EQUILIBRIUM

A situation which exists after the specified conditions have been in effect long enough to affect all ages for the whole exploited life. The assumption of equilibrium is among the most widely used by fishery scientists. In both the Yield-per-recruit model and the Surplus production Model, the values calculated and the predictions made correspond to "equilibrium" conditions where both the environment and the fishing regime (Fishing mortality at age and age-at-first-capture) have been constant for a period equal to the average lifespan of the species, i.e. when ALL year classes have experienced the environmental and fishing regime being considered.. In practice, however, it is considered that "equilibrium" is approximated when the main age classes have experienced such regimes and that the conditions have been constant for half of the lifespan (e.g. about 10 years for hakes, 2-3 years for sardines, 1 year for shrimps and squids). In practice also, because of progressive adjustments in the fishery system (gains in efficiency, shifts in fishing practices, etc.) and variability of the ecosystem (particularly at medium-term scale) equilibrium is rarely reached.