Micropogonias furnieri, Desmarest

Implications for freshwater Micropogonias furnieri View in CoL

Some interesting information regarding the life cycle of M. furnieri from Mirim Lagoon has emerged from these results. Micropogonias furnieri collected in Mirim Lagoon were on average 35 years of age and there were no samples less than 32 years of age. This suggests that most fish in the lagoon were from a few cohorts that recruited to the lagoon over 30 years prior to sampling. Life-history profiles of barium and strontium in otoliths indicated that the fish in Mirim Lagoon entered the lagoon very early in their life history and have remained there for their entire lives as a trapped group. The dominance of the Mirim Lagoon group by a few cohorts of very old fish indicates a sustained failure of reproductive success for this group, and that few individuals have been able to enter the Mirim Lagoon for over 30 years. Recent samplings on Mirim Lagoon found eight 3-4 years old M. furnieri together with almost 100 individuals older than 32 years (Castello, pers. comun.), which corroborates the absence of significant stock renewal in that lagoon. About 32 years before our fish collections, a floodgate was built in the São Gonçalo channel, the only communication between the estuary of Patos Lagoon and Mirim Lagoon. Our results demonstrate that this closure was effective on preventing significant fish migration from estuarine and coastal regions of Rio Grande to Mirim Lagoon, and it is the most plausible explanation for the almost complete absence of young fish inside the lagoon. If M. furnieri could reproduce in the Mirim Lagoon, we would expect that lower strontium concentrations at the otolith core would be more common and an inversion of the age-structure (more young than old fish) should be evident. Previous studies indicate that M. furnieri reproduction and larval occurrence are associated with brackish water ( Macchi & Christiansen, 1992; Sinque & Muelbert, 1997; Acha et al., 1999). Therefore, this species appears not capable of completing its life-cycle within the freshwater environment of the Mirim Lagoon. For this reason, we characterize the Mirim Lagoon M. furnieri as a trapped fish group and not as a population, since reproductive activity is one of the attributes defining the biological concept of population. Given that the floodgate has prevented significant fish movements between Mirim Lagoon and estuarine/coastal regions, and adding the effect of the local fishery activity, that fish group will probably be extinct in the near future.