Fish Identification and Measurements
Fish Identification and Measurements are common activities in our lab. Length and weight data provide important information on annual and seasonal growth rates and condition. For salmon, Otoliths and coded-wire tags (CWTs) are removed from fish which will us their age and what specific stock they belong to. Otoliths are thermally-marked with unique bar code-like patterns induced during the hatchery rearing process, whereas CWTs are implanted in fish snouts. Many fish processed in our lab are from the long-term Southeast Coastal Monitoring project that targets all five species of salmon.
Diet by Stomach Content Analysis
Fish diet studies require detailed examination of stomach contents, a particular area of expertise of our staff. Stomachs are excised and weighed, and the food bolus is teased apart under the microscope to count and identify a wide range of prey using taxonomic keys and qualitative indices. The digested prey is often only 1-2 mm in size and in pieces, making them more difficult to identify than fresh zooplankton. Diet studies are also critical in determining the consumption of zooplankton stocks by juvenile salmon.
Energy Density Determination by Calorimetry
Calorimetry is used to determine the energy content of fish per gram of body weight. This precise measure of condition is determined with a highly sensitive electronic instrument, a micro-bomb calorimeter. Fish are dried to a constant weight and pulverized to a homogeneous powder. A subsample of the powder is formed into a pellet which is combusted in the calorimeter oxygen “bomb” unit. The temperature change measured in the water bath surrounding the “bomb” is then converted to energy units or calories per gram.
Proximate analysis is used to estimate the relative amounts protein, lipid, water, ash and carbohydrate in an organism. Protein, lipid and carbohydrate each contribute to the total energy content of an organism while water and ash only contribute mass. Consequently, the total energy content of a specimen can be reconstructed from its proximate composition. Energy content is frequently used as an expression of a fish’s condition or health status. However, the relative contributions of lipid, protein and carbohydrate to a fish’s total energy vary in response to specific life history demands.
In our lab, we examine proximate composition of marine forage species to understand the interplay between the constraints imposed by the environment and life history strategy on observed energy flux.