Natural mortality of three commercial penaeid shrimps (Litopenaeus vannamei, L. stylirostris and Farfantepenaeus Californiensis) of the Gulf of California using gnomonic time divisions

The estimation of natural mortality (M) is critical for stock assessment and fisheries management. The shrimp fishery is the most valuable one in Mexico and along the Pacific Coast of Mexico, and exploitation primarily targets three species: white (Litopenaeus vannamei), blue (L. stylirostris), and brown (Farfantepenaeus californiensis). It is a sequential fishery, so an appropriate estimate of M for different life stages is required for management purposes. Typically, M is estimated from the exploited stock, which is usually composed of adults, assuming a constant value for M, and this estimate is used for studies of population dynamics, stock assessments and determinations of the status of a fishery. In this study, we estimate M-at-age (i.e. life stage) for each species using the gnomonic time division model. The gnomonic intervals correspond to the actual life stages reported in the literature, whose duration was used for model fitting. The gnomonic model showed that M declines sharply in early life stages but declines to an asymptotic value after reaching maturity, and the model provided biologically consistent estimates of M at each life stage for the three shrimp species. Such estimates may be used with confidence to model the dynamics of sequential shrimp fisheries.