The first approach has been used to evaluate both primary production and secondary production. The activity of the respiratory electron transport system in marine zooplankton has been related to feeding condition and other environmental variables. Currently, studies relating zooplankton growth with the activities of specific enzymes comprise an active area of research. Enzyme activities are known to be sensitive to nutritional status in adult fish, and to scale with body length in both adult and larval fish. Additionally, DNA polymerase activity is known to be responsive to changes in growth rate of Artemia salina nauplii, and may provide another useful marker of cell proliferation and growth in copepods and larval fish.
Ribosomes are the protein synthesizing complexes of the cell.
Ribosome number, and, therefore, the concentration of rRNA, at any given time is directly related to the protein synthesizing activity of a cell. Since the DNA content of a somatic cell is constant, RNA levels may be normalized to a per cell value after dividing by DNA concentration. The relationship between RNA content and growth in copepods, however, appears to be more complex, due to the nature of the molt cycle. Second, the larval period in fish is a single, well-defined life stage characterized by rapid growth. Analysis of growth and nutritional condition in fish larvae, therefore, has been relatively free of complicating factors such as age, gender and reproductive status.