niloticus (Linne), for at least 15 years, and now supports 95 % of the total local catch. Lake Metztitlán, with 640 ha, has received continuous stocking of the hybrid O. Although they because of a temperature reduction, drying up were initially introduced indiscriminately to large basins, after 20 years they have spread to numerous smaller water bodies and have become the most important fisheries species in Mexican continental waters. They became widely distributed throughout natural and artificial water reservoirs in both tropical and temperate regions of the country. aureus (Steindachner) were first introduced to Mexico (Temascal, Oaxaca) in 1964 by Auburn marks could be formed once during the dry University, Alabama. Tilapia rendalli (Boulenger), Oreochromis mossambicus (Peters) and O. This drive us to look ahead and search for structures that can help in the ageing process of this species. This could lead to confusion between ring mark formation due to natural causes and those due to stocking events. Moreover, when these species are subject to stocking, several ring marks can be formed at the farm due to changes in diet, movements between tanks or stress (Britton et al. 1995, Booth and Merron 1996, Gómez-Márquez 1998) or during winter (Weyl and Hecht 1998), or two times of water bodies or sexual maturation (Guzmán 1994, Admassu and Casselman 2000, Jiménez- Badillo 2004). Contrary to temperate areas where annuli are formed each year, the ring marks of otoliths or scales in tropical wild tilapia populations show wide variability. To the fisheries biologist, otoliths, fish scales and hard structures in general are impor tant tools in the understanding of the life of fish and fish populations. Key words: Oreochromis, age, stockings, otoliths, fish scales, annuli, circuli. This proportion was maintained during the study period, and could be of help to calculate an approximate age of juveniles, especially in stocked fish that show multiple ring marks because of manipulation in fish farms and stocking events. The circuli had a continuous regular behavior, with a formation rate of 10.38☐.93 and 11.38☐.95 circuli/month for the first and second cohorts, respectively. The ring marks of the scales and opercula showed great variability, and the sagittae had a significant relationship with length, but it is unclear if at least the first ring mark could be formed at the hatchery and reflect changes in diet and/or tank movements in the fish farm. Since the date of birth of these juveniles was known, the analyses focused on whether the ring marks of the scales, sagittae and opercula or the circuli of the scales could be used to age them. niloticus), and monitored every month for one year. In the period September-November, two cohorts of 1.6 million juveniles of a tilapia hybrid were stocked ( Oreochromis aureus/ O. Vicentina, 09340 México D.F., México Phone: +52 (55) 5804-6585, Fax: +52 (55) 5804-4738.Ībstract: Lake Metztitlán was dried up completely in the spring of 1998 and refilled in August of that year. Romo-Coronel 1ġ Universidad Autónoma Metropolitana-Iztapalapa, Av. niloticus) (Perciformes: Cichlidae)Īna L. Formation of ring marks in stocked tilapia juveniles (Oreochromis aureus/O.
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