Age, growth, and mortality of Opisthonema libertate on the coasts of northwestern Mexico

Abstract

Using readings of 1,214 Pacific thread herring (Opisthonema libertate) otoliths collected at 3 fishing locations off the northwestern coasts of Mexico (Bahía Magdalena, Baja California Sur; Mazatlán, Sinaloa; and Guaymas, Sonora), age was assigned and individual growth parameters, mortality, and exploitation rates were estimated. Up to 5 growth marks were read on otoliths and 6 age groups (0–5) were assigned. Growth marks showed annual periodicity. The age–size data set was supplemented with data of 74 larvae (reared in the laboratory from hatchling to 24 d of age) and data of 96 juveniles (collected from the natural environment, between 48 and 210 d of age). A multi-model approach was used to describe the growth of the Pacific thread herring. Five models were evaluated and their appropriateness was ranked according to the Akaike information criterion. The von Bertalanffy model was the most appropriate model for the 3 fishing areas. Parameter estimates (theoretical maximum length [L_∞], growth coefficient [k], and age at zero length [t₀]) were L_∞ = 18.68, k = 1.13, and t₀ = –0.03 for Bahía Magdalena; L_∞ = 18.63, k = 1.41, and t₀ = –0.02 for Mazatlán; and L_∞ = 18.22, k = 1.44, and t₀ = –0.002 for Guaymas. The differences between estimators by fishing area were significant (likelihood ratio test). Total mortality (Z), natural mortality (M), and the exploitation rate (E) by fishing area were as follows: M = 0.6, Z = 1.93, and E = 0.7 for Bahía Magdalena; M = 0.6, Z = 1.39, and E = 0.6 for Mazatlán; and M = 0.6, Z = 1.4, and E = 0.6 for Guaymas. Results suggest that the O. libertate population off the northwestern coasts of Mexico does not show signs of overexploitation.