The importance of shallow areas as nursery grounds for the recruitment of blue crab (Callinectes sapidus) juveniles in subtropical estuaries of Southern Brazil

Abstract

Estuaries are extremely rich areas that act as nurseries for many species of fishes and crustaceans. This study is the first to address the juvenile distribution of ecologically significant size classes of blue crabs in estuaries from their southern region of distribution. We investigated the spatial and temporal distribution of the populations of juvenile blue crabs in Tramandaí-Armazém estuary (TAE) and Patos Lagoon estuary (PLE), southern Brazil, to determine possible areas of protection. The hypothesis that physical factors can direct the distribution of crabs was tested through correlations of the abundance of different size classes with some environmental parameters, in order to determine the concentration areas of blue crab juveniles. Four sites in each estuary were sampled with a bottom trawl for 18 months. Temporal patterns were more evident than spatial ones in the distribution of blue crabs. There were two recruitment peaks: one during the warmer months of spring and summer and another during the winter. Increased rainfall apparently delayed the first recruitment peak during summer–autumn of the second sampled year. The timing of winter recruitment may be associated with a reproductive cycle delay evidencing the existence of a delicate year-by-year balance in the recruitment. The spatial distribution of the blue crab was discrete in PLE, whereas it was more homogeneous in TAE. Salinity played an important role in the distribution of blue crabs in PLE, where recruits preferred the upper estuary embayments that offered shelter in spite of the lowest salinities. A clear salinity gradient is absent in TAE and was not correlated with the spatial distribution of blue crabs. The upper shallow areas of PLE are critical for the recruitment of the blue crabs, and therefore efforts should be made to preserve this nursery area.