Estuarine spawning of the whitemouth croaker Micropogonias furnieri (Pisces: Sciaenidae), in the Rio de la Plata, Argentina

Most fishes that take advantage of the high productivity of estuaries exhibit offshore spawning, produce great numbers of small pelagic eggs and recruit to estuaries as larvae or juveniles. The reproductive pattern of Micropogonias furnieri (:a planktonic egg spawner) in the Rie de la Plata estuary (36 degrees S,56 degrees W) differs from this. Biological sampling and oceanographic data showed that the spawning area covers a narrow band across the river between Montevideo (34 degrees 50'S,56 degrees 10'W) and Punta Piedras (35 degrees 25'S,57 degrees 10'W) at depths ranging from 6 to 8 m. This area is characterized by strong haloclines, reaching 21.5 units m(-1). M. furnieri eggs were present only below the halocline, in salinities of 9.7-27.3, and at 18.5-20.2 degrees C. Spawning occurred in the innermost part of the estuary, near the upstream edge of the salinity wedge and coinciding with the turbidity maximum (up to 150 mg L-1). The regular spawning of pelagic eggs has been reported in estuaries of southern Africa and Australia characterized by intermittent landlocking. The present results show that a large estuary may also provide the opportunity for successful spawning of pelagic eggs.