| Abstract: A field study of age, growth and mortality of the “boga”, Leporinus obtusidens (Valenciennes, 1847) was carried out in the Yacyretá Reservoir, High Paran´a River, Argentina.We compared age-structure, length–weight relationships and condition factor coefficients in phases previous (1990–1994) and subsequent (1995–1998) to the reservoir filling. In addition, we estimated total and natural mortality, as well as the growth and yield parameters for the whole study period. Comparisons were carried out in four sampling points within the river main channel. Fish were sampled monthly with nine gillnets ranging from 40 to 160mm of total mesh opening (opposed knots). A scale study showed an age range comprised between 1 and 13 years, with ages 4 and 5 the most abundant before reservoir filling, whereas ages 2 and 3 were more frequent afterward. The weight–standard length relationship showed a higher growth during the second phase of reservoir filling. Maximum condition factorwas registered before the spawning period (August and September), being higher after the impoundment, and particularly in the sampling site located within the reservoir. The von Bertalanffy growth coefficient, K, reached 0.12 in both sexes, while the asymptotic length, L∞, was 578 and 547 mm, in females and males, respectively. Natural mortality according to Pauly’s equation reached 0.178 year−1, and the total mortality was 0.36 year−1. The Beverton and Holt yield per recruitment (Y/R) was estimated as 246.54 g, with a maximum sustainable yield of 248.00 g. This result suggests that the species had higher condition after impoundment and is fished near its maximum capacity in the reservoir. |