Wednesday, August 17, 2016

Where are the Children?

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.”
When in doubt, don't blame your sampling plan, or your management of the species, blame some external factor:
 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.”
Which is not a very long time in the life cycle of sturgeon, which live 60 years or more and may only spawn every five years. Could it be related to the Atlantic Multi-decadal Oscillation, which switched from negative to positive about 10 years ago? It certainly affect Striped Bass, which aren't nearly a long lived.
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.”
Blue cats are "new", Blue cats eat almost anything, therefore, Blue cats must be responsible.
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.”
Basically, big fish eat little fish, and the fish of eating size there are, the more there will be big fish of some kind that eat them.
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.
Maybe it's the Sturgeon eating the Blue Cats? It could well be that Blue Catfish are suppressing the recovery of the Sturgeon in the Bay, but I'm a little skeptical when managers reach for one of the things they don't like as the primary cause of lack of recruitment of Sturgeon, instead of all the other myriad changes people have made in the same period.

Wombat-socho's "Rule 5 Sunday: Post-Olympics Edition" is open for business at The Other McCain.

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