An indirect approach of estimating spawning fraction as applied to Sardinops sagax from northern Chile

The spawning fraction of ashes with multiple spawnings and indeterminate fe cundity is normally estimated through biologically intensive and time-consu ming histological procedures, such as the one based on the proportion of fe males with post-ovulatory follicles of 1 day of age. Alternative methods ba sed on macroscopic observations of the gonads are desirable. We conceptuali ze the spawning fraction as an area under a normal distribution of females classified according to oocyte diameters in the more advanced batch. The ar ea depends on the value of two parameters of the normal distribution (mean and standard deviation) and the diameter at which the oocytes start hydrati on, when they are ready to be spawned. This conceptualisation was applied t o Sardinops sagax of northern Chile, for which additional knowledge allowed us to reduce the problem to specifying a single property of the normal dis tribution: the standard deviation. Calibration of the area under the normal curve against the spawning fraction derived from histological information yielded a calibrated standard deviation parameter in a monthly series spann ing one year. This parameter in turn was statistically related to the gonad osomatic index. as a relative measure of gonadic development which is easy to obtain for any given fish population. Furthermore, the gonadosomatic ind ex was shown to be a proper description of size-standardized gonadic develo pment. The equation relating this index and the standard deviation was then used to predict the spawning fraction which was in close proximity to the histologically-derived spawning fraction. Through this approach the gonados omatic index can be used for years in which no histological information is available, providing an indirect way to obtain the spawning fraction when a time series of the index is available.