No Simple Swim: Tracking a Complicated Year for Migratory Fish (Sea Lamprey & American Shad)
- Jill DeVito
- 7 days ago
- 4 min read
When CRC Aquatic Ecologist Dr. Kate Buckman led a sea lamprey nest survey of the Ashuelot River in southern New Hampshire toward the end of July 2025, she explained to the gathered volunteers and CRC staff that the cobble constructions she had seen in Massachusetts tributaries were not quite up to the lamprey’s typical building standard.
Beyond that, she had seen lampreys still spawning during the late June and early July surveys—which were intended to happen after the nesting season. Our group of community scientists didn’t end up seeing any live lampreys, as it turned out, that day on the Ashuelot. But we found very few freshly built nests compared to previous years, and that made us curious. Was 2025 a “bad year” for sea lampreys in the Connecticut River Valley? If so, why might that be? And how did other migratory fish species fare?
The answer, it turns out, is pretty complex. So I asked Ken Sprankle, the USFWS Connecticut River Fish and Wildlife Conservation Office project leader, to help tease apart what happened this spring and early summer. Ken compiles the fish counts reported at each of a dozen fishways designed to allow passage beyond hydro dams on the main stem and major tributaries of the Connecticut River. And he noted that overall, 2025 was actually a pretty good year for our migratory fish. But there were a few exceptions (including the sea lamprey), so we’ll come back to that later.
Spring 2025 Was Cold and Wet
The region experienced abundant rainfall during key points in the spring migration season, which decreased water temperature and increased river flow levels. As Lael Will, a Fisheries Biologist at the Vermont Fish and Wildlife Department, explains, “This is not a bad thing for fish, as water, and cold water are a good thing as opposed to drought years.” Ken Sprankle agrees that a cold, wet spring like this one may prolong the nesting season, which might even help some migratory species. This year, for example, American shad had a pretty good run.
Better Than Average For American Shad
More than 324,000 American shad passed through the Holyoke Fish Lift in 2025. That may not come close to last year’s banner year over 437,500, but it’s above the long-term average of 320,500 over the past four decades. Those that made it all the way into New Hampshire and Vermont started passing through the Vernon fishway in the middle of May, about a week later than they did last year. We’re looking forward to finding out how many fish ultimately made it past the Vernon Hydro Dam, where the count is still being tallied for shad—as well as for the sea lamprey we searched for in the annual CRC lamprey nest surveys.
A Strange Year For Sea Lamprey
Last year in the Ashuelot, CRC volunteers tallied over 150 lamprey nests, including multiple large community nests. Kate reports that while she is still finalizing the data, this year’s excursion resulted in far fewer, with a preliminary estimate of just over 20 that could be confidently identified as new nests. “Based on our observations of late, messy nests in the MA and CT rivers we survey annually, as well as the reduced number of nests in NH, I am curious as to whether the spring rains and higher flows delayed lamprey enough that they were running out of energy and just didn’t travel as far, or if there were just a lot less lamprey in this year’s migration.” says Kate, “Passage numbers and dates can help tease out some of those year to year patterns as well.”
Fewer than 16,700 sea lamprey were counted at the Holyoke fish lift this year. That’s roughly half the long-term average of almost 32,000, and it’s a dramatic drop from last year’s remarkable total over 53,600. So what’s going on here? We’re not quite sure. One thing we do know is the lampreys that made it past the Vernon fishway arrived about two weeks later than they did in 2024. Lael Will notes that “when we have high flows,” it can be “harder for the fish to find the entrance to the ladder.” But she also points out that “sea lamprey numbers are highly variable and can be influenced by both instream and ocean conditions as well as how the fish ladders perform under varying flows.”
Ocean studies of sea lamprey are few and far between, but the population size in a given year may be influenced at least as much by other factors (for example, the availability of host fish species they feed on in the ocean) as by the conditions in the river when they head upstream to spawn. Not only that, but the lampreys that start their lives in the Connecticut River might end up spawning anywhere else along the Atlantic Coast of North America; and in turn, the lampreys that come here to spawn each year could have been hatched in any other East Coast watershed, making predictions of yearly returns more challenging.
To Be Continued
A large number of ecological factors influence the population size of each generation of a migratory species – from the condition of juvenile habitat to the availability of food in the ocean, to the obstacles we humans throw into their path. In fact, it’s worth noting the same hydro dams that make the upstream (and for some species, downstream) river passage more challenging for our migratory fish provide us with the best opportunity for counting them as they funnel through each fishway. So we take advantage of this opportunity to keep our finger on the pulse of migratory populations. And we’ll be back with more information about how our other migratory species fared this year – the alewife herring, blueback herring, American eel, and shortnose sturgeon.
