VERTICAL MIGRATION AND AVOIDANCE CAPABILITY OF EUPHAUSIIDS IN THE CALIFORNIA CURRENT

Abstract

Comparisons of day and night vertical distributions of euphausiids at the same stations make it possible to estimate 1) the extent of vertical migration of the furcilia, juveniles, and adults of the Euphausiacea (Crustacea) species, or 2) their ability to avoid the net in the daytime, or both. Seven species of <i>Euphausia</i> and one of <i>Thysanopoda</i> migrated more than 300 m, rising to near the surface at night. Adults of <i>Nematoscelis</i> (three species) migrated vertically but were bounded at upper limits by the thermocline at all but the most inshore stations. <i>Nyctiphanes simplex</i> and <i>Stylocheiron carinatum</i> were short-distance (ca. 150-0 m) migrants. The extent of vertical migration was greatest in the most offshore central water, intermediate for midcurrent species, and least for coastal. Nonmigrants included all <i>Stylocheiron</i> species (except <i>S. carinatum</i>), <i>Thysanoessa gregaria,</i> and two <i>Nematobrachion species. S. affine, S. suhmi,</i> and <i>T. gregaria</i> inhabited the homogeneous layer. Apparent increases with depth in their daytime densities arise from avoidance of nets in the upper layers. This is minimal at ca. 100 m, where day and night densities are nearly equal.