MODELS OF THE EARLY LIFE-HISTORY OF EUPHAUSIA-SUPERBA .1. TIME AND TEMPERATURE-DEPENDENCE DURING THE DESCENT ASCENT CYCLE

A time- and temperature-dependent model was developed to simulate the descent-ascent behavior of the embryos and early larval stages of the Antarctic krill, Euphausia superba. This model combines laboratory measurements of temperature effects on developmental times, density and physiology of krill embryos and larvae and the observed water temperature structure in the Bransfield Strait-South Shetland Islands region. Simulations with observed vertical temperature profiles from this region show that embryos that develop at temperatures less than 0-degrees-C hatch relatively deep (almost-equal-to 1000 m) or hit the bottom before hatching. The presence of warm (1-2-degrees-C) Circumpolar Deep Water (CDW), between 200 and 700 m, results in hatching depths of about 700 m. The sinking rate pattern characteristic of the embryos of Euphausia superba retains the embryos in the CDW, where development is accelerated. Larval ascent rate through the CDW is rapid, so larvae reach the surface before metamorphosing into the first feeding stage, and have sufficient carbon reserves to drift at the surface for several weeks before needing to find food. These results suggest that the sinking rate pattern characteristic of embryos of Antarctic krill may be part of a reproductive strategy that evolved in response to the thermal structure of its environment. The complementary component of this reproductive strategy is the observed correlation between the distribution of krill schools containing reproducing individuals and the presence of CDW. With this reproductive strategy, the spawning regions of Antarctic krill are in areas where oceanic conditions enhance the probability of survival of its embryos and non-feeding larvae.