PROVIDENCE — To swim up the lower part of the Woonasquatucket River, past several dams, spawning herring must traverse a series of fish ladders before they can reach their breeding grounds upstream.

The ladders gave herring access to areas that had been inaccessible to them for hundreds of years. But measuring the effectiveness of these structures isn’t easy.

So many factors impact the fish that experts and stakeholders who spoke to ecoRI News said knowing the fish ladders’ specific impact on herring population is tricky.

According to data tracked by the Rhode Island Department of Environmental Management, the river herring population waxes and wanes somewhat independent of when fish passage structures were built.

Although populations rose in the Woonasquatucket and Ten Mile rivers, near Gilbert Stewart Mill — off the Narrow River — and in Nonquit Pond after fish passageways were introduced on the dammed waterways, in the past few years populations declined sharply.

“What’s tricky about that is there are a lot of sorts of inputs that have an impact on those fish numbers,” said Patrick McGee, principal fisheries biologist at DEM.

Alewives and bluebacks, the two types of herring found in Rhode Island, are anadromous fish, McGee noted, “so they live most of their lives out at sea, and then they come into the rivers and to the pond and the lakes to spawn.”

Herring tend to move from the ocean to fresh water in March, but can start swimming upstream as early as February. The peak of their season is usually in May and it dwindles into June.

The fish prefer the larger lakes and ponds that exist upstream, habitats that the Industrial Revolution and creation of dams cut off for centuries.

The damming of the rivers led, in part, to historic low herring populations everywhere the fish lived and spawned by the early 2000s, McGee said, which caused stakeholders to push to bring them back.

“What we’re able to concentrate on is getting the habitat open for those fish,” he said, whether that’s been full or partial dam removals, creating natural “fishways,” or installing fish ladders in the past 25 years.

But getting the fish past the dams is only one obstacle.

Herring are commonly caught as bycatch in the Atlantic Ocean. And when they finally get to ponds upstream, they often become prey for bigger freshwater fish.

“They get hammered,” McGee said. “Everything feeds on them, which is why they’re so important.”

The way John O’Brien, policy and partnership specialist at The Nature Conservancy of Rhode Island, described it, the fact that any fish come back to their spawning grounds at all is a miracle in and of itself.

Telling the story of the fish from egg to juvenile to adult, from pond, to river, to high seas, “I mean, it’s incredible, 200 miles of ocean,” O’Brien said. “They’re an amazing creature.”

O’Brien started his career as a laborer in a DEM fishery in 1960s, working his way up the ladder by helping fish get up ladders themselves, eventually retiring as an assistant director in 2006 before joining TNC to help with fish passage projects.

He said that over the decades the approach and effectiveness of the ladders has changed tremendously, usually with lots of trial and error.

The most common type of ladder, called a Denil-style fish ladder, is a Scandinavian structure originally designed for salmon. It’s made of concrete and wooden baffles that form something that looks like underwater steps.

The state started constructing those fish ladders in the ’60s, O’Brien said, but by the early 2000s, when herring populations were at their lowest, scientists knew much more about fish behavior, particularly that the force of water a salmon could endure upstream was much stronger than what a herring could handle.

So, the state and other stakeholders modified the Denil ladders, decreasing their slope to make it easier for the herring to pass through. They have also installed different types of passageways, including pond and weir ladders and “nature-like fish passage facilities,” like the one installed at Bradford Dam in Westerly, which was basically a set of eight small dams that replaced one larger one, to help fish get up the river.

O’Brien said TNC works with the fish engineering team at U.S. Fish & Wildlife Service, who can offer suggestions about a fish ladder’s flow rate to where the entrance of a ladder should be placed.

“They’ll say, this isn’t going to work, because the way they designed it, the entrance to the ladder is too far downstream of the obstruction, which is the dam, and the fish will never find it,” he said. “They’ll swim right by it, right up to the dam.”

O’Brien’s colleague Heather Kinney said many of the fishways in Rhode Island have been modified, including all of the structures in the Saugatucket River watershed, where she’s been studying herring movement and fish passageway effectiveness.

Kinney, TNC’s coastal restoration program manager, tagged hundreds of fish and tracked their movements past different antennae placed throughout the area over a three-year period.

She is still parsing through some of the data, but she was able to see how the fish move through the human-made systems, a progression that one might assume is linear but is often not straightforward.

“Sometimes they hang out at that antenna, pass back and forth 10 times in 10 minutes. Sometimes they move up almost all the way and then all the way back down,” Kinney said.

Those variations can be natural, like to avoid a predator, but they can also be caused by an inefficient fish ladder design, she said.

“We want to make sure that this man-made structure is as easy to pass as possible, because they’re already trucking all the way up this river,” Kinney said, “and the more time that they spend in the space, the less energy they have to spawn.”

Kinney noted dam removal is the best option for the fish but it’s not always possible, either because a dam is still in use or because it would be too costly or hazardous to remove.

“Methods, you know, depending on the fishway, are better or worse, but at least it’s better than nothing at all,” she said.

McGee, who started at DEM as an intern right as some of the fish passageways were opening on the Woonasquatucket River, said he sees the movement of any herring upstream as a victory.

“I remember the first year of the Woonasquatucket being open, going down to those fishways after, again, no fish had gotten upstream there for a century,” he recalled. “In that first year, when all of those fishways opened, we saw fish coming up, and so, I mean, that’s a pretty huge success.”