LAUSR

This study examined the changes in sex ratios and sex reversal rates in pejerrey Odontesthes bonariensis, that occur with the progression of the spawning season in a seminatural setting. Four groups of hatchery-produced pejerrey larvae were stocked in floating cages in La Salada de Monasterio lake (Pampas region), a natural habitat of this species, and reared from hatching beyond gonadal sex determination with minimum human interference. Cage 1 was stocked at the beginning of the spring spawning season and the other cages with monthly delays until cage 4 in early summer. The genotypic (amhy+: XY/YY, amhy-: XX) and phenotypic (testis: male, ovary: female) sex ratios and proportions of genotype/phenotype mismatched individuals were estimated and their relation to water temperature and daylength during the experiment was analyzed by generalized linear modelling (GLM). Water temperature varied between 11°C and 30.5°C and daylength duration between 11 h 22 min and 14 hours 35 min. Sex genotyping revealed nearly balanced sex ratios of XY/YY (46-49.1%) and XX (50.9-54%) fish in cages 2-4 whereas the genotypic sex ratio in cage 1 was clearly biased towards XY/YY fish (60.6%). Phenotypic males ranged from 42 to 54.4% in cages 1-3. Cage 4, in turn, had significantly more phenotypic males (66%). The percentage of XX-males (phenotypic male/genotypic female) was 23.1% in cage 1, decreased to a minimum of 5.4% in cage 2, and gradually increased in cages 3 and 4 to a maximum of 40.7% in the latter. The percentages of XY/YY-females (phenotypic female/genotypic male) were highest in cage 1 (30%) and decreased progressively in the other cages to a significantly lower value (4.3%) in cage 4. These results generally support the findings of laboratory studies on the effect of temperature on sex determination of this species and also provide novel evidence of a XX genotype-specific masculinizing effect of short daylength. This article is protected by copyright. All rights reserved.