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This paper describes the pattern of daylight occurrence of humpback dolphins Sousa chinensis in Algoa Bay, the Eastern Cape, South Africa, between May 1991 and May 1994. Two indices, the Sighting Index (SI) and the Total Recorded Dolphin Presence (TRDP), were used to quantify the dolphins' daylight occurrence. Humpback dolphins display a clear pattern of daylight occurrence in Algoa Bay and can be seen mostly in the morning, and, to a lesser extent, in the evening. This pattern varies little between seasons (with possible exception for the evening hours), follows the time of the solar day, is not related to the tidal cycle and is probably governed by the diurnal cycles of dolphins' prey. [ABSTRACT FROM AUTHOR]

The sympatric Heaviside's (Cephalorhynchus heavisidii) and dusky dolphins (Lagenorhynchus obscurus) are poorly studied in South Africa and are potentially at risk from anthropogenic threats including fisheries bycatch. As part of a larger study, shore-based observations were made in a small bay (~1000 m wide) in the Western Cape, South Africa, during the summer months of 1999-2001 to gather data on their nearshore movements and behavior. Heaviside's dolphins exhibited a diurnal onshore-offshore migration and the sighting rate varied significantly with time of day and brightness of the moon, with numbers being markedly higher before 12:00 and nearer a full moon. These patterns were presumed to be linked to the vertical migration of their principal prey, juvenile hake Merluccius species and Heaviside's dolphins appeared to be resting and not feeding inshore during the day. Sightings of dusky dolphin showed no predictable variation with time of day but their numbers inshore were significantly lower when upwelllng conditions existed offshore. Larger groups of Heaviside's dolphins (especially groups of four) were more active (leaps, etc.) than smaller groups, but behaviour was not linked to environmental factors. The dissimilar responses to environmental conditions suggest that sympatry in these two species is mediated by niche as well as prey differentiation. [ABSTRACT FROM AUTHOR]