INFLUENCE OF THE PYCNOCLINE ON THE VERTICAL MIGRATION OF CHAETOGNATHS IN THE NORTHERN BENGUELA

The vertical distribution and migration patterns of chaetognaths were studied at a sampling station off northern Namibia (18 degrees 00'S, 10 degrees 30'E during a 48 h sampling cycle. The sampling area was characterized by mixing of the Angola current with the northernmost waters of the Benguela current in the surface region. The continuous flow of the Angora current during the study period gave rise to a thermocline and a halocline at a depth of 40 m. The sampling intervals used to study the behaviour of epipelagic chaetognath species under these conditions were 200-100, 100-60, 60-40, 40-20 and 20-0 m. A total of 10 different chaetognath species were captured. Sagitta enflata was the predominant species, with a mean density of 4400 individuals per 1000 m(3) in daytime hauls, accounting for 54.1% of all individuals collected in the samples. Sagitta minima was the next most predominant species, with a mean density of 2400 individuals per 1000 m(3) in daytime hauls, accounting for 16.6% of the chaetognath community sampled. Three maturity stages were considered in analysing possible ontogenetic migrations. Nearly all the species were aggregated above the pycnocline, and stages I and II of many species carried out short migrations in the surface layers. The limited migration pattern, together with a staggered distribution of the different species and stages above the pycnocline, has been interpreted as a space partitioning mechanism to prevent intra- and interspecific competition.