Growth and production of Pisidium amnicum in the freshwater tidal area of the River Minho estuary

Pisidium amnicum is an important indigenous species in the freshwater tidal portion of the River Minho estuary and until the introduction of the non-indigenous invasive species Corbicula fluminea was the most abundant bivalve in this ecosystem. Between January 2005 and August 2006 monthly samples were collected in three sites to study the abundance, biomass, growth and production of P. amnicum. Results indicate that P. amnicum has continuous growth throughout its life span and growth rates were higher during the early phases of development, coincident with the spring and summer months. Life span estimated was 24 months. The annual 2005 growth production of P. amnicum estimated was 2.339 g AFDWm 2 year 1, and the mean annual biomass was 1.594 g AFDWm 2, resulting in a P=B ratio of 1.47 year 1 and a turnover time of 248.7 days. The annual 2005 elimination production estimated was 7.541 g AFDWm 2 year 1, resulting in an E=B ratio of 4.73 year 1. It is concluded that P. amnicum continues to play an important role in the food web in areas where it coexists with C. fluminea. Previous studies showed declines in the indigenous population after the introduction of C. fluminea and currently, areas of high abundance are restricted to small patches in the upper limit of the tidal influence. Consequently, P. amnicum is now under considerable pressure and this situation requires ecological and conservational attention.