Arreguín-Sánchez, F. (2023). Age-specific natural mortality - implications for stock assessment and management of Mexican fisheries. Ciencia Pesquera. 31(1): 43-49.
Francisco Arreguín-Sánchez 1
The instantaneous rate of natural mortality, m, is a key parameter estimated in fisheries stock assessments. Conventionally, m has been assumed to be constant over time and age; however, this assumption typically generates an underestimation of recruitment and an overestimation of spawning stock. If a fish stock is exploited to the limit of its biological production capacity or if it is under recruitment or growth overfishing, then it is important to increase the precision when estimating survivors at critical ages. The estimate of m-at-age/length is based on the notion that m varies with the ontogeny of a species. Almost all methods for estimating m-at-age/length are linked to estimates of individual growth to approximate the ontogeny of species. The gnomonic-interval method allows estimates of M at life history stages that are biologically and ecologically consistent with observed data, and these estimates can be translated to age or length. In Mexico, methods based on length and growth have been applied to some species of Veneridae clams, while the gnomonic-interval method has been applied to species with different life histories, such as the short lifespan penaeids shrimp (Penaeus spp.); long-lived species, such as the top predator red grouper (Epinephelus morio), and the filter-feeding species, the geoduck clam (Panopea globosa); and medium lifespan species with different life histories such as the small pelagics (Sardinops caeruleus); an intermediate trophic level of active predators, such as the giant squid (Dosidicus gigas); and the sea cucumber (Isostichopus badionotus). It should be noted that in the studied cases, the gnomonic-interval method did not consider variability over time, only for life history stages, while methods based on length and growth can be applied to estimate time-variable natural mortality, as in the case of the Venus clam (Chione californiensis). The analysis showed that biased estimates of population size, which occurred due to the assumption of constant natural mortality with ontogeny, were larger for short-lived than long-lived species, while the time varying estimates of m showed a bias of approximately ± 35% in a seasonal time frame for the Venus clam C. californiensis.
Palabras clave: Constant-m; m-at-age/length; ontogeny-dependent-m; Stock assessment; Fisheries Management
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