DISTRIBUTION AND ABUNDANCE OF POST-LARVAE AND JUVENILES OF THE PATAGONIAN SPRAT, SPRATTUS-FUEGENSIS, AND RELATED HYDROGRAPHIC CONDITIONS

This paper reports on the first pelagic survey on the southern Patagon ian shelf. As a result of a joint Argentine-Russian cruise on board th e R/V 'Dmitry Stefanov' carried out in the early autumn of 1992, a maj or nursery ground of Sprattus fuegensis was located in the coastal are a off Santa Cruz and Tierra del Fuego. Forty-nine ichthyoplankton stat ions along 14 transects and seven intertransects were occupied. Biolog ical samples were taken with an Isaacs-Kidd midwater trawl (IKMT) equi pped with a net sounder and flowmeter. Hydrological data were obtained at 78 conductivity-temperature-depth sensor (CTD) stations. Meteorolo gical observations were made concurrently, Real-time sea surface tempe rature (SST) images from NOAA satellites were received on board, twice per day, cloud permitting. Positive stations lay mostly within the 50 m depth contour. Of all coastal stations 71% were positive for sprat post-larvae and juveniles. The density and size of the specimens colle cted increased southwards. Median values of the juvenile distribution varied from 23.6 mm standard length (SL) to the north of the study reg ion to 38.5 mm for stations at 54 degrees S. Highest biomass densities (> 2.8 t per square nautical mile) were found to the south of the Mag ellan Strait, corresponding to IKMT densities of over 1000 specimens p er haul. Other important components of the Isaacs-Kidd collections wer e the post-larval and juvenile stages of Sebastes oculatus and Patagon otothen tessellata.To obtain a higher resolution view of the distribut ion and abundance of juvenile sprat, the area was simultaneously surve yed using an EK-400 echosounder with a SIORS echointegrator, The acous tically derived sprat densities were higher than those obtained from I KMT sampling; however, a significant correlation (r = 0.98; alpha < 0. 01) was obtained between IKMT and echointegration data. The large amou nt of post-larvae and juveniles collected and the very high values of echointegration registered, particularly in the area of the strait and channels, are indicative of a major nursery ground of the species alo ng the Patagonian coast. Juvenile production in the area was estimated at 1.32 X 10(9) individuals. Differences in the diel patterns of the vertical migration of adult and early juveniles are described. Growth increments in otoliths were used to estimate the age of sprat post-lar vae and juveniles. A Laird-Gompertz model was fitted to the length-at- age data. The spawning periodicity of the species was determined by ex amining the temporal distribution of birth dates from otolith-aged spe cimens. There was an indication of a major spawning peak in late Decem ber, and a secondary one in mid-January. Available information on the reproduction and early life history of the species is analysed in rela tion to the obtained results. The importance of the low-salinity water outflow in the Magellan Strait for egg dispersal and larval retention is discussed.