On the role of frontal systems of the Argentine Sea as spawning grounds for fishes.

MACCHI, G.J.; ACHA E. M.; PÁJARO, M.; MILITELLI, M.I.; RODRIGUES, K.A.

Lisboa, Portugal

Simposio; Symposium on Reproductive and Recruitment Processes of Exploited Marine Fish Stocks; 2007

Background Most fishes inhabiting the Argentine Sea and adjacent waters southward 33° S show a wide spatial distribution. Nevertheless, their main spawning activities occur in small areas inside this region. Selection for reproductive aggregation sites depends on the physical, chemical and biological properties of these environments. Thus, we analyze physical and biological patterns searching for potential bio-physical couplings defining reproductive grounds selection for several species. Methods Biological data corresponding to ten bone fish species (Merluccius hubbsi, Engraulis anchoita, Brevoortia aurea, Micropogonias furnieri, Cynoscion guatucupa, Percophys brasiliensis, Cheilodacthylus bergi, Ramnogaster arcuata, Micromesistius australis y Sprattus fueguensis), representative of different ecosystems from the Argentine Sea, were analyzed jointly with  oceanographic data. This information was taken during research cruises carried out between 1991 and 2005. Macroscopic and histological ovarian analyses were used to assign the maturity stage of each sampled female. The criteria employed to estimate the incidence of spawning individuals was the presence of hydrated oocytes or new postovulatory follicles. Oceanographic analysis was performed on CTD data (salinity and temperature) collected during the cruises. Results From the ten species analyzed, six showed the main spawning activity during spring and summer, one in middle winter, two at the end of this season and one in autumn. M. hubbsi showed two fishing stocks with spawning peaks in different seasons, one in spring- summer (southern stock) and the other in autumn – winter (northern stock). Oceanographic characteristics of the zones where gravid fishes aggregate evidence that spawning activities were coincident with areas of maximum salinity and/or temperature horizontal gradient (frontal zones). The most important oceanographic systems were: a) the Río de la Plata bottom salinity front (35° - 36° 20`S), where spawning of M. furnieri, B. aurea and R. arcuata occur, b) a bottom thermal front south of the Buenos Aires Province (39° - 41° S), where spawning of P. brasiliensis and C. guatucupa take place, c) the northern Patagonia frontal system (42° - 45 S), where the Patagonian stock of E. anchoita and the southern stock of M. hubbsi spawn, and d) the shelf-break front south of Malvinas Isalnd (52° - 53° S), where spawning of M. australis and S. fueguensis were registered.. Conclusions Spawning of the species analyzed was in general coincident with frontal zones. This fact was associated with the physical and biological properties of these systems: consistent patterns of high chlorophyll abundance have been reported for the frontal systems of the Argentine Sea, showing them as productive places suitable for larval feeding and growth. Moreover frontal dynamics could act as retention mechanisms reducing egg dispersion after spawning, while in other situations, they may facilitate the movement of larvae from the reproductive areas to the main nursery grounds.