Natural mortality estimates are commonly computed from empirical methods or catch curve analysis, and their values are assumed constant for age or size in a population; however, estimates of natural mortality usually vary spatially, temporally, or by size and age. Several factors affect natural mortality rates, such as predation, disease, senescence, cannibalism, starvation, or environmental factors. Seven gnomonic time divisions (GTD) were used to estimate the natural mortality of Panopea globosa for specific portions of its life history: 1, egg to trochophore larvae (24 h); 2, early larvae (6.5 days); 3, late larvae (11 days); 4, early juvenile (35 days); 5, juvenile (3–9 mo); 6, late juvenile (1–2 y); and 7, preadult to adult (47 y). The statistical procedure based on gnomonic time divisions assumes units of time increase as a constant proportion of time elapsed from the end of the previous biological stage; in this manner, the method estimates a vector of natural mortality values by dividing the life cycle into specific time-based subunits. The results provided the following values of naturalmortality atGTD: 1 = 537.42/y; 2 = 230.32/y; 3 = 134.35/y; 4 = 33.58/y; 5 = 2.55/y; 6 = 2.21/y; and 7 = 0.046/y. The consistency of the estimates derived were compared with previous reports of mortality rates and yielded similar values. The gnomonic time method proved to be particularly effective in estimating natural mortality based on the specific life history and life span of the geoduck.
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Vol. 34 • No. 1