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    Antarctic neritic krill Euphausia chrystallorophias: spatio-temporal distribution, growth and grazing rates

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    E.A. Pakhomov (Ukraine) and R. Perissinotto (South Africa)
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    Dynamics of distribution, growth, life span and feeding were studied in the endemic Antarctic euphausiid Euphausia crystallorophias in the central part of the Indian sector of the Southern Ocean during the austral summer, from 1977 to 1990, and in the Lazarev Sea during the summer 1990/91. Both larvae and adult E.crystallorophias were found in abundance in shelf waters of 100-500 m depth. Maximum abundances of larvae (up to 7688 ind.∙m-2) and adults (up to 1267 ind. ∙m-2) were found in the Prydz Bay region. Size frequency analysis indicated that the generation time was ~ 3 years in the Indian sector of the Southern Ocean. Total life span of E.crystallorophias may exceed 5 years in the Prydz Bay region and 4 years in the Cosmonaut Sea. Analysis of size distribution by sex showed that male E.crystallorophias had a shorter life span than females, although their growth rates were similar. Assuming growth for only 180 days per year, mean growth rates ranged from 0.070-0.075 to 0.019-0.022 mm per day during the first and the fourth year, respectively. Von Bertalanffy growth curves calculated for different areas were similar to those obtained by Siege1 (1987) in the Antarctic Peninsula region. In the Prydz Bay and the Cosmonaut Sea, spawning of E.crystallorophias appears to peak from the end of November to early December and may extend to the beginning of January. Most larvae were in the stage of metanauplius/calyptopis I at the beginning of January and calyptopis III/furcilia I during February. The year-l cohort was identified in January-February and exhibited a mean length in the range 10.8-16.8 mm. In situ feeding rates were estimated in the Lazarev Sea using the gut fluorescence method. Ingestion rates during austral summer l990/91 ranged from 52 to 471 ng (pigm)∙ind-1∙h-1 in adults and from 2.5 to 25.2 ng (pigm)∙ind-1∙h-1 in calyptopis III larvae. Total population impact on the phytoplankton stock varied between 160-2860 and 215-652 μg (C)∙m-2∙d-l for adults and larvae, respectively. This is equivalent to 0.06-1.12% and 0.02-0.07% of total daily production. In areas with dense E.crystallorophias swarms, however, daily consumption rates may attain levels as high as 13.6-96.5% of daily primary production. The longterm monitoring of different populations of E.crystallorophias allowed the identification of covariance patterns between its abundance, spawning success and the formation of coastal polynyas, especially in the Prydz Bay region.