In this study we report the abundance, fecundity and an index of mortality of Oithona\ud similis across a large latitudinal and temperature range within the Southern Ocean.\ud The abundance of O. similis was strongly related to temperature and to depthintegrated\ud (0-100m) chorophyll a (Chl a), abundance increasing with increasing temperature (and therefore increasing latitude) and Chl a. In situ total egg production rates and fecundity per female were significantly and positively related to temperature and Chl a. Egg hatch times lengthen as temperature decreases and in sac spawning species the next batch of eggs cannot be produced until the previous clutch hatch.\ud Consequently, O. similis fecundity rates must rapidly decline at low temperatures,\ud especially below 5oC. In situ fecundity rates were compared with a model of\ud maximum fecundity, and were generally much lower, thus suggesting strong food\ud limitation across the region studied. However the relationships of in situ and\ud maximum rates to temperature were similar, confirming the importance of\ud temperature. Further, as time taken to develop from egg to adult also rapidly extends\ud with declining temperature, it is increasingly unlikely that O. similis will be able to\ud 2\ud maintain its population against typical field mortality. Our findings have broad\ud implications for the lower temperature range and hence geographic limits of O.\ud similis, but also for the distribution of other sac spawning copepods and planktonic\ud species generall
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