Sample Processing and Identification

Plankton Volume Determination

Splitting plankton sample with a Folsom Plankton Splitter    Voluming a plankton sample

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.

Identification Methods

        Sample jar

First, all of the fish eggs, larvae, cephalopods (squids and octopods), and lobster larvae are sorted out from the other plankton in the sample jars. Then an ID expert looks at each animal under the microscope and identifies it to the lowest taxon possible, usually to the species level. The numbers of individuals are counted. For target species, such as Market Squid, Pacific sardine, Northern anchovy, Pacific hake, Bocaccio and Cowcod rockfishes, and Pacific mackerel, we measure the length as well. Eggs of some species, such as Pacific sardine, are further classified into developmental stages.

         Researcher identifying ichthyoplankton

The developmental stages as well as the larval lengths are used to construct a mortality curve for each of the species. The mortality curve is one of the components that contributes to the formulation of the biomass estimate. This and other data are used to support management of these species.

Once the samples are identified, counted, and measured, they go into the collection archive and the data are sent to the data management group.

Reference Library

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 fishes covering the region from approximately Oregon through Baja California. Nearly all the Larval Lab staff were principal contributors to the publication of this Atlas.

1Moser, H. G. (ed.) 1996. The early stages of fishes in the California Current region. CalCOFI Atlas 33. Allen Press, Lawrence, Kansas. 1505 pp.



Last modified: 12/24/2014