INFLUENCE OF THE PYCNOCLINE ON THE VERTICAL MIGRATION OF CHAETOGNATHSIN THE NORTHERN BENGUELA

The vertical distribution and migration patterns of chaetognaths were studied at a sampling station off northern Namibia (18 degrees 00'S, 1 0 degrees 30'E during a 48 h sampling cycle. The sampling area was cha racterized by mixing of the Angola current with the northernmost water s of the Benguela current in the surface region. The continuous flow o f the Angora current during the study period gave rise to a thermoclin e and a halocline at a depth of 40 m. The sampling intervals used to s tudy the behaviour of epipelagic chaetognath species under these condi tions were 200-100, 100-60, 60-40, 40-20 and 20-0 m. A total of 10 dif ferent chaetognath species were captured. Sagitta enflata was the pred ominant species, with a mean density of 4400 individuals per 1000 m(3) in daytime hauls, accounting for 54.1% of all individuals collected i n the samples. Sagitta minima was the next most predominant species, w ith a mean density of 2400 individuals per 1000 m(3) in daytime hauls, accounting for 16.6% of the chaetognath community sampled. Three matu rity stages were considered in analysing possible ontogenetic migratio ns. Nearly all the species were aggregated above the pycnocline, and s tages I and II of many species carried out short migrations in the sur face 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 int ra- and interspecific competition.