Climate change hurts Louisiana’s oysters and shrimp
18th February 2019 · 0 Comments
By Susan Buchanan
Contributing Writer
Greater New Orleans is expected to be even hotter ten years from now. And the impacts of climate change on local seafood production will grow. Scientists in Louisiana say these changes could reduce supplies of fresh oysters and shrimp.
Last week, Julie Lively, associate professor and fisheries specialist at Louisiana State University AgCenter and Louisiana Sea Grant, said in the short term, more severe weather could negatively impact shrimp and oysters because of their dependence on temperatures and water salinity.
Louisiana accounts for over a third of the nation’s oyster output and well over a third of its shrimp landings. The state’s oysters have partly recovered from a hit they took after the BP spill in 2010. Shrimp landings are down, however. 
Climate change has already affected south Louisiana’s seafood, Earl Melancon, emeritus professor of biological sciences at Nicholls State University in Thibodaux, said. Sea level rise, higher atmospheric temperatures and sudden changes in the thermometer, along with hurricane intensity, increased bouts of heavy rain – and the opposite, greater droughts – will become more pronounced.
Environmental parameters work in combination with one another and have real consequences for seafood production, Melancon said. As changes in weather become more extreme and less predictable, they may one day create a tipping point, or an unstoppable situation, for commercial seafood, he said. Plaquemines Parish, with its fleet of vessels and fishers, is ground zero for the ways that climate impacts the biology, ecology and economics of shrimp and oyster production, he said. That’s of interest in New Orleans, where plates brim with Plaquemines seafood.
An estuary is a highly productive and dynamic habitat, Melancon said. Its resilience is based on an ever-shifting mix of fresh and salt water in seasonal cycles. But climate can alter these dynamics and create imbalances.
What will sea level rise do to Louisiana’s oysters and shrimp? Coastal marshes will be increasingly drowned by tides, Melancon said. Water-logging can kill vegetation and its roots, resulting in wetlands loss. During their early lives, juvenile shrimp migrate into estuaries from the Gulf and use wetlands to escape from predator fish. Wetlands loss reduces their chances of survival. And with fewer wetlands, increased tidal amplitudes and currents flush juveniles into the Gulf before it’s their time.
Oysters are immobile after a two-week, swimming larval stage, Melancon said. Once they settle on the bottom, they’re dependent on water currents to bring them food for filtering and waste removal and to disperse their larvae during spawning. Wetlands loss changes water currents and dramatically alters the ability of reefs to sustain oysters. Soil no longer binds, and sediment can bury and suffocate a reef.
Lively at LSU said continued subsidence with sea level rise can cause additional land loss. “Shrimp thrive with our marsh edge, but as more of it sinks, there’s less habitat,” she said “This can lead to salinity changes, which oysters can only tolerate in small changes. Shrimp populations may move based on salinity and temperature changes, but oysters can’t just move.”
So what about rising and changeable temperatures? Water temperatures are a driving force for shrimp movement out of marshes and shallow waters into deeper bays and the Gulf, Melancon said. As water temperatures rise, shrimp growth rates may increase. But with an earlier warming, the spring brown shrimp will be forced into deeper, cooler bays and Gulf waters before they reach commercial sizes.
Cold fronts will be less predictable in the future, he said. Cooler, earlier low-water temperatures will force juvenile, fall white shrimp into deeper, warmer bays and Gulf waters before they reach commercial sizes.
Oysters have two spawning peaks in Louisiana and other Gulf states. Oysters mature and spawn based on rising water temperatures in the late spring and early summer, and spawn again in declining water temperatures in the fall and early winter. “As weather and temperatures become more random and unpredictable, we’re seeing changes in spawning patterns of oysters, with one or both spawning peaks often lost or severely diminished,” Melancon said.
Morgan Kelly, assistant professor of biological sciences at Louisiana State University, said, “From our research on oysters, I can tell you that warming temperatures from climate change are likely to exaggerate the negative effects of fresh water.” Oysters have a “sweet spot” at about 15 parts per thousand salinity, which translates to about one-half freshwater and one-half seawater. Too much freshwater and oysters will close up and stop feeding, she said. On the other hand, too much saltwater, and they’re more susceptible to diseases and predators.
“Warming temperatures and low salinity act like a bad drug interaction,” Kelly said. “Up to a certain point, oysters can handle one or the other. But warm temperatures and low salinity lead to much greater stress than either one alone.”
What will more intense hurricanes do? Late-spring season brown shrimp and fall white shrimp will be flushed out of marshes with high tides, Melancon said. Oyster reefs will be buried by mud from wind and wave activity. And wetlands will be destroyed.
Lively said increased storms could cause phy-sical damage to oyster reefs and more sedimentation on reefs.
What can excessive rain do? Kelly said the number of big rain storms is expected to increase in the southeastern United States, causing pulses of fresh water and adding to stress on oyster reefs. This may occur more often when oysters are getting ready to breed.
Melancon said juvenile brown shrimp need salinity of about 25-percent seawater strength to grow and survive in commercial numbers. Excessive, prolonged rain will decrease salinity and stunt their growth. White juvenile shrimp can tolerate lower salinity but they need at least moderate salt levels to grow and survive. Oysters require salinity of about 25-percent seawater strength for larvae to survive.
What impacts will droughts have? Little or no rainfall can increase salinities and tide heights when there’s a southeast wind east of Mississippi River and a southwest wind west of the river, Melancon said. This situation can be somewhat favorable initially and can stimulate growth, but eventually predators will become more prevalent. Shrimp and oysters will be pressured by predators and by an increase in parasites, influencing survival and reproduction.
In shrimp, a common Protozoan parasite can attack the musculature of the abdomen, or tail meat, and produce a cottony appearance, Melancon said. Growth of this “cotton shrimp” is stunted, affecting its maturation. For oysters, a Protozoan parasite, known as Dermo, invades tissue fluids through the body, and by the second year of an oyster’s life can impact reproduction and survival. Oysters are left skinny and shriveled when they’re near death. Neither of these Protozoan parasites is harmful to humans, however.
Lively said prolonged drought would hurt the growth of shrimp. Oysters and shrimp would be negatively impacted by extended droughts with elevated temperatures. Indirect effects like algal blooms, diseases and bacteria could harm the health of shrimp and oysters and might make them unfit for human consumption.
What about river diversions and climate change? The state’s planned diversions of the lower Mississippi River will add to the impacts of climate, affecting oysters in particular, according to scientists. Diversions will build land by diverting river water into open-water wetlands. Sediment from the river can smother oyster beds and greatly change salinity. The river’s pollutants will hurt beds too.
As local seafood supplies shrink, consumers will look for alternatives. “Farmed is replacing wild caught, and the majority of it’s from Asia—Indonesia, China, India, Vietnam and other countries,” Julie Lively said. As it is, over 90 percent of shrimp consumed in the United States is imported now, with most of it produced by farming.
This article originally published in the February 18, 2019 print edition of The Louisiana Weekly newspaper.
