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Teaming Up

Fish larvae
Fish larvae

Larvae (from top) of American eel, American menhaden, and Atlantic Croaker. Scale bar is 5 mm. ©Eric Hilton/VIMS

Virginia Marine Resource Bulletin
Volume 40, Number 1, Fall/Winter 2008
By Jessica Smits with additional reporting by Margaret Pizer

It’s an act of nature that goes largely unnoticed. Every year, larval fish—barely visible to the naked eye—leave their birthplace in the offshore Atlantic and make their way into the waters of the Delaware and Chesapeake bays. Survivors of the journey find food and shelter in the bays’ nursery areas and, ultimately, grow to become important parts of the ecosystem. Not to mention its recreational and commercial fisheries.

And although this movement ultimately determines what fishermen find at the end of their lines, we know little about how the larvae find their way to each bay. What forces drive their movement? Do these forces differ between the Delaware and Chesapeake bays? And what could an increased understanding of them mean for fisheries management?

These are questions that keep researchers like John Olney, chair of the Fisheries Science Department at VIMS, up at night—quite literally. For much of the past year, Olney and members of his research team, including VIMS scientists Brian Watkins, Pat Crewe, and Ashleigh Rhea, have gotten up at all hours of the night, trudged to the end of the VIMS research pier, and collected samples of larval fish from the York River. Likewise, biologists from the University of Delaware have conducted similar late-night fishing trips at the mouth of the Broadkill River in Delaware Bay.

“This is not very sophisticated sampling. We stand at the end of a dilapidated pier and throw a net over the side,” says Olney. “But it’s cost effective and we end up with huge quantities of fish larvae.” The sampling is done at night because that is when the larval fish are most active in the water column. Once they are caught in the researchers’ nets, the tiny larvae are carefully sorted, counted, and preserved to create a weekly record of larval abundances and sizes.

By comparing these data over the course of years and between the two bays, the researchers hope to get a better handle on when and where larvae are coming in to each estuary. Data from this simple record of larval abundances and sizes will be fed into a complex oceanographic model that should give scientists a better idea of why the larvae arrive when they do.

This effort is the work of a team of ichthyologists, physical oceanographers, and computer modelers from Delaware, Maryland, and Virginia that has come together to combine data from the two estuaries. Funded by Sea Grant programs in each of the three states with additional support from the National Oceanographic and Atmospheric Administration, the scientists are collaborating to study patterns in the abundance and movement of fish larvae (ichthyoplankton)—particularly Atlantic croaker (Micropogonias undulates), Atlantic menhaden (Brevoortia tyrannus), and American eel (Anguilla rostrata). These species spawn offshore, then enter the Delaware and Chesapeake bays in fall and winter.

The results of this regional project should help fisheries managers better understand fluctuations in fish populations—an essential variable in harvest quota decisions. Fish populations can vary wildly. One year may bring a large group of larvae into the estuaries, while the next year may bring relatively few.

Given this uncertainty in what scientists call recruitment, it’s a challenge for managers to set harvest guidelines that support fisheries and maintain sustainable populations. Understanding factors that guide young croaker, menhaden, and eel into Delaware and Chesapeake bays should help managers make more informed decisions.

In addition to shore sampling at VIMS pier and on the Broadkill River, during the 2007–2008 recruitment season, the team conducted plankton tows across the mouths of both bays aboard the Research Vessel Hugh R. Sharp. By comparing data gathered on board the RV Sharp with shore-based station data, researchers got an idea of how accurately the shore-based stations depict the abundance of fish larvae moving into the estuary. It turns out that shore samples and vessel-based samples give similar results in terms of larval abundances and sizes, so for the next two years, the shore sampling will continue weekly to create a longer record of information about where the larvae are going and when.

Although plankton tows and station sampling give insights into the timing, abundance, and kinds of larvae moving into the estuaries, they do little to uncover the factors that direct this movement. To get a clearer picture of possible relationships between larval fish movement and physical conditions, researchers turn to oceanographic instruments that measure properties such as salinity, temperature, and the direction and speed of water movement.

The scientists hypothesize that these physical factors—winds, tides, and freshwater flow—influence circulation patterns and interact with larval behavior to determine when and how the larvae move into the bays.

To test the specifics of their hypotheses, Elizabeth North, a fisheries oceanographer at the University of Maryland Center for Environmental Science (UMCES) Horn Point Laboratory, uses data gathered on the cruises, as well as data recorded year-round by Delaware Bay and Chesapeake Bay Observing System stations. North builds computer models that explain possible mechanisms of larval movement into the two bays. The simulations will help the researchers understand how winds, tides, and river flow pull or push larvae into the bays.

Will these fundamental drivers have the same effect in both the Chesapeake and the Delaware? Maybe not.

The two bays have markedly different physical conditions, North says. Tidal currents at the mouth of the Delaware can be over one-and-a-half times stronger than those at the mouth of the Chesapeake, while freshwater inflow to the Chesapeake is four times greater than in the Delaware. Since freshwater is less dense than saltwater, North thinks it’s a factor that could affect circulation at the estuary’s mouth, which could in turn affect how fish enter the estuary.

Being able to compare the two bays, North says, will greatly extend knowledge of the recruitment process. “Comparison gives you power.” Understanding the differences and similarities of how larvae move into the Chesapeake and Delaware will give insight into the environmental and physical conditions that drive coastwide population fluctuations—insight that would be harder to achieve by looking at just one bay, she says.

Preliminary results indicate that the timing and size of larval fish at entry differs between the Delaware and Chesapeake bays.

While glass eels seem to enter both bays at similar times and sizes, croaker entering Delaware Bay are larger on average than those entering Chesapeake Bay and the opposite is true of menhaden larvae—they are larger as they ingress into Chesapeake Bay. Presumably, larger larvae are older, and the differences in size may reflect differences in spawning locations. If larvae have to travel farther from the spawning location to one bay than to the other, they should be older and larger when they reach the more distant bay. Researchers also found that peaks in abundance of the different types of larvae happened at different times in the two bays.

Thus, says North, differences in larval distributions may have implications for understanding stock structure and spawning patterns of adults, factors that ultimately control juvenile recruitment and how coastal fish populations may respond to climate change. Menhaden larvae, for example, have been becoming scarcer in the past twenty years in Chesapeake Bay, but not in Delaware Bay. The hope is that North’s models will help researchers understand how changes in spawning location and timing could interact with physical conditions to cause such a change.

Graphs of fish larva abundance.

Abundance patterns of fish larvae in the Chesapeake and Delaware Bays. Click on the image to view a larger version.

It is this type of large-scale comparative analysis that the Delaware, Virginia, and Maryland Sea Grant programs had in mind when they developed the funding opportunity for scientists in the three states to design a joint project that addresses an issue of regional priority. An opportunity that, North says, is unique in fostering collaboration, rather than the usual competition between research labs in the region. The Delaware-Chesapeake comparison is one component of a broader study comparing larval abundances and sizes along the coast from New Jersey to North Carolina.

In addition to Olney and North, the project’s leaders include physical oceanographers Bill Boicourt from UMCES Horn Point Laboratory and John Brubaker from VIMS and fisheries biologists Ed Houde from UMCES Chesapeake Bay Biological Laboratory and Tim Targett from University of Delaware College of Marine & Earth Studies. VIMS ichthyologist Eric Hilton has joined the research team for the two-year continuation of the project. The late Richard Garvine from University of Delaware was also a part of the research team, and the project will serve as the master’s thesis work of University of Delaware graduate student Ed Hale.

Working with four institutions across three states and two bays does have its challenges. Developing compelling scientific questions and combining them into a proposal, “That’s fun stuff. That’s like planning a party,” North says. The hard part, she notes, is coordinating administrative logistics, things like overhead and budgets, for their respective state’s Sea Grant programs.

The result has led to rewarding science says marine biologist Targett. “It allows these programs to do things that are larger than any one program could do alone.”
Olney agrees that the collaborative aspects of the project make it special. “One of the strengths of this program is that we’ve combined modeling, physical oceanography, and biology,” he says. This should allow the team to really get to the bottom of questions that span the three disciplines. Until they do, the late-night sampling, sorting, and cataloging of larvae will continue.

This article originally appeared as an online NOAA Research Spotlight (http://www.oar.noaa.gov/spotlite/archive).