Charleston Gyre

Daily At-Sea Log February 3rd

February 3, 2003

What is ichthyoplankton and why is it important to our mission?

By Jon Hare, Chief Scientist

Most marine fish have complex life history
cycles. Complex means that a species of fish has different life stages that occupy different habitats. The most common life cycle in fishes is adults – eggs – larvae – juveniles – adults, although there are variations on this theme. Typically many small, planktonic eggs (~ 1 mm) are released into the water during spawning; ( above: Hake larvae, Urophycis )
planktonic means that the eggs at the mercy of ocean currents. An individual fish can produce tens of thousand to millions of eggs each year. The eggs hatch and the larvae are also planktonic. Larvae feed and grow from ~1.5 mm to ~20 mm and then transform into juveniles. This transformation is not as dramatic as the butterfly’s metamorphosis, but is marked by a varying degree of change in body plan, feeding, and habitat. In the case of flounder, the head twists 90o and the fish goes from swimming in the water (like most fish) to laying on its side on the bottom. After the juvenile stage, which lasts for months to years, fish then join the adult breeding population in adult habitats.

(below: Horned Whiff larvae,
Citharichthys cornutus)
The planktonic stages of fish, those at the mercy of ocean currents, are termed ichthyoplankton from the Greek root for fish – ichthyo. So why is ichthyoplankton important? If all the millions of eggs and larvae survived to become adults, we would literally be swimming in fishes. But, all of the eggs and larvae don’t survive, and mortality during the first year of life typically exceeds 99.99%. In other words, out of 100,000 eggs spawned, fewer than 10 will survive their first year of life. Thus, the surviving eggs and larvae are the exception rather than the rule, and the factors that create the exceptions, the surviving ichthyoplankton, have very important consequences to adult fish population.

Eggs and fish larvae die in many ways. They can starve. They can be eaten. They can be transported away from suitable habitats. The survivors are those lucky few who get enough to eat, avoid the myriad of predators, and are transported favorably by ocean currents. These few larvae then enter juvenile habitats and must again get enough to eat and avoid predation. After making it through this early life cycle gauntlet, the surviving fish join the adult population and reproduce, initiating the cycle again. Determining which factors dominate early life cycle survival will provide important information as to the health of fish populations. Bad years could be marked by not enough food or cold temperatures. Good years could be caused by favorable winds or few predators. Understanding the factors that affect early life cycle survival will contribute to NOAA’s mission of building sustainable fisheries and sustaining healthy coasts.
(below: the researchers wash the caught plankton into a holding container)

This cruise finds us north of the Charleston Bump studying ichthyoplankton. Why are we working here? The Gulf Stream runs northward along the edge of the southeast United States continental shelf. After passing over the Charleston Bump, the Gulf Stream often veers to the east and then back to the northwest. This veering sets up counter-clockwise rotation, or cyclonic rotation, and deep, nutrient rich water is sucked up towards the surface. The area of semi-permanent cyclonic rotation is termed the “Charleston Gyre” and we are exploring whether the “Charleston Gyre” is a good place for the survival of ichthyoplankton, or fish eggs and larvae.

The cool nutrient rich water that upwells in the center of the “Charleston Gyre” fuels the growth of single celled plants called phytoplankton (phyto is the greek root for plant). The plants are food for zooplankton, which are small animals living in the plankton (zoo is the Greek root for animal).
(below: Plankton samples)
Many different types of animals make up the zooplankton: chaetognaths, salps, copepods, amphipods, isopods, pteropods, and many more. Zooplankton in turn is larval fish food. So, the “Charleston Gyre” may be a good place for the survival of ichthyoplankton because of all the food. However, fish larvae are also food themselves, and the “Charleston Gyre” may be a good place for the predators of fish larvae (for example chaetognaths and ctenophores). Finally, once formed, the “Charleston Gyre” moves with the Gulf Stream along the edge of the shelf and another “Gyre” forms in its place. What happens to the water and fish larvae in a “Charleston Gyre” is unclear. Ichthyoplankton may be trapped and not able to get to juvenile habitats. Alternatively, the “Charleston Gyre” may move along the shelf and supply fish larvae to their appropriate juvenile habitats. In the end, we hope the data we gather will allow us to discover if the “Charleston Gyre” is a good place for fish larvae.

Career of the day

Marcy Hutchinson, Physiology and Ecology Technician
Marcy actually works for a company called ORISE and is contracted by NOAA. She got her Bachelor’s degree in biology with a concentration in marine biology. She went on to get her Master’s degree, also in biology. She said she had always been in interested in the ocean but chose her career for the “fun factor”.

Some of Marcy’s duties include sorting and identifying zooplankton, acquiring and analyzing chlorophyll samples and help write cruise reports. She’ll be working this summer in a marine protected area in the Dry Tortugas.

The thing she likes the most about her job is the field work and being outdoors. What she doesn’t like is getting seasick! She decided that if she could be any sea creature it would be a dolphin. Why? They’re fast, graceful, intelligent and seem to have a lot of fun.

Ph. 843-953-7263
Project Oceanica
Dept. of Geology & Environmental Geosciences
College of Charleston
Charleston, SC 29424
Fax 843-953-7850