Diet and trophic ecology of the tiger shark (Galeocerdo cuvier) from South African waters

Knowledge of the diet and trophic ecology of apex predators is key for the implementation of effective ecosystem as well as species-based management initiatives. Using a combination of stomach content data and stable isotope analysis (δ15N and δ13C) the current study provides information on size-based and sex-specific variations in diet, trophic position (TP) and foraging habitat of tiger sharks (Galeocerdo cuvier) caught in the KwaZulu-Natal Sharks Board bather protection program. This study presents the longest time-series and most detailed analysis of stomach content data for G. cuvier worldwide. Prey identified from 628 non-empty stomachs revealed a size-based shift in diet. Reptiles, birds, mysticetes, and large shark species increased in dietary importance with G. cuvier size, concomitant with a decrease in smaller prey such as batoids and teleosts. Seasonal and decadal shifts in diet driven primarily by changes in the importance of elasmobranchs and mammal (cetacean) prey were recorded for medium sized (150-220 cm) G. cuvier. Both stomach content and stable isotope analysis indicated that G. cuvier is a generalist feeder at the population level. Size-based δ13C profiles indicated a movement to offshore foraging habitats by larger G. cuvier. Calculated TP varied by method ranging from 4.0 to 5.0 (TPSCA for stomach contents) and from 3.6 to 4.5 (TPscaled and TPadditive for δ15N). Large (> 220 cm) G. cuvier did not feed at discrete trophic levels, but rather throughout the food web. These data provide key information on the ecological role of G. cuvier to improve the accuracy of regional food web modelling. This will enable a better understanding of the ecological impacts related to changes in the abundance of this predator

Movement patterns and catch trends of the diamond ray Gymnura natalensis (Dasyatidae) in South African waters

The diamond ray Gymnura natalensis is endemic to southern Africa where its preference for shallow coastal habitats makes it vulnerable to recreational shore-based angling. Although it makes up approximately 1% of the shore-based tag numbers, little is known about its movements, reproduction or population status in South Africa. This study used three independent long-term (34–41 years) datasets, including tagging by recreational anglers, competitive shore angling catches and shark net catches, to investigate the species’ movements, catch composition and population status in South Africa. Of the 3 739 individuals tagged (1984–2018), only 30 (1%) were recaptured after an average of 487 days at liberty. The majority (60%) of the recaptures occurred within 10 km of the release site, while 7% had moved more than 1 000 km along the coast. The longest recorded movements (1 577 and 1 756 km) were undertaken by adult rays tagged in the Western Cape Province moving to KwaZulu-Natal Province (KZN). The competitive shore angling catch (1977–2018; n = 9 150) from KZN was dominated by adult rays caught north of Durban, while the shark net catch in KZN (1981–2018; n = 584) was dominated by juvenile rays primarily from the central beaches of Durban. All the datasets exhibited strong seasonal trends with most catches taking place in summer. A risk assessment confirmed a stable to increasing population trend over four generations, suggesting that the population sampled along the east coast of South Africa should be classified as Least Concern