Thesis (MTech (Oceanography))--Cape Technikon, Cape Town, 2004 / Determining lipid content for sardine (Sardinops sagax) in the Southern Benguela Ecosystem (SBE), through applying a method of fat staging, is a relatively easy method to use as a biological time-series to assess the condition of sardine. The condition of fish is an important indicator for fluctuations in the population size of post-recruit fish. Depending on the amount of fat associated with the stomach, one of seven fat stages was assigned to each sardine. Visually assigned fat stages were then converted to a lipid content value, given as a percentage of wet body mass, by applying a conversion equation to these fat stages. Time-series of lipid content and gonadosomatic index (GSI) were generated using general linear models (GLM). Results from these GLM's explained 34% and 39% of the observed variance in lipid content and GSI respectively. Monthly and annual least square means (LS mean) were derived from these GLM's to show seasonal variability in lipid content and GSI, for the period 1996 to 2003. Clear seasonal patterns in both lipid content and GSI were observed. Lipid content showed a decrease, but an interannual trend in GSI could not be observed, for the time series. The study area, which ranged from the Orange River mouth (28°40'S and 16°30'E) on the west coast to Port Elizabeth (25°37'E and 33°57'S) on the east coast was divided into four smaller zones to allow for spatial tests. Zone 1 being on the west coast and Zone 4 on the east coast. Both lipid content and GSI showed strong spatial differences, with lipid content being at highest levels in Zone 4 throughout the time series, and the highest GSI levels being observed in Zone 1.

The long-beaked common dolphin, Delphinus capensis (Gray 1828), is one of the most enigmatic predators feeding in the annual sardine run (Sardinops sagax) off the coast of KwaZulu-Natal (KZN), South Africa. In recent years, unpredictable inter-annual variations in the timing, spatial extent and intensity of the sardine run have been documented, possibly resulting in changes in the suite of prey available to the common dolphin during winter. Although the diets of a number of predators during the sardine run have been studied in detail (e.g. sharks and flying seabirds), little is known about the diet of long-beaked common dolphins during this period. Each year, a low number of common dolphins are incidentally caught in the anti-shark nets in the waters of KwaZulu-Natal. These captures provide a valuable source of data on selected aspects of the ecology of the long-beaked common dolphins along the KwaZulu-Natal coastline. The objective of this study was to provide new dietary data for the common dolphins feeding in the waters of KwaZulu-Natal during winter over the period 2000 to 2009, as well as to determine if any dietary changes had taken place since the common dolphin diet was last assessed, over 15 years ago. Stomach contents from 95 common dolphins (55 females, 40 males) caught between 2000 and 2009 were analysed and compared to historical data from dolphins caught between 1974 and 1992. Mesopelagic fish and squid dominated the diet, with 23 fish and 5 squid species represented in adult dolphins. Multidimensional Scaling (MDS) indicated that there was no resource partitioning between adult male and female dolphins. Numerical analyses indicated that there was a shift in the principal prey species consumed by the dolphins over the past decade, particularly during the winter. Prior to 1992, sardine comprised up to 49% of the total stomach contents, while chub mackerel (Scomber japonicus) was the dominant prey item (66% by mass) recorded in the stomach contents over the period 2000 to 2009. The shift in the relative contributions of sardine and mackerel in the diets of the dolphin appeared to correspond to fluctuations in the availability of the two principal prey species. Between 2000 and 2009, the diversity of the dolphins' diets was highest during the sardine run, reflecting the presence of a wide suite of predatory teleosts in the waters of KwaZulu-Natal during the annual sardine run. Conversely, prior to 2000, the diet was dominated by sardine during the peak of the sardine run, whilst diet diversity increased after this period. Apart from sardine and chub mackerel, elf (Pomatomus saltatrix), maasbanker (Trachurus delagoa), strepie (Sarpa salpa) and flying fish (Exocoetid sp.) also formed important components of the diet both prior to 1992, and over the last decade. Blubber thickness was assessed as an indicator of animal condition. No significant change in blubber total weight (R² = 0.0016, N = 185), nor dorsal, lateral or ventral blubber thickness (R² = 0.3146, R² = 0.0003, and R² = 0.0003 respectively, N = 78) was seen over the last 30 years (1970 to 2009). Results of stable isotope analyses conducted on tissue derived from the teeth of D. capensis indicated that there has been no significant shift in the trophic position (δ¹⁵N) and potential prey consumed (δ¹³C) over the corresponding period. These data would suggest that the long-beaked common dolphins along the KwaZulu-Natal coastline can be considered as opportunistic predators generally consuming the most abundant prey species available locally. As common dolphins feed opportunistically, this dietary shift appears to indicate changes in the shoaling characteristics of the most abundant fish prey in KwaZulu-Natal during winter. Given the “Data Deficient” status of the long-beaked common dolphin on the IUCN Red Data List, and the strong climatic forcing of the sardine run, such dietary data have important implications for their conservation in the light of expanding fisheries and climate change.