Like their counterparts at lower latitudes, many Antarctic midwater fish make extensive, diel vertical migrations. However, Antarctic species make these migrations in the absence of two of the three selective advantages generally ascribed to this behaviour. Because the water column is nearly isothermal, there is no metabolic bonus to be gained from thermal differences in habitat depth. Likewise, because their prey do not perform significant vertical migrations, the fish migrations act to diminish rather than enhance their feeding opportunities. For Antarctic midwater fish, the sole advantage of these migrations appears to be the avoidance of visually-cued predators in the upper part of the water column. An evaluation of the relative importance of the three selective factors supports the suggestion that predator avoidance may also be the principal driver of diel vertical migrations for midwater fish at lower latitudes.
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