In order to effectively manage human impacts on marine mammals, we need to know how many there are in the management area. We also need to know if this number is changing over time: is the population going up or down? The best way to get this information is to carry out surveys to estimate the abundance of whales or seals in specific areas. This must be done repeatedly over time to determine if the population is rising or falling.
Surveys are carried out to generate estimates of absolute abundance, which is the number of animals in a specific area at a specific time. In some cases estimates of relative abundance (a fraction of absolute abundance that is assumed to be constant) are also useful. If surveys are repeated over time, trends in abundance (whether the number of animals is increasing or decreasing or stable) can be estimated. The main use of these estimates is for the management of whale and seal populations. They are also used for general environmental monitoring and ecosystem research.
“Management” does not mean telling whales what to do! It is actually the management of human impacts on whale populations, mainly direct catch (hunting) and indirect catch (bycatch, ship strikes), but also other impacts such as pollution and climate change. The main goal of management is usually to ensure that human impacts on marine mammal populations are sustainable, meaning that they do not cause the populations to decrease below a pre-defined threshold. To do this, estimates of abundance are combined with past, present and projected future catch levels in a population model, a mathematical model that mimics the response of the population to catch. This enables managers to set allowable direct and/or indirect catch levels that will not endanger the population.