Biologists alarmed over lack of young Atlantic sturgeon in surveys
Some point to an abundance of blue catfish, but recruitment issues may go back more than a decade
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Biologists have been surprised in recent years about how many big Atlantic sturgeon they are finding around the Chesapeake Bay. But rather than celebrating, they have become increasingly alarmed about what they are not seeing: a new generation of young sturgeon.
While finding more adults is certainly good news, biologists say they have seen little evidence those sturgeon have successfully produced significant numbers of offspring in recent years that would be critical if the endangered species is to make a comeback in the Chesapeake.
“To get any kind of recovery, the best thing you can do is to increase that first year of survival,” said Dave Secor, a fisheries biologist with the University of Maryland Center for Environmental Science. If young fish survive, he said, “you can actually realize very rapid recovery, even for a species like sturgeon.”
That’s something that biologists working with sturgeon around the Bay say they haven’t seen, perhaps for a decade or more. Many blame the absence of young sturgeon on a rampant population of introduced blue catfish, which they say could be consuming eggs and newly hatched fry, or outcompeting them for habitat.
But researchers who study the catfish dispute that, and even sturgeon specialists acknowledge they lack concrete evidence.
Whatever the cause, a decade of “no to nil” recruitment, Secor said, “could indicate a kind of shift in the environment. That would be worrisome if you just didn’t see any recruitment for 10 years.”
The new concern has dampened what biologists thought was, until recently, a good news story. When the Bay population of Atlantic sturgeon was listed as an endangered species in 2012, the James River adult population was thought to be small, but surveys in recent years suggest it may actually number in the thousands.
No sturgeon were known to exist in the Pamunkey River, but now biologists calculate that it may have its own population of about 300 adults. Adults have also been seen on suspected spawning grounds in the Nanticoke River, and have been tracked into potential spawning areas in other rivers.
Initially, biologists thought that meant sturgeon were better off than previously assumed. Now, they’re worried that the big fish — sturgeon can reach lengths greater than 10 feet, weigh more than 300 pounds and live for decades — may be even more imperiled than they realized.
“I had been saying the James River was the sign of a great recovery,” said Matt Balazik, a fisheries biologist with Virginia Commonwealth University. He has netted hundreds of adult sturgeon on the river — including males and females that have clearly released eggs and sperm — but he has seen no evidence of little fish, or “recruits” to the overall population. Since 2012, he has captured 190,000 young fish of other species, but the number of recently spawned young-of-year fish that he has caught is zero.
“Even if we were the worst fishermen in the world,” Balazik said, “with all the sampling that we have done, we should have gotten at least one if the young ones were there at any number whatsoever.”
The lack of juvenile production may go back much further than 2012, though. In recent years, Balazik has seen the average size of spawning adults slowly increase as few younger, small adults return to spawn — a sign that spawning activity is not producing young fish that survive.
Problem may be older than a decade
Sturgeon don’t mature and return to their native river to spawn until they are around 8–12 years old, which, to Balazik, suggests the poor production of young fish may go back more than a decade.
But there is evidence sturgeon were successfully producing young before then. In the mid-1990s, a short-lived reward program paid watermen to report any sturgeon they captured in Virginia’s rivers, and dozens of juvenile fish only a few inches long turned up. In recent years, Balazik said he has revisited the places where watermen caught those sturgeon, and fished with the same type of gear, to no avail.
He, like a number of other sturgeon researchers, strongly suspects the problem stems from the river’s brimming population of blue catfish, a nonnative predator introduced to the river in the 1970s, but whose population has exploded in recent decades.
Blue catfish accounted for 72 percent of the 190,000 fish captured in Balazik’s juvenile surveys, and they blanket the areas where sturgeon are thought to spawn. Balazik believes the catfish may be eating small sturgeon, as well as sturgeon eggs and larvae.
“We don’t have a smoking gun,” he acknowledged, “but it is like a simple common sense thing. You have a generalist predator that will eat anything, and it is documented they will eat anything.”
He’s not alone. Biologists only recently documented the existence of a sturgeon population on the Pamunkey River, a York River tributary adjacent to the James, but are already worried about its future.
As on the James, they have found no juvenile fish in recent years, and few small adults. Adults generally join the spawning population when they are about 4.5–5 feet long. But on the Pamunkey, few fish are that small — only 6 percent in three years of sampling, said Jason Kahn, an endangered species biologist with the National Marine Fisheries Service.
“In a healthy population, it would be the other way around,” Kahn said. “You would see most of them being that size range and dropping off as they get older.”
He cautioned that in other areas along the East Coast, young sturgeon often have proven difficult to find, so “just because we don’t find them doesn’t mean they are not having some success.”
And the presence of a few small adults suggests that at least some young fish are surviving. “It is not absolutely zero, it is just that there is not enough to make the population recover,” Kahn said. “It may be that we are on a declining trend for a while into the future.”
There are other problems that could affect small fish, such as a periodic zone of low-oxygen water near West Point, where the Pamunkey and Mattaponi rivers merge to form the York, which Kahn said might be an important habitat area. But, he added, that appears related to a local industrial discharge and wouldn’t explain the same overall juvenile trends in both the James and Pamunkey.
As a result, Kahn’s concerns have steadily risen about the huge number of blue catfish he sees in Pamunkey spawning areas and in habitats where they would expect to find young sturgeon, potentially forcing them into less desirable areas which, in turn, could reduce survival.
Other evidence has recently emerged, as well. Last year, DNA from multiple sturgeon were found in the stomachs of two blue catfish captured on the Pamunkey spawning grounds. But sturgeon DNA showing up in blue catfish doesn’t necessarily mean the catfish ate sturgeon eggs or larvae — just that the blue catfish ate something which, in turn, had eaten them.
“What we’ve got at this point with the catfish is a whole lot of smoke,” Kahn said. “And my suspicion is that eventually we will find the fire.”
Biologists working with blue catfish agree there is smoke, but are more dubious about the fire.
“Nobody has seen eggs, larvae, any other (parts of a) sturgeon in a catfish,” said Bob Greenlee, a fisheries biologist with the Virginia Department of Game and Inland Fisheries. He noted there are many other species that are more likely to eat the eggs, including white perch and common carp — and they have co-existed with sturgeon for centuries.
Don Orth, a Virginia Tech fisheries biologist, heads a research team that has examined the stomach contents of 16,000 blue catfish since 2012 as part of an effort to understand the impact of the invasive species on native fish populations. Although many of those fish were taken from sturgeon spawning areas, no bits of sturgeon have been found in a blue catfish.
Eggs that show up in catfish stomachs are almost always from other blue catfish. “It is during the breeding season of the catfish,” Orth said. “They are raiding each other’s eggs.”
Orth said that, generally, what turns up in a blue catfish is whatever is abundant in the area, whether it is vegetation or other fish.
Orth also noted that along the Gulf of Mexico, where blue catfish are native, they inhabit the same areas as the Gulf sturgeon — a close relative of the Atlantic sturgeon.
Greenlee also disputes the idea that a decline in sturgeon recruitment over the past decade coincides with an increase in blue catfish during that same period.
He said blue catfish numbers in areas where sturgeon spawn in the James probably peaked in the 1990s — well before 2005, the approximate time sturgeon biologists believe recruitment began to decline. Further, he said, the numbers of catfish have declined in the James over the last decade, and have stabilized — and possibly declined — in the Pamunkey.
“I don’t think that line of logic holds,” he said.
Weather may be a factor
Populations of anadromous fish like sturgeon, striped bass and shad — which live most of their lives in the ocean but return to their natal freshwater rivers to spawn — tend to be highly dependent on good “year classes” when reproduction meets favorable weather conditions conducive to the survival of large numbers of young.
Greenlee said biologists have long noted that survival of young for other species — such as largemouth bass and white perch — were particularly sensitive to weather conditions on the York and Pamunkey rivers. “They all have recruitment variability. Some of them only have very occasional strong year classes,” he said.
Because there are so few sturgeon to produce young, Greenlee and Orth both suggested, they are particularly sensitive to fluctuating conditions, and they said poor sturgeon recruitment may be more closely linked to weather than blue catfish.
Nonetheless, Orth said, because the sturgeon population is so low, “any potential source of sturgeon mortality should be investigated.”
More light on the issue could come from a third river — the Nanticoke, which drains parts of Maryland and Delaware on the other side of the Bay. While blue catfish are present in the Nanticoke — and support a fishery — they only arrived during the last few years and their abundance is lower than in the James and Pamunkey.
So far, biologists have caught only 15 adult sturgeon on the Nanticoke during the last two years, a fraction of what’s been seen in the other two rivers.
Chuck Stence, a fisheries biologist with the Maryland Department of Natural Resources, said several of the males from the Nanticoke were in the 4–5 foot range, which could suggest they’ve had successful reproduction and young survival within the last decade.
Biologists have not looked for young sturgeon on the river, Stence said, but are planning new efforts this year to look for them — and for signs of spawning, such as eggs.
“The good thing on the Marshyhope (the Nanticoke tributary where many of the sturgeon were found) is there is some fishing pressure on the blue cats now, to kind of keep the numbers at bay,” he said.
That means, biologists said, that monitoring the Nanticoke population may provide some indication about whether catfish are playing a role in controlling sturgeon populations. “Blue catfish are definitely in the system, so we could watch for the evolution of this,” said Secor, who called concerns about the impact of blue catfish on sturgeon “a viable hypothesis, absolutely.”
A successful Nanticoke population could also be beneficial to the Pamunkey sturgeon because initial genetic testing suggests that the two populations may be closely related, and spawning-age fish have been tracked moving back and forth between the two rivers.
“It may be a situation that if it is a catfish problem, and if the Nanticoke doesn’t really have a catfish problem, that may be a lifeline for the Pamunkey,” Kahn said. “So there
are a lot of positives that can come
out of that.”
If blue catfish are the culprits, it’s unclear what could be done. In some places, reducing predators has been part of the effort to protect endangered fish. On the West Coast, for instance, sea lions have been killed to protect endangered salmon runs.
But reducing the huge blue catfish population in some Bay tributaries to a level that would control predation could be a challenge. States have promoted a commercial blue catfish fishery, but some scientists say that removing enough to dramatically affect their overall population might flood the market.
Another option could be a hatchery operation that rears sturgeon to a size that would allow them to evade predation. But that would be a particular challenge in places like the Pamunkey and Nanticoke where populations are small because it would be difficult to get enough fish, with enough genetic diversity, to avoid inbreeding.
Ironically, only a few years ago biologists thought hatcheries might be the only way to bring sturgeon back but that idea was abandoned as biologists began discovering wild sturgeon populations,
“It would be a real shame, now that we’ve discovered natural spawning, that we would have to revert back to an artificial propagation strategy,” Secor said. “Hopefully they are cryptic enough that maybe they can get around the blue catfish.”
- Category: Fisheries
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