Presence of two whitemouth croaker (Micropogonias furnieri, Pisces: Sciaenidae) groups in the Río de la Plata spawning coastal area as consequence of reproductive migration

Individuals of Micropogonias furnieri were sampled from artisanal fisheries catches in summer, when the main spawning aggregation occurs in fishing grounds off the coast of Montevideo, Uruguay, from December 1987 to March 1988. Morphometric and age analysis of otoliths from 738 fishes, showed the presence of that two groups during the spawning season. Sagittal weight and age were the variables contributed the most to separate the groups using principal component analysis prior to removing the size component and the allometric effects. The size frequency distribution between groups differed substantially. An important proportion of older/larger fish with heavier otoliths arrived earlier to the area and moved away towards to the end of the reproductive period. Age, otolith weight regressions and von Bertalanffy growth functions were significantly different (p < 0.05) between groups. A reproductive migration could be the cause of the presence of two different groups at different times in the spawning coastal area. This segregation by groups during spawning and the dispersal of the spawning effort through time might have important implications for the survival of different cohorts of eggs and larvae in a changeable environment in the Rio de la Plata coastal area.