No Simple Swim: Tracking Population Declines in Alewife and Blueback River Herring
- Jill DeVito
- Aug 28
- 6 min read
Updated: Sep 5
Meet the alewife and the blueback herring–but don’t feel bad if you have trouble telling these two species apart. The alewife typically has a bigger eye relative to its body size, but even an expert can’t confirm a species identification without cutting the fish open. And these two have a lot more in common than their looks. Together known as “river herring,” the alewife and blueback were once spectacularly abundant here in the Connecticut River watershed. They both spend most of their lives in the Atlantic ocean, and they each got their start in a river.
You may be familiar with other anadromous fish—like sea lampreys and many species of salmon, which migrate to the ocean as juveniles and swim upstream as adults to spawn.
River herring are anadromous too; but unlike the sea lamprey, alewife and blueback herring can survive the upstream spawning journey and swim back downstream. Ideally, a river herring in the prime of its life will return to its saltwater habitat after laying or fertilizing eggs. This round trip is too exhausting for an individual fish to complete every year—but it might return to spawn twice or even three times over the course of its life. In fact, in healthy populations of alewife or blueback, many adults should be doing just that.
But here in the Connecticut River, river herring populations are imperiled. Not many fish are making it back from the ocean to spawn, and only a tiny fraction of those have made the journey more than once. Like our other migratory species, they are challenged by obstacles like hydro dams and habitat degradation in river systems. But alewife and blueback herring may be in even greater danger from the human-made hazards they face in the ocean. Ken Sprankle and his team at the USFWS Connecticut River Fish and Wildlife Conservation Office, along with other members of the Connecticut River Migratory Fish Restoration Cooperative, are trying to help change that.
A Terrible Year for Blueback Herring
I was curious how the river herring had fared during this year’s migration in the Connecticut River, so I turned to Ken Sprankle for help with understanding their situation. Ken compiles annual fish count reports from hydro dam passage facilities to keep track of how many individuals of each species are coming through each fishway on their way upstream. But river herring populations are in such precipitous decline here, he says the fish passage counts “are not really informative to us anymore.”
For example, this year only 144 blueback herring passed through the Holyoke Fish Lift. Thirty-five years ago (before the sudden collapse of the commercial river herring fishery in southern New England), Holyoke passed around 400,000 bluebacks in a typical spring. And alewife herring have not passed through Holyoke at all in recent decades. Both species are “functionally extirpated” from that part of their historic range; they’re just not making it that far upriver anymore.
Fortunately, Ken Sprankle has another way to track the river herring populations. His team spends most of each spring out on the water, using an electrofishing boat to survey for migrating alewife and blueback herring on major tributaries in the southern portion of the Connecticut River valley. But Ken reports that 2025 was not a great year for these fish. “My blueback herring catch rate across all monitored sites in 2025 was higher than the 12-year low of our program last year, but still well below the same time period mean rate. We also sample alewife but at much lower levels; approximately 15% of our sample data is comprised of alewife. Given the lower sample sizes, inferences are more difficult to make."
How Many, and How Old?
To understand how a population is doing, the first thing a biologist wants to measure is abundance over time. How many herring made the migration each year compared to previous years? It takes a lot of work to gather this type of data over the long term. Researchers need to get past year-to-year fluctuations to identify a trend of growth or decline. But this work is crucial to help inform fisheries management decisions and potential restoration measures.
Ken’s team has been sampling for twelve years now. And when they calculate the number of blueback herring captured relative to their sampling effort (how much time is spent trying to catch the fish), they find that in the past few sampling seasons their catch rate has been low. That picture could be complicated by high river levels resulting from heavy spring rains in recent years, because it’s harder to find fish in a larger volume of water. But the team has other clues they can use to solve the mystery of what’s happening with the herring. These clues are hiding in the bodies of the sampled fish.
Ear bones (right) are used to determine the age of sampled river herring (photos by Alix Kaplan).
Beyond tracking how many fish swim upriver each year, the key to understanding the future of a population is to reveal its age structure. In other words, Ken’s team needs to know how many of these fish are coming upriver for the first time, and how many have survived to make the journey more than once. So how does a scientist measure the age of a herring? “To do that,” Ken says, “we need to extract bones out of their skulls.” The sampled fish are put on ice and brought back to the lab. There, a researcher removes two tiny, flat, Pac-Man shaped ear bones from each fish and prepares them for analysis under a microscope. These ear bones are called otoliths, and—much like a tree—they contain growth rings that can be counted to reveal how many seasons the fish survived.
That’s how we know that most of Ken’s sampled fish are young. About 60% of the blueback herring collected in his study are age four or younger, and fewer than 7% are at least seven years old. Ken says the maximum life span of a river herring is about fifteen years, but his team doesn’t catch fish older than ten. The age structure of this population is concerning because older, larger female river herring often lay more eggs than younger females. The super survivors are the ones with the greatest potential to help grow the population. And these days, in our region they’re not living long enough to do that.
Deadly Encounters with Ocean Fisheries
It’s been almost twenty years since the Atlantic States Marine Fisheries Commission (ASMFC) banned commercial and recreational fishing for river herring in southern New England. And in the intervening years, habitat restoration projects have made the Connecticut River watershed a safer place for fish to swim and spawn. So why haven’t these stocks recovered? As it turns out, river herring appear to be coming up against the greatest obstacle to their survival in the habitat where they spend most of their lives: the ocean.
While alewife and blueback herring are no longer targeted by fishing operations, they are still captured accidentally by nets intended for other fish. And they rarely survive the experience. The Atlantic States Marine Fisheries Commission acknowledges that bycatch is a serious problem for river herring populations that are already severely depleted, and the New England Fisheries Management Council had proposed a now paused amendment process to help address this issue in our region (See the Connecticut River Conservancy's 2024 comments on the amendment here).
So, based on their own studies along with data collected by fisheries, members of the Connecticut River Migratory Fish Restoration Cooperative are advocating for new regulations in the southern New England fishery. These would include stricter limits on bycatch, and increased monitoring efforts to enforce them. They also recommend fishing prohibitions in specific areas during certain times of year. And they have reason to hope such efforts will succeed, because similar measures have helped restore river herring stocks in Maine.
"These two species are resilient and have responded well to restoration measures under the right set of conditions,” Ken says. “As resource managers we are trying to better define limiting factors that are difficult to study in the marine environment.”
Dr. Kate Buckman, Aquatic Ecologist at the Connecticut River Conservancy, is hopeful too. “River herring play important ecological, economic, and social roles in our watershed, and we’d like to see those roles regain the prominence they historically had. The work of the Cooperative is critical to understanding what is impeding river herring population recovery. I’m hopeful that the time-series data in the river, combined with knowledge of what they are facing in other ecosystems, will lead to effective management changes that enhance survival and spawning success for these beautiful fish.”
This article was the second in a series related to documenting 2025 fish migration in the Connecticut River. You can also read the first article about sea lamprey and American shad.