Life cycle, natural mortality and exploitation rate of Totoaba macdonaldi

Natural mortality rate (M) is a crucial parameter for fish and other species. In fisheries management it is common practice to obtain estimates of M using one of several empirical formulas available in the literature. However, when using these formulas, an important question arises: for which life stage or age does the estimated rate pertain to? In the present work, gnomonic stage-specific mortality rates are estimated for Totoaba macdonaldi, a vulnerable fish species of the gulf of California. The gnomonic method is based on knowledge of the species life cycle; it tracks numbers surviving during exponential decay of a cohort under specific restrictions. An average weighted mortality rate of 0.108/year is then computed for adults and senescent fish subjected to poaching. This allowed us to ascertain status of the population calculating exploitation rate (80%). It is concluded that the totoaba population is under heavy overexploitation.