Parasitism in the black-spotted croaker (Protonibea diacanthus) (Teleostei: Sciaenidae) off northern Australia

The overall objective of this thesis was to assess the parasitism in the black-spotted croaker, Protonibea diacanthus (Teleostei: Sciaenidae), to report on the presence and abundance of new and known parasite species and to decipher the complex interactions and impacts that these organisms have with their marine host, P. diacanthus and the surrounding environment. Protonibea diacanthus is a large, fast-growing, site-attached marine fish that is of significant value to Indigenous, recreational and commercial fishers in northern Australia. The fish for this study were collected from two locations based near river mouths (Peron Islands and Sampan Creek) and two location near offshore islands (Caution Point and Mitchell Point), locations which experienced vastly different environmental conditions across the seasons. Through various research approaches the parasite assemblage of P. diacanthus has been explored for the first time and has been utilised to identify future focal areas for fisheries management.
Marine ecosystems support a multitude of biodiversity, stretching from primary producers to top order predators, inclusive of course of the countless parasitic organisms interacting with all. Large, predatory marine fishes can harbour a rich and diverse parasite fauna. One of the main challenges is that for most fish, surveys of the parasite faunas are scanty. Building on the few previous reports of parasitic organisms in this sciaenid, both morphological and molecular techniques were applied in this study to identify and describe the parasitic organisms present. Within the parasites infecting P. diacanthus were two new species of monogeneans, Diplectanum timorcanthus n. sp. and Diplectanum diacanthi n. sp.; many other parasites representing several genera still require further investigation. Parasites are remarkably diverse and adaptive, but without current taxonomic publication of new and existing organisms, ecological parasitology and life histories will remain unsolved.
Contributing to the parasitic assessments, stable isotope analysis was used to report on the trophic interactions between P. diacanthus and its associated parasites. Values of host nitrogen (δ¹⁵N) and carbon (δ¹³C) were measured against adult ectoparasites, adult endoparasites and larval cestode plerocercoids, with results demonstrating how different parasite species occupy different trophic levels and lie either above, below or alongside their P. diacanthus host. The levels of isotopic depletion and enrichment expressed by parasites were various and signatures seemed to depend on the individual parasite feeding ecologies and site of residence within the host. The stable isotope ecology of P. diacanthus and its parasites has provided inference on the connectivity and functionality of the marine ecosystem.
The use of parasites as biological tags is well established for understanding patterns of fish population connectivity. This practice was applied in the current study where an assessment on the potential impacts of spatial and temporal variability on the P. diacanthus stock and the biological parameters of the parasitic fauna were considered. The four populations of P. diacanthus investigated were clearly delineated by parasite assemblage, however temporal variability during the tropical wet season was highlighted as a confounding factor when identifying nearshore stocks. In order to move towards a sustainable P. diacanthus stock, appropriately scaled spatial management and a more thorough assessment of temporal changes in the stock is required to avoid localised depletion. Despite the dynamic nature of parasite communities this study has further shown that parasites can be ideal indicators of host population variance and movements and that both temporal and spatial considerations must be applied when deciphering the complex biology of living organisms.
Environmental change is upon much of the worlds marine ecosystems and with this comes imminent need to understand the ecological implications and potential risks to fisheries and the surrounding biomes. Through parasite presence and abundance modelling, this study has shown that short-term environmental variation has an impact on the abundance and distribution of parasites. Specific parasite genera illustrated variable responses to short-term seasonal environmental changes, however the overall parasite presence and abundance was influenced by changes in water temperature, salinity level and nutrient composition. This study has described a positive correlation between P. diacanthus body length and parasite species richness, introducing concerns for parasite abundance as warmer temperature shifts lead to smaller fish sizes. The discussion surrounding the influences of short-term seasonal environmental variation and fish host size, is a window into the potential future of parasitism when exposed to longer term climate variation. Additional deliberations for the sustainability of P. diacanthus and their associated parasites loom as the species faces continued fishing pressure and increasing climate-induced changes. It is vital that the climate sensitivity of parasitic organisms, their marine hosts and the ecosystems in which they inhabit is measured and understood.
An improved awareness of marine parasitism is warranted and the importance and research versatility of these organisms presented in this thesis has strengthened the incorporation of parasitic applications to future marine fisheries studies. These organisms, their hosts and the ecosystems surrounding them, are of significance and this study has paid overdue attention to what remains the largest contributor to biodiversity, parasites.