Frequently Asked Questions About Ichthyoplankton
Detailed information about ichthyoplankton on the west coast.
What Are Ichthyoplankton?
It is important to study ichthyoplankton because the abundances of eggs and larvae of several species have been demonstrated to be good indicators of the transient spawning population size of the adults. Determining the abundance of eggs and larvae in an area is usually less expensive to do than sampling the adults. For species such as sardine and anchovy, egg and larval counts are a good indicator of population size. Thus, we can use the egg and larval data as a way to monitor trends in population abundance of the adults. We are able to tell when populations are declining, often more rapidly than if we were just monitoring adults.
For species that are not captured by a fishery, monitoring their population trends by monitoring their eggs or larvae can provide an indication of a healthy or stressed ecosystem. It is unlikely that we would have an idea of the abundance, growth or decrease of these species in any other way.
How Do We Collect Them?
There are two methods that researchers use to collect plankton at sea. The first is to tow a net through the water to catch the plankton, and the second is to pump the water aboard the vessel, and sort the plankton from it.
Plankton tows
The objective of a plankton tow is to obtain a quantitative sample of the zooplankton occupying the water column from a desired depth at the time of the tow. We collect plankton from a research vessel using three main types of net tows:
A PairoVET is a small, fine mesh net that is lowered into the water to 70 meters when the ship is stopped and towed vertically to the surface. It is used primarily to sample planktonic fish eggs.
Bongo tow
A Bongo tow is named because the nets look like bongo drums. The net is towed obliquely through the water while the ship is underway from 212 meters to the surface, effectively sampling the layer of water that nearly all the ichthyoplankton resides in.
Manta Tow
A manta tow is different in that the net is dragged just along the surface of the water while the ship is underway. Some larvae are specialized to live at the surface, such as mahi-mahi, grunion, and flying fish.
When the nets are brought back on board the research vessel, a scientist uses a hose to rinse the net, flushing the plankton to the very bottom, or cod end of the net. The contents from one side of the bongo are then rinsed into a glass jar, and a 5% buffered formalin solution is added to preserve the sample. The plankton from the other side of the bongo tow is preserved in 95% ethanol, which is a better preservative for genetic analyses and for ageing studies. All the samples are then brought back to the laboratory for sorting and identification.
CUFES
In addition to net tows, plankton is collected while the research vessel is moving using a Continuous Underway Fish Egg Sampler, or CUFES. Water is pumped aboard the vessel from 3 m depth at 640 liters/min. The water is sent through a concentrator where it passes through a net, and the plankton is diverted to a collector. While CUFES is running, a data logger is recording the date, time, and position for each sample as well as other environmental data from the ship’s sensors (e.g. wind speed, direction, SST).
How Do We Identify and Process Ichthyoplankton?
Plankton Volume Determination
For net tow samples, laboratory processing begins with plankton volume determination of the bongo net sample that is preserved in formalin. Two volumes are measured by displacement for each sample: total volume and "small volume". Total volume includes everything in the sample except any non-planktonic organisms such as juvenile and adult fish, large cephalopods, and pelagic crabs.
The total volume is determined by measuring the volume of plankton and formalin together, and subtracting the volume of formalin remaining once the plankton has been strained out. "Small volume" is determined using the same method, however, small volume is the total volume with the large plankters removed (salps and jellies).
If the volume of the small plankton component of a sample is large, after voluming the sample is fractioned using a Folsom Plankton Splitter. Usually, only one split is done to yield approximately 50% subsamples, which are then re-volumed. To reduce sorting time only one subsample is sorted. The sample selected for sorting is taken alternately from the left and right sides of the plankton splitter to avoid any bias.
Once the samples are identified, counted, and measured, they go into the collection archive and the data are sent to the data management group.
To aid the scientist in making an identification, we have a library of reference specimens accumulated over the last 50 years of nearly all the eggs and larvae, at all stages of development, that we might possibly encounter. In addition, scientists utilize CalCOFI Atlas No. 331, which is an illustrated reference guide to the identification of larval fish covering the region from approximately Oregon through Baja California.
Literature Cited
- Moser, H. G. (ed.) 1996. The early stages of fish in the California Current region. CalCOFI Atlas 33. Allen Press, Lawrence, Kansas. 1505 pp.
What Publications and Projects Are We Involved In?
Publications
The CalCOFI ichthyoplankton abundance data also are published at approximately yearly intervals as NMFS Technical Memos, and the data have provided the basis for numerous papers published in a variety of scientific journals. The many advances in fish egg and larval identification made in the larval lab over the years have been documented in a descriptive atlas: Moser, H. G. (ed.) 1996. The early stages of fish in the California Current region. CalCOFI Atlas 33. Allen Press, Lawrence, Kansas. 1505 pp.
Projects
In addition to the CalCOFI surveys and Fisheries Resources Division resource surveys such as the annual Pacific Sardine biomass cruise, larval lab personnel participate in, and/or analyze the resulting plankton samples from a variety of other research programs such as: the Southern California Nearshore Ichthyoplankton survey (2004-2005), the Cowcod Conservation Area high-resolution ichthyoplankton and oceanographic surveys (2002-2005) and the Marine Ecological Reserves survey (1998-1999). These surveys provide an efficient and cost-effective means for monitoring abundance trends of cowcod, bocaccio, and other fish in non-routinely trawled or untrawlable areas.
Are There Opportunities for Students?
Opportunities for student internships as well as volunteer and paid positions in the Larval Lab are available on a regular basis.
Types of Projects
Students can choose among a variety of projects, including:
- Learning the systematics of fish
- Learning various ichthyoplankton sample analyses
- Learning how to curate the Collection Archive
Who Can Apply?
We accept currently enrolled high school and college students.
Time Commitment
We ask students to work a minimum of one month, at least 15 hours a week. Depending on the project and the student's goals, however, some positions are available for several years.
In the past we have worked with students from:
- Local high schools (Stella Maris Academy, MET, Francis Parker)
- Harvey Mudd College Upward Bound
- UCSD Work Study
- Hire A Youth Program
For more information on student opportunities, please contact william.watson@noaa.gov or sherri.charter@noaa.gov.