.

Now, both are under review to see if they warrant protection under the federal Endangered Species Act.

In May, the Atlantic States Marine Fisheries Commission, which manages species that migrate along the East Coast, released new stock assessments that confirmed the poor condition of both species.

The assessments found stocks of both river herring and eels to be “depleted” with populations at or near all-time lows, and continuing to decline in many places.

River herring catches peaked at more than 60 million pounds a year in the late 1960s, mostly because of large, offshore fisheries, and have since declined 93 percent. Stocks have persisted at historic low levels in most places in recent decades.

River herring harvests were halted as of Jan. 1 in most places, including the entire Bay, to comply with new ASMFC regulations. But the stock assessment expressed concern that other offshore fisheries may still catch large numbers of river herring as bycatch.

Annual eel catches peaked at more than 3.5 million pounds along the coast in the late 1970s, and have declined to less than 1 million pounds a year recently. Most of the catch provides bait for other fisheries, including crabs and striped bass.

The stock assessment expressed concern about continued fishing pressure on all ages of eels, but worried particularly about a likely uptick in poaching “glass eels” — small translucent eels that first reach the coast months after spawning — to supply aquaculture markets in Asia, where they reportedly bring $1,000 a pound. Poaching is thought to be a “serious problem” but enforcement of regulations is poor, the report said.

Because river herring and eels have been low-priority species, they also share a legacy of poor data.

River herring, which live most of their lives in the ocean but return to their native rivers to spawn, have historically been managed as individual river stocks. Of more than 200 river-specific stocks along the coast, assessment information was available for only 52. Of those, 23 stocks were declining, one was increasing, and there weren’t enough data to determine the status of the other 28.

In the Bay, the assessment found too little information to establish trends in any Virginia tributary. In the Maryland river with the best data, the Nanticoke, stocks were considered to be depleted and declining. In the Potomac, stocks were considered depleted, but no trend could be established.

Unlike river herring, eels are considered to be a single stock because they spawn together in the ocean before migrating into rivers where they live most of their lives.

Reliable data about eels is also poor, the assessment said, but it concluded that “the prevalence of significant downward trends in multiple surveys across the coast is cause for concern.” Eel numbers in the Chesapeake show no consistent trend, it said.

The assessments classified river herring and eels as depleted, rather than overfished in part because of such poor data, said Kate Taylor, fishery management coordinator with ASMFC. “There is a lot of uncertainty in the information that we have, and we see these other stressors that are impacting the populations beyond fishing.”

Both species have lost large amounts of habitats because of dam construction. Eels, which can slither around some dams, often suffer large rates of mortality when they are full-grown and trying to migrate through power turbines. A nonnative parasite infecting eels may also be affecting survival, and there are concerns that changing ocean conditions may be affecting the distribution of eel larvae along the coast.

Likewise, dams and a number of other factors are likely affecting river herring, including changing ocean patterns; predation, including consumption by nonnative species; water withdrawals; ocean bycatch; and habitat losses.

Nonetheless, the assessments represent a bit of a milestone, Taylor noted. The data in an earlier eel stock assessment were so poor the assessment didn’t survive peer review. The last river herring stock assessment, completed more than two decades ago, had data for only a dozen rivers. She said both assessments were “starting from square one.”

Both eels and river herring are under consideration for listing as threatened species under the federal Endangered Species Act. If they are listed, federal rules would drive much of what happens with them.

But, Taylor said, “if the listings don’t occur, we still want to make sure that we have management in place that will reduce mortality and not contribute to any further declines in the populations.”

The ASMFC, which this year closed most state river herring fisheries, is working with regional Fishery Management Councils, which regulate catches in federal waters more than 3 miles offshore, to monitor and reduce bycatch of river herring in other fisheries.

Management recommendations for American eels will be made at the next ASMFC meeting in August. Those could include proposals for further curbs on fishing.

The river herring stock assessment and supporting information is also going to the National Marine Fisheries Service, which is conducting the endangered species review. A recommendation is expected later this summer.

The eel assessment may also be used by the U.S. Fish and Wildlife Service, which is undertaking that endangered species review. Its decision is not expected until after the river herring decision.

If they are listed under the act, they would join Atlantic and shortnose sturgeon as the only other native Bay species on the list.

American Eels

American eels spawn in the Sargasso Sea in the Atlantic Ocean. Their larvae are carried by currents to the East Coast, where they transform into unpigmented “glass eels” as they near land, gradually becoming pigmented “elvers,” then yellow-greenish “yellow eels” for most of their lives, then becoming dark “silver eels” before they migrate back to the Sargasso to spawn. They live most of their lives in estuarine and river waters, and those that migrate into the most distant headwater streams can live to be 30 years old, attain lengths of more than 3 feet and weigh more than 8 pounds. Eels die after spawning.

Although their range extends from Greenland to the northeast coast of South America, their abundance has fallen dramatically at the northern end of their range.

The sex of eels is not determined until they are several years old, and eels that move farther upstream in rivers, and farther to the north, tend to disproportionately become females. The loss of northern eel populations, and obstructions to migration in rivers, has raised concern that the number of spawning females in the population might be reduced.