Fish Population Dynamic and Stock Assessment

1.4.3. Fish abundance

Fish abundance or population size can be expressed as either the number of fish or the total fish weight (or biomass). Increases in the amount of fish are determined by body growth of individual fish in the population, and the addition or recruitment of new generations of young fish (i.e. recruits; recruits from the same year are said to comprise a year-class [or cohort]). Those gains must then be balanced against the proportion of the population removed by harvesting (called fishing mortality, F) and other losses due, for example, to predation, starvation, or disease (called natural mortality, M). In stock assessment work, removals of fish from the population are commonly expressed in terms of rates within a time period. The fishing mortality rate is a function of fishing effort, which includes the amount, type, and effectiveness of fishing gear and the time spent for fishing. Catch per unit of effort (CPUE) is an index showing the ratio of a catch of fish, in numbers or in weight, and a standard measure of the fishing effort expended to catch them.