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| Fisheries
Many of our marine fishery stocks have been in serious decline for years. Returning stock levels to sustainable sizes requires an understanding of how overfishing, variable recruitment, and habitat alterations affect fish populations. This Delaware Sea Grant research priority focuses on efforts to achieve sustainable fisheries, as well as enhance aquaculture production, in the Mid-Atlantic region. This research involves linkages between fisheries science, water quality, the health of coastal habitats, and marine policy. The following Fisheries projects have been funded by Delaware Sea Grant for the 2003–2005 period:
For additional information, please also see our latest annual report.
From an economic standpoint, the blue crab is the most valuable shellfish in the Mid-Atlantic region, so when the crab harvest takes a downturn, local residents want to know why. Recent field surveys indicate that blue crabs may be overfished in both estuaries, which has resulted in stringent and controversial catch limits on the fishery. However, natural forces — wind and rainfall — also can impact the crab population, according to Sea Grant research conducted by UD marine biologist Charles Epifanio and his colleagues. In previous Sea Grant studies in Delaware Bay, Epifanio and colleague Richard Garvine, a physical oceanographer at the UD College of Marine Studies, determined that once the tiny, larval crabs hatch in July and August, they get swept out of the bay and onto the continental shelf by the Delaware Coastal Current, Summer winds then push the crabs home, back into their bay nursery grounds.
If river flow is at a minimum due to drought, wind has a greater effect in shuttling the crabs back into the bay," Epifanio says. "Thus, the supply of larval crabs may be highest in drought years." The scientists have discovered that the larval crabs often occur in the bay in large, distinct patches. "We think these patches may be formed through the synchronized spawning of large aggregations of female crabs," Epifanio notes. In the current Sea Grant research project, which includes physical oceanographer Charles Tilburg on the research team, the scientists are testing their hypothesis using satellites to track the crab patches, coupled with intensive sampling operations at various locations in the bay. This spatial data will help them refine a unique mathematical model that considers the genesis, maintenance, and transport of patches of blue crab larvae in time and space. The novel tool should aid state regulators in determining optimum strategies for managing the blue crab fishery.
Hypoxia and Estuarine Nursery
Habitat Quality: An Experimental and Modeling Approach Linking Low Dissolved
Oxygen with Fish Survival and Growth
UD fisheries scientist Tim Targett and his graduate students have built a state-of-the-art aquarium system that is a key component in a regional Sea Grant effort to assess the effects of low-oxygen conditions, called hypoxia, on fish in coastal nursery grounds. Targett's collaborators include Jim Rice from North Carolina State University and Ken Rose from Louisiana State University. "Fisheries management organizations have place a strong emphasis
on the importance of identifying 'essential fish habitat' — those
waters critical to fish for spawning, feeding, and growth to maturity,"
says Targett. "In our regional project, we're working to develop
a simulation model that can predict the impact of changing oxygen conditions
on the growth, survival, and distribution of several major species including
young weakfish, summer flounder, Atlantic menhaden, and spot." So far, the team's research indicates that there is a significant decrease in growth rate for summer flounder at 3.5 milligrams of dissolved oxygen (DO) per liter or less at both 20°C and 25°C. Diel cycling of dissolved oxygen, which occurs throughout the day in estuaries (cycling between 2.0 and 11.0 milligrams of oxygen per liter), significantly reduced summer flounder growth rate only at 25°C. There was no evidence suggesting that summer flounder are capable of acclimating to low DO in terms of increased growth rate during prolonged exposure. The scientists' behaviorial experiments indicate that summer flounder are able to distinguish between DO levels and show avoidance of very low DO levels. Additional research is now under way to determine the levels that elicit an avoidance response. "When we're done, we should have the framework for quantifying
the complex relationships between water quality and fish populations,"
Targett says.
A Versatile New Technique
for Genetic Monitoring of Marine Enhancement Programs Applied to Oyster
Restoration in Chesapeake Bay
The once-thriving Atlantic coast oyster industry — based on the Eastern oyster (Crassostrea virginica) — is approaching commercial extinction. The Delaware Bay industry has been almost flat-lined for half a century, while the Chesapeake Bay industry now is nearing the point of collapse. While much of the damage was done initially by overharvesting and habitat degradation, additional losses have occurred during the last several decades due to two oyster diseases: Dermo (Perkinsus marinus) and MSX (Haplosporidium nelsoni). In response to the crisis, watermen, concerned citizens, scientists,
resource managers, and non-governmental organizations have been working
together to restore oyster populations, most notably in the Chesapeake
Bay. Numerous oyster reef construction projects are under way, using
wild or hatchery-produced oysters that will, it is hoped, reproduce
and contribute new oysters to once-productive areas.
"The use of genetically improved lines to enhance local wild populations involves not only the construction of oyster reefs, but also the hatchery propagation of large numbers of selected oysters for seeding these reefs," says UD marine biologist Patrick Gaffney, a principal investigator on the regional project. "So it's essential to determine which methods will yield the greatest seed survival in order for stocked oyster reefs to act as sources of oyster larvae for replenishing adjacent areas. Because it's impossible to track the dispersal and survival of oyster larvae using physical tags, we are using genetic markers for this purpose." In previous research, Gaffney and his team developed a reliable, cost-effective
genetic technique for monitoring the survival and propagation of genetically
distinct Gulf Coast oysters planted in the Chesapeake Bay in 1997. They
are now working to make the technique applicable in situations where
the planted seed is derived from local populations of oysters rather
than from genetically distant sources.
Impact of
Hypoxia on Quality and Quantity of Estuarine Nursery Habitat: Patterns
of in situ Growth and Swimming Avoidance Activity Costs in Estuarine-Dependent
Fishes
To advance the next phase of development of Sea Grant's model of fish habitat quality (see Project R/F-21), UD marine biologist Tim Targett and his research team now must "ground truth" their lab data in order to provide a concrete reference with which lab results can be compared and model predictions can be validated. No data currently are available on the effects of chronic low dissolved oxygen conditions (hypoxia) on field growth rates of either juvenile summer flounder or weakfish. This project seeks to fill that void. "Activity rates are a large and highly variable component of fish energy budgets," Targett says. "A comprehensive investigation of swimming speeds and the associated energetic expenditure for young summer flounder and weakfish is needed for input into our ongoing modeling effort." Targett and his team now are working to establish growth indices and biochemical correlates of growth for juvenile summer flounder and weakfish in Pepper Creek (Indian River Bay, DE) over a range of temperature and ration level combinations. Targett views Pepper Creek as a model system for studying the effects of eutrophication-related environmental fluctuations on estuarine nursery habitat quality. Previous and ongoing field investigations there (funded by the Center for the Inland Bays) have shown that severe diel-cycling dissolved oxygen (DO) conditions occur between April and September, when peak abundances of juvenile summer flounder and weakfish are found in the nursery area. The ultimate goals of this research are to provide information to resource managers on the impact of hypoxia on nursery habitat quality and quantity and to support recommendations for protecting and restoring fisheries habitats. "The project should refine our understanding of the direct, indirect, and interactive effects of hypoxia on nursery ground quality; improve model parameterizations and predictive power (capability); and thus facilitate interpretation of model outputs on the impact of low dissolved oxygen on the growth of young fishes in estuarine nursery grounds," Targett says.
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   Copyright © University of Delaware College of Marine & Earth Studies and the Delaware Sea Grant College Program |
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