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Fish Measurements (19KB)

Collection Methods

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Once collected, fish are usually placed into a holding container filled with ambient water. As an anesthetic, tablets that generate carbon dioxide are added to the water. Only a few fish are anesthetized at a time to minimize mortality due to prolonged sedation.

After being anesthetized, each individual fish is sacrificed by a sharp blow to the base of the skull and rinsed in the field with native water to remove sediment, detrius, vegetation, or any foreign substance. After rinsing, each fish is weighed to the nearest gram and measure for total length to the nearest millimeter.

A scale sample or a bony spine is taken for age determination. Knowing the ages of organisms in a composite sample helps interpret contaminant concentrations. For soft-rayed fishes, a scale sample of about 20 scales are taken from the side of the fish, above the lateral line, and slightly anterior to the midpoint of the length of the fish. spiny-rayed fishes, the scale sample is taken from the area below the lateral line, near the tip of the oppressed pectoral fin. For fish without scales, a pectoral fin spine is clipped from the fish.

The processing procedures for samples used for trace elements differ from procedures for samples used for synthetic organic compounds.

Trace Elements
For trace elements, only the livers are used in analysis. While wearing vinyl gloves, the body cavity of each fish is opened with a precleaned stainless steel scalpel and blade or stainless steel scissors, with care taken to avoid touching the liver. The sex and maturity of each fish is then recorded. The liver is exposed, and then the liver is carefully excised without puncturing the gall bladder. A different set of instruments are used in excising the liver from those used to open the body cavity because the exterior of the fish is assumed to be contaminated. The liver is then placed in a glass sample container and covered with a teflon-lined cap. Once all eight livers have been dissected, they are weighed and this weight recorded. At least 5g of sample material is needed, 10g being optimal. If the eight livers do not provide 5g of mass, more livers will have to be added to the compositie. The sample jar is then labeled with the sample identification number, date, location, species of fish, and the analyses to be performed. It is placed in a plastic bag and frozen in the field with dry ice. Finally, the sample is shipped frozen to the laboratory.

Synthetic Organic Compounds
For synthetic organic compounds, a composite of eight whole fish of one species is used as the sample. The body cavity is opened to determine the sex of the fish. For large fish, each individual is wrapped in foil and then placed in a plastic bag and labeled. Then the sample is placed in a second bag with the other fish in the sample. For small fish, all 10 fish can be wrapped in one aluminum foil packet. The composite sample is then frozen on dry ice and held frozen at -20 C or colder.