Population structure, mortality, and recruitment of Macrobrachium amazonicum (Heller, 1862) (Caridea: Palaemonidae) in the eastern Amazon region, Brazil

Macrobrachium amazonicum (Heller, 1862) displays marked ecological plasticity as it is found in a variety of freshwater environments, from inland waters to estuaries. We analyzed a population located in the eastern Amazon region in northern Brazil. At the study site, Parananema Lake, Parintins, Amazonas state (02 degrees 40'52 '' S, 56 degrees 47'15 '' W), the species completes its life cycle in a freshwater environment influenced by the dynamics of the Amazon River. In addition to describing the population structure, this study examined data on the mortality, recruitment, and sex ratio of the population. Data were collected between March 2013 and February 2014, during which 1,300 individuals were sampled, including 386 males and 911 females (of which 155 were ovigerous). No significant difference in the mean sizes of males and females was found, and females outnumbered males in every month of the collection period. The results indicate that M. amazonicum reproduces continuously throughout the year at the study site. Peaks of different magnitudes were observed in the population size of this demographic group, with the largest occurring during the river's low-water season. These reproductive pulses resulted in bimodal monthly histograms occurring precisely in months following the observed recruitment modes. The females in this location reached maturity while they remain small in size. The smallest ovigerous female measured 5.1 mm (CL), which may be related to rapid gonadal development or extreme environmental conditions. Mortality analyses indicate that the species is not being overexploited locally, meaning that the obtained results and values were not affected by pressure from fishing. Our results confirm the existence of variations between estuarine and inland populations of M. amazonicum and indicate that the dynamics of the Amazon River, which is unique in its size and water volume, influences the life-cycle strategies of the species in the study area.