Fish predation on brachyuran larvae and juveniles in the Pinheiros river, Guaratuba Bay, Paraná, Brazil

Fish predation is thought to exert an important influence on the demographical dynamics of larvae and juveniles of estuarine brachyuran crabs but few studies have investigated this phenomenon in nature. In this study, fishes were captured during full moons (in January and February 2005), when many brachyuran species are known to release their larvae. Samples were carried out in the Pinheiros river, Guaratuba Bay, Paraná, Brazil. The stomach contents of collected fishes were surveyed to determine the species that are most likely to prey on brachyuran immature forms in this location. Two techniques were used to capture fishes: manual samplings, using a 5-mm mesh size net (1.8m depth), and bottom trawling with a 20-mm mesh size net. The collected specimens were fixed using 10% formaldehyde and preserved in 70% ethanol. A total of 2941 fishes of 43 different species were collected and the stomach contents of 962 individuals were analyzed in laboratory. Food items were identified and quantified. The fish species captured using manual nets that showed the highest number of preyed zoeae per stomach was the clupeid Platanichthys platana (Regan, 1917). Sphoeroides testudineus (Linnaeus, 1758) and Centropomus parallelus Poey, 1860 showed the highest levels of predation on megalopae. Of the fish species captured using trawl nets, Genidens genidens (Cuvier, 1829) showed the highest average number of zoeae per analyzed stomach and Bairdiella ronchus (Cuvier, 1830) was the most important predator on brachyuran megalopae and on young juveniles. The obtained data indicate that brachyuran crab restocking efforts performed in the Guaratuba Bay should include strategies to avoid or, at least, reduce the access of fishes to the released early juvenile stages, given the potential substantial losses caused by fish predation. Considering that, in general, predation potential was 8.5 times lower in juveniles than in megalopae, releasing immature forms produced in laboratory in the juvenile phase or releasing megalopae into tethered areas and maintaining than under this condition until they reach the juvenile phase may be tested as a method to reduce losses given to fish predation.