VERTICAL MIGRATION AND AVOIDANCE CAPABILITY OF EUPHAUSIIDS IN THE CALIFORNIA CURRENT

Area of Science:

• Marine biology
• Crustacean ecology
• Oceanography

Background:

• Euphausiids (Crustacea) are key zooplankton with complex vertical migration behaviors.
• Understanding their diel vertical migration (DVM) is crucial for marine ecosystem dynamics.
• Day-night distribution comparisons reveal migration extent and net avoidance.

Purpose of the Study:

• To quantify the vertical migration extent of various Euphausiacea species.
• To assess the influence of net avoidance on daytime euphausiid densities.
• To correlate migration patterns with oceanic zones and environmental factors.

Main Methods:

• Comparative analysis of day and night sampling data for euphausiid vertical distribution.
• Identification of species-specific migration depths and ranges.
• Assessment of thermocline influence on migration boundaries.

Main Results:

• Seven *Euphausia* and one *Thysanopoda* species migrated >300m, reaching surface waters at night.
• Adult *Nematoscelis* species showed thermocline-limited vertical migration.
• *Nyctiphanes simplex* and *Stylocheiron carinatum* were short-distance migrants (0-150m).
• Migration extent correlated with offshore-to-coastal water masses.
• Non-migrants included several *Stylocheiron* and *Thysanoessa* species.
• Apparent daytime density increases at depth were attributed to net avoidance, minimal at 100m.

Conclusions:

• Euphausiid vertical migration is species-specific and influenced by oceanic conditions and depth.
• Net avoidance significantly impacts population density estimates, particularly in upper layers.
• Diel vertical migration plays a critical role in nutrient cycling and predator-prey interactions in marine environments.