Re-examining alewife populations in N.H. rivers

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For a species so significant to both the human and natural food webs, there is still a lot of mystery surrounding alewives.

They arrive each spring, swimming upriver from the sea to spawn. Unlike salmon that die afterwards, alewives can spawn multiple times throughout their nine-year lifespan. But in N.H., no one really knows where they prefer to spawn, how many juveniles survive, where they go after leaving the river or what determines their frequency of upstream migrations.

The problem, it seems, is that alewives aren't returning like they used to. This poses a rather large dilemma for the recreational and commercial fishermen that target alewives, not to mention the numerous other aquatic and avian species that rely on the foot-long fish for food.

UNH professor of biological sciences David Berlinsky is working on a N.H. Sea Grant-funded project encompassing both field and lab work to address some of the fundamental questions about the life history of this species.

"We can't understand the factors affecting alewife population declines without first understanding the where's, when's, how's of spawning, larval feeding and up- and downriver migration," Berlinsky said.

There is currently a moratorium on alewife fishing in the nearby states of Rhode Island and Massachusetts. Alewives are federally and N.H. state-listed as a species of special concern. In decades and centuries past, hundreds of thousands, perhaps millions of alewives made the journey up N.H. rivers in spring when water temperatures warm to 12 degrees C — about 52 degrees F. Now, the numbers are so low that N.H. Fish and Game biologists often hand-count the fish in some rivers rather than relying on electronic counting devices. "The error associated with the electronics is large, and when you only have a few thousand fish, if your estimate is off by another few thousand it can really skew your population estimates," explained Kevin Sullivan, biologist with N.H. Fish and Game. This work is also part of Sullivan's master's research — he works closely with Berlinsky to assess alewife populations and determine their ability to ascend fish ladders that bypass the dams.

UNH researcher stands in fish ladder water looking at alewives in net.

Counting the fish

Every day from late April until early June, Kevin and his N.H. Fish and Game biologist colleague Mike Dionne park their pickup truck close to the dams to monitor populations of alewives on local rivers: the Lamprey, Oyster, Cocheco, Exeter, Winnicut and Taylor. The ladders on these rivers, called standard denil ladders, are set up like a long cement runway sloping gently upward. A bit of water diverted from the dam flows through wooden-framed baffles that create alternating runs and resting pools, giving fish the opportunity to move up as slowly or quickly as they need.

Biologists have the top of the ladders blocked off so the alewives congregate rather than emptying into the above-dam portion of the river. Wearing chest waders and using a seine net to gather them up, Sullivan and Dionne stand in the ladder's hip-high water to count the fish and then transfer them above the dam in the river in hopes of allowing their upstream migration to continue.

Sullivan said this year they counted more than 7,000 alewives in the Oyster River — an improvement over last year's 2,500 count but still a far cry from the 157,000 alewives caught in 1992.

When asked what's changed to cause this drop in numbers, Sullivan indicates the impoundment above the dam, a quiet pool in the river. "The impoundment is getting shallower with more sediment settling out," he said. "The water quality is likely poor just above the dam, the water is warmer, and often throughout the year there isn't even any water going over the dam. That might be the cause, although we don't know for sure."

The ladder is also a limited passageway: during the biggest pulse of the upriver migration, alewives often have to line up and wait to get into the ladder because the entranceway can't accommodate all of them at the same time. While they wait to get inside, they are vulnerable to predation by striped bass, gulls and cormorants.

Tagging studies

The team tagged fish to determine the efficacy of the standard denil ladders' ability to allow upriver fish passage. They used simple Floy tags similar to the plastic t-bar tags for clothing. The Floy tags have identification numbers on them so if the tagged fish are recaptured, they can determine the number of unique trips taken by the fish up river. While inexpensive, Floy tags yield limited data on the number of recaptured fish.

Some nearby rivers like the Lamprey have large enough alewife runs to warrant using the electronic tags, despite their technological inaccuracies. Sullivan and Berlinsky tagged 622 alewives with PIT tags, small devices that display an identification number when scanned with a reader, and fitted another 93 alewives with radio tags. A series of eight PIT antennas were placed in the first dam on the Lamprey and two on the second dam upriver. Radio antennas were secured to the top of the ladders at both dams and along the rivers just above the dams.

The tags have provided some interesting data: There are some fish that successfully make it up the ladder, some that don't, and curiously there are some that travel back and forth within the ladder without immediately exiting on either side. When nightfall comes, those still in the ladder quickly exit downstream, perhaps to try again the next day or perhaps not. This is an enormous waste of energy for the fish, Sullivan said.

Searching for the eggs

Still, the ladders provide some unintended benefits for researchers. Many fish that don't make it up the ladder simply choose to spawn at the base of the dam, making the job of finding the eggs slightly easier than the proverbial needle in the haystack. Sullivan, Dionne and Berlinsky wanted to see where the eggs were settling for those fish. They used egg collection tiles — three strings of five, one-foot square steel plates tethered together and leashed to rebar secured in the river sediments — in hopes of capturing the sticky alewife eggs as the current takes them further downstream. Wading into the river, Sullivan and Dionne pulled up the tiles and found dozens of tiny eggs dotting the steel surface. A quick glance underfoot revealed eggs clinging to the rocks recently exposed when the tide backed out.

In the Exeter River, there are far more alewives spawning below the dam than the number of fish successfully ascending the ladder and spawning upriver, Sullivan explained. For a species that prefers to spawn in freshwater, the dam on this river is clearly an obstacle they may not be able to overcome, even with the help of the ladder.

"Wasted efforts," Sullivan said of the eggs attached to the rocks below the dam. Many of the eggs will dry out and die before the tide comes back in. Had these fish been able to access the freshwater above-dam portion of the river, the eggs would not be subjected to the tides that alter the height of the water and leave the eggs lethally exposed to the air twice a day. The alewives that hatch from eggs settling below the low-tide line face the daunting task of adapting to saltier below-dam water immediately post-hatch.

Previous experiments noted that juveniles can adapt to full-strength seawater in about 45 days. Berlinsky's lab experiments are indicating they adapt much more quickly than that — perhaps in as few as 10 days. This is a key piece of information for the eggs found below the dams in tidal rivers like the Exeter. If juveniles hatch out below the dam and are capable of adjusting to a more saline environment in a short amount of time — say, the amount of time it takes for the water flows to push them into even saltier water downstream — then they could survive and therefore should be counted as part of the population.

By only counting the adults, biologists could miss one or more years of poor alewife recruitment — when juveniles survive to grow into adults. It's only when the adult populations begin to crash that they find out there's a problem, and by that time it's often too late.

Biologists can get a fairly accurate estimate of adults who make it through the ladder, and they can get an idea of where the eggs are found below the dam. Yet the most useful information — the number of juveniles that survive and leave the river to swim to the sea — remains elusive. "We still need a good measure of productivity," Sullivan said.

Sullivan notes implementing an alewife stocking program might be a necessary next step if populations remain low in the state. After a year of experiments at the UNH Aquaculture Research Center (ARC), Berlinsky has established aquaculture protocols that would enable alewives to be raised in captivity for stock enhancement purposes. Using fish captured from the nearby ladders, Berlinsky set up a series of tanks at ARC to determine the best methods for spawning alewives, rearing the larvae and growing their food — mostly rotifers and Artemia. If N.H. or any other state along the Atlantic coast decides it wants to begin a stock enhancement program for alewives, they now have the protocols established to raise the fish they'll need.

In the future, Berlinsky and Sullivan hope to continue their tagging efforts and egg sampling to help locate specific spawning areas within the rivers.

"Developing an annual productivity index of juvenile alewives and blueback herring will help researchers and state fisheries managers better understand, monitor and hopefully restore their populations to historic levels," Sullivan said.

 You can watch a video about this research: