The day and night vertical distributions of calanoid copepods in the western Gulf of Mexico, with reference to feeding relationships.

The day and night vertical distributions of total zooplankton and calanoid copepods were examined to a depth of 1000m at a station in the western Gulf of Mexico. Interrelationships between abundant species with similar and varying feeding habits were analyzed as possible factors contributing to vertical distribution and migration. The vertical distribution of the zooplankton at this station in the western Gulf was generally comparable with published results on vertical distribution in tropical oceanic regions. The greatest volumes of zooplankton and copepods in this study were found in the upper 50m and they decreased rapidly with depth. Zooplankton populations in the water column appear to co-exist through a complex assortment of interactions. Interspecific relationships may be important in determining vertical distributions and migrations. The most abundant particle grazing calanoid species were concentrated at 50 m at night and were generally distributed in deeper layers during the day. The common carnivores were most abundant in the upper 200m of the water column during the day and at night. These distributions suggest that partial escape from predation, through periodic movements into the deeper layers where carnivore densities are relatively low, may be an important advantage of vertical migrations.