Seasonal Reproductive Patterns and Length at Maturity of Sympatric Oncorhynchus mykiss and Salmo trutta in Lake Galletué, Chile

Introduced salmonids support recreational fisheries but can threaten native freshwater biodiversity, requiring reproductive evidence to inform management. We compared seasonal reproductive patterns and length at maturity of the introduced rainbow trout (Oncorhynchus mykiss) and brown trout (Salmo trutta) in Lake Galletué, Chile. We examined 250 trout collected during 10 sampling campaigns from March 2018 to May 2019 using biological measurements, macroscopic maturity staging, and complementary ovarian histology. Seasonal patterns and limnological associations were tested using multivariate permutation methods. Salmo trutta dominated catches (68.4%) and was generally larger than O. mykiss. Both species showed maximum GSI in April 2018 and May 2019, indicating maximum gonadal investment. Ovarian histology revealed mature oocytes, postovulatory follicles, and atresia, providing evidence of reproductive development and previous ovulation, although some macroscopic stages were not clearly distinguishable microscopically. Female and male L50 values were 38.8 and 34.2 cm in O. mykiss and 45.9 and 52.1 cm in S. trutta, respectively. Seasonal multivariate variation was significant in both sexes of S. trutta, whereas environmental associations were restricted to female O. mykiss and lacked consistency across analyses. The fishing season overlaps the observed autumn maximum in gonadal investment, but sampling gaps preclude identifying exact spawning dates. These findings provide a preliminary comparative baseline for evaluating management alternatives under explicit recreational fishery and native biodiversity conservation objectives.