The parasite assemblage of Scomber japonicus (Houttyun, 1782) off South Africa

Master Thesis


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In South Africa, knowledge of marine parasite diversity is lacking and is often ignored or underutilised. Parasitology has several potential applications in fisheries management, pollution monitoring, aquaculture and general community ecology. With increased knowledge and understanding, the role that parasites play in the marine ecosystems of South Africa is gradually being exposed. This study aimed to document the parasite assemblage of Scomber japonicus (commonly known as chub mackerel) off South Africa, and to determine which host characteristics (size, sex and region) influenced parasite infection indices. This species is a small to medium sized, pelagic fish, that has a cosmopolitan, anti-tropical distribution and with populations showing large-scale, environmentally dependant migratory behaviours. It is found off South Africa throughout the year but is most abundant between austral spring and summer. Thanks to their diverse diet and wide array of predators, S. japonicus is an ecologically important species, and although it was historically important in the South African purse-seine fishery with large catches taken in the 1960’s and 1970’s it is no longer, although small amounts are taken as bycatch. A total of 152 fish ranging between 99 and 514 mm (FL) were sampled in this study and were found to host a total of 16 parasite taxa, 9 of which were identified to species level and 6 to genus level, as well as cysts that were not identified. The parasite assemblage was made up of two nematode species [Anisakis simplex (Rudolphi, 1809) and Contracaecum sp. (Railliet & Henry, 1912)], six digenean species [Lecithocladium sp. (Lühe, 1901), Opechona bacillaris (Molin, 1859), Nematobothrium faciale (Baylis, 1938), Halvorsenius sp. (Gibson, MacKenzie & Cottle, 1981), Didymocystis sp. (Ariola, 1902) and a metacercarean], three monogenean species [Pseudokuhnia minor (Goto, 1984), Kuhnia sp. (Sproston, 1945) and Grubea cochlear (Diesing, 1858)], one acanthocephalan species [Rhadinorhynchus pristis (Rudolphi, 1802)], one cestode species [Tentacularia coryphaenae (Bosc, 1802)], two myxozporan species [Kudoa thyrsites (Gilchrist, 1924) and Ceratomyxa sp. (Thélohan, 1892)], one copepod species [Clavellisa scombri (Kurz, 1877)] and one unidentified cyst species [Cyst 1]. Whilst no new host records were recorded in this study, N. faciale and Halvorsenius sp. are new locality records. Generalized linear models were used to determine which host characteristics most influence the prevalence and infection intensity of the six most prevalent parasite taxa (A. simplex, Lecithocladium sp., O. bacillaris, P. minor, R. pristis and Cyst 1). All showed significant relationships between size and either prevalence or infection intensity or both. This was attributed to the fact that larger, older fish have had more opportunities to get infected than smaller, younger fish, as well as the different diets of adult and juvenile S. japonicus which, along with the fact that adults and juveniles tend to school separately, means that the level and diversity of parasites that they are exposed to are different. Three parasites, A. simplex, P. minor and R. pristis also showed significant spatial variation in either prevalence or infection intensity. The prevalence of A. simplex and the infection intensity of P. minor decreased with in an eastward direction, while the prevalence of R. pristis increased eastwards. The spatial trends in the prevalence of A. simplex and R. pristis were predicted to be driven by the diet of the fish, and the intensity spatial trend observed in P. minor infections was predicted to be driven by environmental factors. An analysis of the gill preference of P. minor revealed that the outermost gills, furthest from the spinal cord were favoured sites of infection. The driver behind this trend was not definitively identified, however space availability was removed through standardization and water flow is suspected to be the main factor affecting gill arch selection. Using data from Oliva et al. (2008), the parasite assemblage of S. japonicus off South Africa was compared to the parasite assemblages of populations of this species in Brazil, Peru, Chile and Portugal using Nonmetric Multidimensional Scaling modelling (NMDS) and an ANOSIM. The NMDS plot showed that all populations were unique. The South African and Portuguese populations (R statistic = 0.28) as well as the Chilean and Peruvian populations (R statistic = 0.32) were the most similar, while the Brazilian population was the most dissimilar from the other populations analysed. The SIMPER analysis revealed that 16 parasite taxa account for 80% of the dissimilarity between the six populations of S. japonicus. This result supports the conclusions made by Oliva et al. (2008) that extended separation is the main driver of interspecific differences in the parasite assemblages of this species. This study has increased our knowledge of South African marine biodiversity and of the ecology of South African chub mackerel, and further demonstrated how parasites can be used to elucidate the taxonomic status of their hosts.