Influence of the thermocline on the vertical migration of medusae during a 48 h sampling period

Abstract

The vertical distribution and migratory movements of epiplanktonic medusae were studied at an oceanic station off northern Namibia (18°00'5 110"30'E). Samples were colleded from five separate depth strata (200-100 m, 100-60 m, 60-40 m, 40-20 m, and 20-0 m) using a multiple opening and closing RMI 1 x6 net over a 48-h period. The area was charaderized by mixing of the Angola Current surface waters with the nor-thernmost subsurface waters of the Benguela Current. During the sampling period there was a continuous flow of water from Angola, giving rise to a strong thermocline between 20 and 40 m depth. A total of 17 species of medusae were captured, of which Aglaura hemistoma, Liriope tetraphylla, and Solmundella bitentaculata were the most abundant. Both the number of species and the number of individuals were higher on the second than on the first day of sampling, chiefly in the 20-0 stratum. Differences in abundance appeared to be related to the patch size of each species. The depth diistribution of the medusae population was charaderized by the existence of two assemblages, one associated with each current, and by the presence of the thermocline, which acted as a boundary between the two water masses and did not facilitate migratory movements of the .most abundant species. The non-migratory distribution pattern of most of the species was attributable to the high concentration of potential prey items and to the absence of predators in the layer above the thermocline throughout the sampling period.