ABSTRACTThe Patos Lagoon Estuary (PLE) is the main nursery ground for the pink shrimp Penaeus paulensis, accounting for more than 40% of its total landings in Brazil. The spatial distribution and recruitment of shrimps in estuaries are usually non-uniform and influenced by the frequency and intensity of marine water intrusions. Recruitment of penaeid shrimps in tidal-regulated estuaries often presents a pattern where smaller individuals are concentrated in the lower estuarine regions associated to the saltwater influence. Conversely, in the PLE tidal variation is negligible and mainly influenced by rainfall and wind direction, controlling the freshwater discharge and saltwater intrusion. Understanding size distribution and its relation to the ontogenetic trends in habitat use is relevant for fisheries assessment and for species conservation. Additionally, the high fishing pressure applied to this stock has led to overexploitation and its associated consequences, such as shifts in size structure by removing the larger individuals, which in turn hinders the renewal capacity of the population. Therefore, our first hypothesis is that the long-term exploitation of the shrimp stock has resulted in the erosion of large size classes. The second hypothesis of this investigation is that the size distribution of shrimps in this estuary will not present a characteristic pattern, as previously described for other penaeids in tidal-regulated environments, since the unpredictability of environmental shifts could change the life strategy during the recruitment process. Continuous sampling of pink shrimp was carried out monthly with a beach seine net at five shallow sites (PL1–PL5) for the years 1996 to 2012. Temperature, salinity and transparency data were also obtained. Sampled shrimps were measured and grouped in 10 size classes of 10 mm interval. A factorial ANOVA was used to evaluate the spatial variation of the size distribution frequency along the estuary during 16 years, and a cluster analysis was performed to identify similar sites according to size composition. Shrimps were not distributed homogeneously in the PLE (P < 0.05), and no significant pattern related to distance from the mouth of the estuary, temperature or salinity was evident. In spite of not presenting a significant difference in size composition between sites, it was possible to observe a clear tendency of higher frequency and abundance of smaller individuals in the upper estuarine areas. The dominance of small juveniles in the upper estuarine areas is probably related to the erratic pattern of saltwater penetration and freshwater discharge, resulting in a nearly unpredictable pattern of size distribution. The cluster analysis grouped two stations (PL3, PL4) with greater size diversity. These areas were identified as favourable for catching large-sized individuals. Conversely two sites (PL1, PL2) were characterized by a high frequency of small individuals and should be taken into consideration when establishing fishing restricted areas. The long-term analysis of size structure indicated that the mean size of individuals decreased approximately by 25% after the first four years of the survey, suggesting overexploitation of this species in the region, and that fishery is removing an important fraction of the population. The conservation of large individuals is vital for stock recovery, since this group represents better chances of survival and better reproductive performance.