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We investigated population and social structure of the franciscana dolphin, Pontoporia blainvillei, an endemic and the most endangered cetacean of the southwestern Atlantic Ocean. We analyzed samples from the Rio de la Plata estuary obtained in Uruguayan waters and from the Atlantic Ocean obtained in both Uruguayan and Brazilian waters. Mitochondrial and microsatellite DNA markers were used to study differentiation between the estuary and the ocean and the association between kinship and social group structure. Although multilocus analyses suggested that franciscanas are structured into 2 subpopulations (K = 2, divergence among clusters: F(ST) = 0.06, P = 0.002; R(ST) = 0.3, P = 0.001), mitochondrial markers did not support such divergence (F(ST) = 0.02, P = 0.12; Ф(ST) = 0.06, P =0.06). However, these units are not entirely segregated geographically. Regarding social structure, some groups are composed by first-order related individuals (R ≥ 0.5, P < 0.5). Overall, the data suggest that matrilines could be the social unit in this species. We argue that the divergence found could be associated to local adaptation and social structure, resulting from either feature leading to a recent divergence or reflecting equilibrium between local differentiation and gene flow. This evidence supports considering franciscanas from the Rio de la Plata estuary a discrete management unit.

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The franciscana dolphin (Pontoporia blainvillei) is the most threatened small cetacean in the southwestern Atlantic Ocean. Its incidental capture in the Uruguayan fisheries of the Atlantic oceanic coast (AOC) has been recorded since 1940 and was systematically studied from 1974 to 1994, providing the most recent by-catch per unit of effort estimate (BPUE) for the AOC region (0.0064 franciscanas per 1000m net per fishing set). The lack of new by-catch data during the past 12 years has led to a data gap in by-catch estimates for the species in Uruguay. The current study was developed in two separate stages, a first stage (July 2004 -December 2005 designed to identify fisheries that interact with franciscana, and a second stage (2006), designed to determine new BPUE and franciscana mortality estimates for the selected fisheries. During the first stage, 13 artisanal fisheries of the Uruguayan coast were visited monthly, while in the second stage five fisheries (including the Rio de la Plata estuary and the AOC) were selected for monitoring. During 2006, 26 fishermen recorded all the information related to each fishing event, allowing the estimation of fishing effort calculated in linear units multiplied by hour (FE L ). We also estimated a fishing set (FE S ) based by-catch rate which allowed the comparison with previously reported results. For 2006, the BPUE L (based on FE L ) was estimated at 0.0020 franciscanas per 1000m net per hour and the BPUE S at 0.0286 franciscanas per 1000m net per fishing set. The BPUE L was extrapolated to the surveyed fishing fleets during 2006, resulting in a mortality estimate of 289 (95% CI: 266-350) franciscanas. Based on the fishing dynamics reported herein, we consider the BPUE L the most accurate estimate of by-catch for the Uruguayan coast. This research updates the values of BPUE in the AOC after a 12-year information gap and includes the first by-catch evaluation in the Uruguayan estuarine coast. We highly recommend an abundance estimation of the species to complement the information reported herein in order to know the current status of franciscana dolphin population in Uruguay.RESUMEN: El delfín franciscana (Pontoporia blainvillei) es el pequeño cetáceo más amenazado de Atlántico Sudoccidental. La captura incidental de franciscanas en las pesquerías uruguayas de la costa del Océano Atlántico (AOC) ha sido registrada desde 1940 y relevada sistemáticamente entre 1974 y 1994; brindando el último valor de Captura Por Unidad de Esfuerzo (BPUE) en la AOC uruguaya (0,0064 franciscanas por1000m de red por lance). La ausencia de nuevos valores de BPUE durante 12 años produjo una carencia de datos sobre la captura incidental de la especie en Uruguay. El presente estudio fue realizado en dos períodos, el primero (Julio 2004-Diciembre 2005) con el objetivo de identificar las pesquerías que interactuaban con la franciscana, y el segundo período (año 2006) para determinar la BPUE y mortalidad de la franciscana en las pesquerías seleccionadas. En el primer período se visitaron mensualmente ...