Diet of albacore (Thunnus alalunga) and yellowfin tuna (Thunnus albacares) off the south-west coast of South Africa

Master Thesis


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Considering the magnitude of albacore and yellowfin biomass, these two economically vital tuna species apply a considerable amount of predation pressure on prey communities off the South African coast. Nevertheless, little is known about their respective diets in this upwelling region. Fish Aggregating Devices (FADs), otherwise used globally, are banned in South Africa therefore giving us a unique opportunity to examine the natural diet of the tuna surrounding our shores. Sixty-one stomachs were sampled from recreational fishing competitions in May 2016 and 2018 off the south-west coast of South Africa to investigate the diets of albacore and yellowfin tuna. The fork length (mm) and wet weight (g) of the fish were recorded. The importance of each prey in the diet was estimated by the Index of Relative Importance (IRI). Each sample specimen was cut open, had its entire stomach carefully removed and frozen for later dissection and analysis. Prey items were initially grouped into fish, cephalopods and crustaceans and later identified to the lowest possible taxonomic level, counted, and weighed. The difference in diet between the species was investigated and modelled by Analysis of Similarity (ANOSIM), Non-Metric Multidimensional Scale (NMDS) and Principal Component Analysis (PCA). The feeding strategy was determined by Costello's Diagram. The 43 albacore and 18 yellowfin tunas ranged in fork length from 740 to 959 mm and 803 to 1720 mm, respectively. The most important prey class based on IRI in the diet of albacore was crustaceans (6162.01), followed by cephalopods (3672.47). For yellowfin the highest IRI was cephalopods (6269.39) followed by fish (3977.53) and unlike the diets of yellowfin in other parts of the world, crustaceans were numerically a very low prey item making up just 15.84% of the diet. Yellowfin, the larger of the two tuna species consume larger prey items and are more opportunistic than albacore, showing a greater vertical feeding range, by diving deep for cephalopods and surface feeding on offal from hake trawlers. Intra-species variations for both tuna proved to be smaller than the difference between the two species, with no substantial changes in either diet based on fish size. Unlike diets of tuna species in other parts of the world, the data suggest that southern African albacore and yellowfin were less dependent on fish (possibly due to the lack of FADs in the case of yellowfin), but more likely due to the higher availability of crustaceans and cephalopods at the upwelling front. The magnitude of the role of cephalopods in the Benguela ecosystem is likely to be underestimated.