When oysters hatch, the microscopic larvae are totally at the mercy of the environment. Controlled by time and tide, they move where and when the water takes them, sometimes landing far from where they were spawned. When they finally come to rest, the place where they settle may or may not be a suitable place for them to grow.
Two researchers, marine ecologist Ruth H. Carmichael of the Dauphin Island Sea Lab and coastal physical oceanographer Keyong Park of Texas A&M University in Galveston, have combined their expertise to discover more about how these processes affect the larvae of oysters and other commercially important seafood populations as they make their first journeys.
Park, formerly a faculty member at the University of South Alabama and the Sea Lab, began a hydrodynamic model of Mobile Bay in 2003, incorporating data on freshwater discharge, wind, water level, current velocity, salinity and temperature. The model outlines the paths the water commonly takes as it flows through the area.
This project will expand the flow data to include the eastern Mississippi Sound and northern Gulf of Mexico to see how those bodies of water interact with the bay. The researchers will test and perfect the expanded model by tracking the movement of oyster larvae in the area.
“I’m interested in how and why water moves,” said Park. “Because as it moves, it carries things in it, with it.”
Those “things,” he said, can be anything from mercury to storm debris to shellfish larvae. In fact, his model was used extensively during the 2010 Deepwater Horizon oil spill to determine how oil would impact Mobile Bay.
Wild oyster populations are on the decline in fisheries around the world and they are missed for a variety of reasons.
“There are tremendous efforts nationwide for restoration of oyster stocks,” said Carmichael. “Their reefs provide habitats for other species and actually protect the shoreline from erosion.”
New model expands to Sound, Gulf
The new model will provide a thorough diagram of probable temperature and salinity levels in different parts of Mobile Bay, the Mississippi Sound and the Gulf of Mexico. Resource managers working to rebuild oyster stocks in the area could use the model to identify suitable habitats.
“If flow dynamics are such that we can show oysters won’t settle there, the other stuff doesn’t matter,” Carmichael said. Data that show the best — and worst — places to build reefs will benefit state wildlife managers and environmental groups interested in boosting oyster populations.
Park explained that Mobile Bay has naturally occurring hypoxia at the bottom that has been confirmed in historical accounts of the area. The hypoxia intensifies during summer times when the wind dies down. Park said the model will help scientists understand that phenomenon, the impact modern urbanization might have on it and how it might affect larval oyster movement.
Charting the flow mechanics of the region has other applications as well. Some municipalities have septic and stormwater drainage systems that empty into the bay area. The team can use the model to track where biological contaminants or otherwise compromised water might go. This knowledge will help identify areas that are unsuitable for oyster reefs.
An earlier version of the model has been used already by area police and U.S. Coast Guard in search and rescue missions.
Two graduate students are assisting Park and Carmichael on the project. Myeong “Max” Han, a Ph.D. student at the Sea Lab, is collecting data for Park to help him populate the flow model. Haley Nicholson, currently working toward her master’s degree, is working with Carmichael to collect biological data from the oysters in the lab and in the field.
Tracking stained oyster larvae
One component of the study involves directly tracking larval movements by capturing marked hatchery-reared and natural larvae at sites throughout Mobile Bay. The first round of oyster larvae has been released after first being stained with a fluorescent dye in the lab. The dye, nontoxic and invisible to the naked eye, has never been used in this large of a field operation before. The goal is to recover as many of the stained larvae as possible to better identify the path oysters took after their release.
Carmichael stated that the odds are against them finding many of the stained larvae. Most larvae of any species are eaten before they reach the juvenile stage, so they might not live long enough to be collected, but even one recovered larva will provide direct movement data that have never before been collected. The team has placed settlement plates throughout the model area and will check them regularly for both the stained and any wild larvae to build their dataset. Any oyster larvae found will further the project; the stained larvae just have the added benefit of a known starting point.
All of the biological and hydrodynamic data gathered during the project will further refine the model and inform fisheries experts in their management and restoration efforts. In this manner, scientists will help oysters find sites for safe, healthy reefs and oysters will help scientists better understand local hydrology.