A large purse-seine fishery is located in the highly productive southern Benguela System off the western Cape. Purseseine fisheries have been prone to collapse worldwide and management practices have met with limited success. Predators offer potential as biological indicators yielding information on the status of fish stocks. The rationale behind this proposed usage was previously loosely-defined. The premise that some variable of seabird biology is related to some aspect of fish biology about which information is required was critically examined. The diet of the Cape Gannet was monitored monthly at Lambert's Bay and Malgas Island in the western Cape from 1977 to 1986. Gannets ate shoaling fishes (mainly Cape Anchovy, South African Pilchard and Saury) , measuring 29-429 mm Lc, which were available during the day at the surface and hake offal scavenged from demersal trawlers. Comparison with purse-seine fishery catches and the distributional ecology of the fishes suggested that the contributions of epipelagic fishes to the diet of the Cape Gannet were related to their availability and abundance, but that the occurrence of mesopelagic fishes (adult Redeye Roundherring and Onderbaadjie) in the diet of gannets was not related to their availability or abundance. Availability of epipelagic fishes was apparently lowest during late winter and spring; at longer time scales, availability was apparently lowest between 1983 and 1985 during the period from 1978 to 1986. Differences in the diets of breeding and nonbreeding gannets were small. Gannets from the colony at Lambert's Bay fed primarily north of Cape Columbine. These birds fed in cool inshore waters on juvenile fishes (mainly Cape Anchovy). Gannets from Malgas Island fed primarily south of Cape Columbine. They fed in cool inshore waters on juvenile epipelagic fishes (mainly Cape Anchovy), in warmer waters offshore on large epipelagic fishes (Saury and adult pilchard) or scavenged hake at trawlers offshore. Adult South African Pilchard were preferred prey. Epipelagic prey were selected in preference to mesopelagic fishes and hake. Significant correlations between the percentage of pilchard in gannet diet and purse-seine fishery catches suggested that gannets were reliable monitors of the trend in pilchard stocks at low biomass levels. The Saury is a poorly known species and its availability to gannets was reviewed as an example of the indirect use of the diets of predators in understanding the ecology of prey species, and functioning of ecosystems. Future directions for research were suggested. However extensive use of data from predators in fisheries management awaits the development of techniques which can use qualitative data. Nevertheless, it was concluded that the species composition and length of prey in gannet diet and the mass of regurgitations and the proportion of birds which regurgitated food were related to the abundance of epipelagic fish prey generally and South African Pilchard in particular. / Thesis (Ph.D.)-University of Natal, Pietermaritzburg, 1987.