Distribution and abundance of post-larvae and juveniles of the Patagonian sprat, Sprattus fuegensis, and related hydrographic conditions

Abstract

This paper reports on the first pelagic survey on the southern Patagonian shelf. As a result of a joint Argentine-Russian cruise on board the R/V ‘Dmitry Stefanov’ carried out in the early autumn of 1992, a major nursery ground of Sprattus fuegensis was located in the coastal area off Santa Cruz and Tierra del Fuego. Forty-nine ichthyoplankton stations along 14 transects and seven intertransects were occupied. Biological samples were taken with an Isaacs-Kidd midwater trawl (IKMT) equipped with a net sounder and flowmeter. Hydrological data were obtained at 78 conductivity-temperature-depth sensor (CTD) stations. Meteorological observations were made concurrently. Real-time sea surface temperature (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 collected increased southwards. Median values of the juvenile distribution varied from 23.6 mm standard length (SL) to the north of the study region to 38.5 mm for stations at 54 °S. Highest biomass densities (> 2.8 t per square nautical mile) were found to the south of the Magellan Strait, corresponding to IKMT densities of over 1000 specimens per haul. Other important components of the Isaacs-Kidd collections were the post-larval and juvenile stages of Sebastes oculatus and Patagonotothen tessellata. To obtain a higher resolution view of the distribution and abundance of juvenile sprat, the area was simultaneously surveyed using an EK-400 echosounder with a SIORS echointegrator. The acoustically derived sprat densities were higher than those obtained from IKMT sampling; however, a significant correlation ( r = 0.98; α < 0.01) was obtained between IKMT and echointegration data. The large amount 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 along the Patagonian coast. Juvenile production in the area was estimated at 1.32 × 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-larvae and juveniles. A Laird-Gompertz model was fitted to the length-at-age data. The spawning periodicity of the species was determined by examining the temporal distribution of birth dates from otolith-aged specimens. There was an indication of a major spawning peak in late December, and a secondary one in mid-January. Available information on the reproduction and early life history of the species is analysed in relation to the obtained results. The importance of the low-salinity water outflow in the Magellan Strait for egg dispersal and larval retention is discussed.