-In Amazonian fisheries, the silver arowana, Osteoglossum bicirrhosum (Cuvier 1829) is heavily exploited for human consumption as an adult, and for the aquarium trade as a small juvenile (yolk sac juvenile mainly). The periodicity of annuli formation on otoliths and growth variability of the silver arowana were studied in different river river-basins of the Peruvian Amazon between 2006 and 2009. Transverse stained sections of 606 individual otoliths were analysed from four different river-basins, the Amazonas, Ucayali, Napo and Putumayo, of which 554 could be interpreted. These belonged to 274 females ranging from 15 to 91 cm (standard length) and 280 males ranging from 30 to 91 cm. In addition, yolk sac juveniles of known age were collected to improve growth modelling. Monthly proportions of stained otolith edges validated the formation of a single annulus per year in two different river-basins with lagged hydrological cycles: the Amazonas-Ucayali and the Putumayo. Stained growth mark counts resulted in a longevity estimate of at least 16 years for the silver arowana in the Peruvian Amazon. This fish grew quickly during the first two years, and asymptotic growth was reached after four to five years, except in the Putumayo where it was reached slightly earlier. Results showed no significant growth dimorphism between sexes within basins, but indicated significant growth differences among river basins. Silver arowanas measured, on average, between 38 and 40 cm at the end of their first year. Length-at-age differences among river basins increased with age to reach over 14 cm (and >3 kg) after 7 years between the faster and slower growing populations (Amazonas and Putumayo, respectively). The growth differences observed emphasize the need for further investigation on the population structure of this species as, although these differences might merely be phenotypic responses to contrasted environmental conditions, they could alternatively reflect the existence of several populations with distinct genetic and life history characteristics. The consequences of such differences would be very important for the management and conservation of this fragile and extensively exploited species.

Summary The aim of this study was to assess the variability of reproductive characteristics of the silver Arowana, Osteoglossum bicirrhosum, in four sub‐basins of the Peruvian Amazon: the Amazonas, Ucayali, Napo and Putumayo rivers. An illustrated, detailed gonadal maturation scale is proposed for a clearer determination of the reproductive status of male and female Arowana. The breeding season occurred during the dry season and early rising waters in the Amazonas and Ucayali, whereas it was restricted to the dry season in the Napo and Putumayo. A same female can reproduce more than once during the breeding season. Age at maturity was reached during the second year, at sizes varying between 43–45 cm for males and 46–50 cm SL for females. Important variations in fecundity were observed between fish of the four sub‐basins, whereas mean oocytes diameter and mass did not vary significantly, suggesting differences in reproductive investment. Together with previously reported variations of growth patterns in the same sub‐basins, these results suggest the existence of distinct populations in the Peruvian Amazon, with important consequences for management and conservation, but also for future aquaculture development programs.

El objetivo de este estudio fue determinar la diversidad y abundancia de ectoparásitos en las branquias de 80 ejemplares de arahuana Osteoglossum bicirrhosum (Cuvier, 1829) provenientes del lago “El Dorado”, “Reserva Nacional Pacaya-Samiria”, Loreto, Perú. Las branquias de cada ejemplar fueron colectadas y colocadas por separado en solución formalina y agua (1:4000) y preservadas en refrigeración a 4°C hasta su análisis en el laboratorio. El análisis consistió en la localización, conteo, colecta y fijación de los ectoparásitos de cada arco branquial. Los resultados muestran que el 100% de los ejemplares analizados estuvieron infestados con uno o más ectoparásitos. Fueron identificados un total de cinco especies de Monogenoidea: Gonocleithrum cursitans Kritsky & Thatcher, 1983, Gonocleithrum aruanae Kritsky & Thatcher, 1983 y Gonocleithrum coenoideum Kritsky & Thatcher, 1983 y dos de Copepoda: Ergasilus sp. y Therodamas elongatus (Thatcher, 1986). Gonocleithrum cursitans fue el ectoparásito con mayor prevalencia (100%). Therodamas elongatus es reportado por primera vez en el Perú parasitando a ejemplares de O. bicirrhosum.