June 5, 2001 COLLEGE STATION, Texas -- It's a hard life -- too young to swim, being tossed this way and that by the waves and tides, the ever-present danger of becoming a fish's lunch, the limited time to get into shallow water to avoid being sucked back into the Gulf of Mexico, the search for a patch of sea grass in which to settle.
Red drum larvae face a lot of challenges traveling from the Gulf of Mexico, where they are hatched, into the estuaries that serve as their nursery grounds. A recently completed Texas Sea Grant-funded study examined the role physical factors, such as winds, tides and currents, play in moving red drum larvae from the Gulf into the nursery grounds of Aransas and Corpus Christi bays.
Doctoral student Cheryl Brown said the study found physical factors play a key role in explaining why red drum larvae only settle in certain parts of the bays. Instead of evenly distributing red drum larvae around the bays, the winds, tides and currents tend to concentrate the larvae in certain areas, she said.
"There tends to be high settlement where sea grasses are in close proximity to the inlet when there is also a high supply of larvae," she said.
Researchers say the results of this study can be applied to other marine animals -- such as Gulf shrimp -- that spawn in the Gulf but rely on estuaries as nursery grounds. Also, when combined with the results of other research, the findings suggest that variations in currents and winds may cause fluctuations in the size of fish populations along the entire Gulf coast.
Using a computer model to simulate the exchange of water and fish larvae through Aransas Pass, researchers found that many of the "fish larvae" settled in an area of sea grasses just inside the pass. Successful larvae use pathways close to shore to enter the estuaries, Brown said. Researchers also found that 60 percent of the fish larvae make it into the inlet at some time.
"This means that this accumulation of larvae next to the shore and their transport down the coast is an efficient way of getting them into the bays," Brown said. "We didn't expect that many to make it in. We were pretty surprised."
Researchers were also surprised to find an abundance of "fish larvae" close to the coastline.
"We found that we need to move inshore," she said. "We need to study the distribution of larvae in the surf zone. In addition, we think that this may be a common transport property for other inlets."
Texas A&M University oceanographer George Jackson said the winds, tides and currents concentrate the larvae near inlet openings and then the larvae are slurped through the passes in pulses. The estuary acts as a big pump, taking water from close to shore, expelling it through the pass in a jet and then sucking the water back into the inlet, he said.
Brown said this information could help fishery managers better identify essential fish habitats that are important to preserve. By knowing where tides and currents will carry fish larvae, managers can protect these areas as well as access to them.
"If we substantially change the patterns, it could result in more larvae being transported to regions that don't have suitable habitat," she said.
The work also may let scientists know how changes in sea level and weather patterns could impact fish and their critical habitats, she said.
"We could also begin looking at climate change and how it will affect wind patterns along the coast," she said. "With models such as this, we can see what these larger scale changes in wind patterns may mean for recruitment."
This study is part of an ongoing effort combining biological information and insights with ocean physics to further the understanding of variability in fish populations.
While this study emphasized the role physical factors play in a fish's life, Jackson said, it is a collaborative effort with biologists Scott and Joan Holt of The University of Texas Marine Science Institute, Port Aransas. Biologists can use the study's results to help them understand why fish larvae are more abundant in certain regions, he said. It will also give scientists the opportunity to study how fish in different bays are connected.
The National Sea Grant College Program is a partnership of university, government and industry, focusing on marine research, education and advisory service. The Sea Grant Program is a practical, broad-based effort to promote better understanding and use of marine resources through research, education, extension and information transfer.
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