Information such as the number of trips needed to catch a fish, the amount of fish caught, and the mass of fish collected is gathered by these agencies. They use the information for researching the factors affecting the availability of the fish. A common way to record overfishing is with a measure called the exploitation rate. It uses biomass, the total weight of a selected population of organisms, as an indicator of fish or invertebrate abundance. The exploitation rate calculation takes into account the natural reproductive rate of a particular organism and the natural rate of death needed to maintain the organism's population.
Researchers then evaluate the biomass loss due to fishing.
Overfishing is then defined as an annual rate of catch that lowers the population of the organism. Regular overfishing can decline an animal population to the point of extinction. Factors such as pollution and loss of habitat must also be taken into account when estimating overfishing. However, it is not always possible to predict unexpected losses due to environmental catastrophes. So, overfishing may occur even when the calculations indicate safe levels of fishing.