FEEDING AND VERTICAL MIGRATION OF THE CHAETOGNATH SAGITTA-FRIDERICI (RITTER-ZAHONY, 1911) IN THE SOUTHERN BENGUELA DURING SPRING 1987, WITHNOTES ON SEASONAL VARIABILITY OF FEEDING ECOLOGY

The feeding biology and the vertical migration of Sagitta friderici we re examined over 24 h at two stations in the southern Benguela during spring (October) 1987. Together with studies conducted during summer ( February 1991) and winter (May 1984), they serve to allow valuable gen eralizations of the biology and ecology of this abundant chaetognath. Populations migrate vertically and feed nocturnally, although the timi ng and the extent of migration vary between studies. S. friderici exhi bits ontogenetic layering and the cross-shelf distribution of maturity stages differs, suggesting that it is able to take advantage of cross -shelf water movement in order to maintain populations in the nearshor e waters of the West Coast. S. friderici prey almost exclusively on co pepods (cannibalism is rare), and there is a positive relationship bet ween the lengths of predator and prey that is influenced by the size s tructure of the prey environment. This casts doubt on the validity of a chaetognath species-specific relationship between predator and prey size. S. friderici selects its prey on the basis of size, and not spec ies. Daily ration is related linearly to prey density, so reflecting t he low density of prey and providing support for theoretical predictio ns regarding ingestion rates under oceanic conditions. The impact of S . friderici predation on the copepod assemblage is generally less than 3% of the standing stock, although it could be much higher under cond itions of low copepod biomass and poor secondary production.