Diel vertical migration of the prawn larvae of Macrobrachium amazonicum (HELLER, 1862) in a Central Amazonian floodplain lake, Careiro Island, Brazil

Prawn larvae of Macrobrachium amazonicum have been collected during six diel cycles in the epilimnion (surface and 2 m) from three different habitats of the Lago do Rei, Careiro Island. The larval distribution showed a significant increase in density at 22 hours regardless of season or habitat, which suggests a diel vertical migration pattern. The thermal stratification of the lake (mean differential of 0.5 °C between the surface and 2 m and 0.6 °C between 2 m and 4 m) suggests that this phenomena could be only partly related to a metabolic efficiency resulting from prawns residing at cooler depths during the day. In shallow tropical waters, predation by visually dependent planktivores can be a more important factor in the adaptative significance of the diel distribution of the macrozooplankton than a low thermal stratification. The diurnal feeding activity found in the predominant planktivores fish species of the Careiro lake reinforces this hypothesis

Fishery potential of Macrobrachium amazonicum in Central Amazonia (Careiro Island): abundance and size variation

Abundance and body size variations of the prawn Macrobrachium amazonicum (HELLER, 1862) have been studied in a Central Amazonian floodplain lake, on Careiro Island. The highest number of captures and smallest sizes have been observed beneath floating macrophyte meadows. This suggests that these meadows serve a nursery habitats. The densities registered in the Amazon river were three times lower than in the lake. Maximum densities were recorded during low-water periods, while minimum densities were found during high water periods. These results point to a relationship between current force and catchability. In the floodplain lake, prawn abundance seems to be related to macrophyte expansion during the periods of rising water; the lowest captures were registered at high water, which is probably due to the spatial dispersion as the water volume increases. River prawn populations are characterized by larger sizes than the ones in the lake. They show a well-marked pattern of seasonal variation in size, with larger sizes found during periods of rising water and smaller sizes predominating during high-water periods. On the contrary, lagoonal prawns show maximum sizes during high-water, and smallest sizes during low-water periods. Despite its good fishing potential (up to 200 g/trap), the native prawn is not exploited in the Amazonas, but imported from the State of Pará. Artesanal prawn fishing in floodplain lakes, or extensive culture in ponds for carnivorous fish or human consumption, can be a commercially lucrative activity for sustainable regional economic development

Variation in egg size of the fresh-water prawn Macrobrachium amazonicum (Decapoda: Palaemonidae)

Egg size of the fresh-water prawn Macrobrachium amazonicum varied among ecologically isolated or geographically distant populations of the Amazon basin. The mean volume of both recently spawned nonpigmented eggs and older pigmented eyed eggs was smaller in prawns from the Tocantins River (0.14 and 0.20 mm3, respectively) than in those from the middle Amazon (0.17 and 0.25 mm3). The largest eggs (0.19 and 0.27 mm3) were displayed by females from the Iquitos and Guapore Rivers. Since the egg volume was independent of female body size, it was attributed to population specific characteristics. The egg size increased with the distance of the sampling site from the ocean, suggesting a progressive divergence of this species from a typical littoral population to an inland form, in a still active fresh-waterization process similar to the pattern evolved among other species in the genus Macrobrachium