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Monthly samples of Pacific sierra Scomberomorus sierra were taken during nine years (1998–2000 and 2002–2007) with gillnets in Navidad Bay, Jalisco (Mexico). The sex ratio for the whole sampling period was not significantly different from 1 (F:M, 1:0.9). Monthly trends of the gonadosomatic index, the percentage of individuals with ripe gonads, and mean oocyte diameter indicate that this species reproduces in the period from April to June (spring). This reproduction period is also supported by the microscopic characteristics of gonads (highest percentage of oocytes in advanced vitellogenesis phase and high concentration of spermatozoa in lobular lumen and testis main duct). Total length (TL) ranged from 26.0 to 102.8 cm, with a mean of 53.3 cm. Mean maturation lengths calculated for females and males were 59.3 and 56.4 cm TL, respectively. Most individuals caught had a length which was smaller than the mean maturation length indicating that the fishery is based mostly on juvenile fish.
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