Behavioral observations made on fish have revealed remarkably diverse reproductive strategies, including polygamy by both sexes. Still, to date, most Neotropical species remain unstudied as to whether the observed reproductive behavior in natural populations correlates with their genetic mating systems. Here, we investigated the genetic mating system of a wild population of Prochilodus lineatus settled in the Middle Uruguay River basin. By using sibship reconstruction and parental inference methods based on microsatellites’ genotypes, we inferred 45 females and 47 males as potential parents of the 87 larvae analyzed. We found evidence supporting polygamous mating in both sexes: while a high percentage of males (44.7%) fertilized the eggs of one female, 55.3% of the inferred males fertilized eggs of up to four females. Likewise, while 44.5% of the inferred females had their eggs fertilized by one only male, 55.5% of females were fertilized by multiple males. The estimated proxy of the effective population size (Nb) was 126, exhibiting moderate to high levels of genetic diversity. The genetic evidence contributed in this study complements earlier behavioral observations of formation of spawning nuclei of aggregating breeders, which may be promoting a polygamous mating strategy in this long-distance migratory fish.

Freshwater fish present unique challenges when one attempts to understand the factors that determine the structure of their populations. Habitat fragmentation is a leading cause of population decline that threatens ecosystems worldwide. In this study, we investigated the conservation status of genetic variability in the Neotropical catfish (Pimelodus maculatus). Specifically, we examined the structure and genetic diversity of this species in a region of the Upper Uruguay River fragmented by natural barriers and dams. There was no genetic structure among the four sites analyzed, indicating the existence of only one population group. A combination of environmental management and genetic monitoring should be used to minimize the impact of impoundment on panmitic populations of migratory fish species.

Salminus brasiliensis is a potamodromous fish species that occurs in southern South American rivers. In spite of its ecological and economic relevance, information regarding the reproductive biology of S. brasiliensis is still scarce. This study used data from 18 years of continuous sampling in the Upper Uruguay River Basin, analyzing 718 adult fish (307 males, 243 females, 168 undefined) captured at different months of the year. The results showed that the reproductive timing for S. brasiliensis is wide in the Upper Uruguay River, with the occurrence of mature fish between the month of August and March and spawned individuals between July and May of the next year. These results were sustained by the increase of gonadal somatic relationship (GSR) from August. The reproductive timing of S. brasiliensis in the Upper Uruguay River may start between the middle winter and early spring (from late July to late September), and may extend until the late summer and middle fall (from the middle February to early May). These findings contribute to information on the general biology of S. brasiliensis and provide valuable knowledge to management programs and to conservation efforts of this fisheries resource.