Young freshwater shrimp are stirring in the murky Mississippi River, cued by warming water temperatures to head upstream in great darting hordes each night. Soon, the adult females will stir too, carrying masses of orange eggs hundreds of miles downstream and releasing their progeny where the Mississippi meets the Gulf of Mexico.
This amazing and still-mysterious life cycle put the Ohio River Shrimp, Macrobrachium ohione, in harm’s way last summer as a tsunami of BP oil lapped endlessly at the Mississippi's mouth and Louisiana shores.
“Last year’s oil spill called attention to it,” said Paul Hartfield, a Jackson, Miss.-based biologist with the U.S. Fish and Wildlife Service.
The amphidromous river shrimp live and breed in freshwater, historically as far upstream on the Mississippi as St. Louis and hundreds of miles up the Ohio River. The adult females carry their fertilized eggs downstream, almost to the Gulf of Mexico, releasing them to drift into salt water and develop for a month or so. Then the adults and their young head back upstream to breed again.
“It’s one of the great migrations on the planet,” said Tyler Olivier, Hartfield’s research colleague and a doctoral candidate at the University of Louisiana at Lafayette, as he rested in the shade of willows during a boat survey near Vicksburg, Miss. Because the young shrimp need to live in salt water for a period to successfully molt from larval stages into adulthood, their life cycle means they must travel up to 1,000 miles twice each year.
While last year that meant river shrimp swam straight into the “heart of darkness,” as Hartfield puts it, the future of the species has been uncertain for decades. They may be one of the most abundant animals in the Mississippi, serving as a crucial food source for birds and fish, but it is also one of the least understood. The species used to be served in New Orleans restaurants up until the early 1900s. It was caught for bait as well.
One scientific journal noted that the annual harvest of river shrimp in the Mississippi declined from nearly 1,000 tons in the 1930s to about 2 tons a year by the 1970s. That’s the same period that the U.S. Army Corps of Engineers began intensive management of the Mississippi – installing rock dikes along the shores, straightening the channel and erecting hundreds of miles of levees to accommodate barge traffic and facilitate farming of what was once a 25-million-acre floodplain forest. That river management also was a big reason Louisiana has lost more than 1 million acres of coastal marsh since the 1930s, as land-building sediments are shunted offshore instead of being allowed to rebuild marshes every year.
No one knows if the river shrimp decline can be linked to intensive river management, but Hartfield and others suspect it is. The species seems to be mostly extirpated from the upper parts of its range in the Mississippi and now exists mainly south of Memphis.
Hartfield and Olivier began regular trapping of river shrimp last year near Vicksburg, or more than 400 miles upstream from the Gulf of Mexico. They caught them in wire mesh traps as far as 480 miles upstream, north Greenville, Miss. Their research is not funded by BP but normal operating funds they receive. They’ve already learned several things about the shrimp’s movements, including that they seem to move on the bottom of the river during the day, when they are more subject to predation. They will compare their data with last year to see if they can detect differences in abundance because of the oil spill. We may never know how much the oil spill added to their decline, Hartfield said.
River shrimp are not related to the Gulf shrimp. They are more related to crustaceans like crayfish. And they can walk like them, too, and climb up or around small dams and other obstructions. Other migrations of amphidromous shrimp have been seen throughout the world. Some, in the Caribbean and elsewhere in the tropics, occur in much smaller rivers and cover much shorter distances. Others occur in the Amazon, Ganges and other large rivers.
The males and juveniles measure about an inch or two long. The adult females can be up to four inches long.
Hartfield and Olivier will keep catching shrimp this spring and through next winter.
Said Hartfield: “Here’s one of the most basic elements of the Mississippi River food chain, and we don’t know anything about it.”
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