Parasite assemblages are dynamic communities which can vary within an individual host species based on both temporal and spatial factors. Despite this, parasites can be ideal indicators of host population variance and movements, however, the type of parasites, or parasite community, utilised as biological/ecological tags, must be carefully considered. Utilising the entire parasite community of a host allows for more thorough and higher-level conclusions on host population structuring to be drawn4,12. Determination of the extent of temporal stability of a parasite community across different seasons, such as those in this study, is needed to evaluate the suitability of parasites in stock structure studies1. It is evident from this study that there is temporal variability in parasite assemblage composition among seasons that has the potential to confound stock structure assessment if not adequately considered. Despite this temporal variability, the results of the current study are consistent with the overall findings of Taillebois et al.12, who reported populations of P. diacanthus off northern Australia to be highly complex and spatially distinct. However, we also found some evidence of mixing of regional populations during the wet season, with overlap in parasite assemblage composition evident between the two offshore sites (Caution Point and Mitchell Point), suggesting that P. diacanthus may disperse more widely during the wet season than other times of year.
The climatic conditions of the tropical wet season are significantly different to those of the dry and build-up seasons where many environmental factors remain unchanged over a long duration. During the dry season, river flow regimes are characterised by varying degrees of flow cessation, or intermittency15, with subsequent changes in salinity levels and water temperatures13 compared to the wet season when rivers are in spate and combined with strong tidal currents create high turbidity levels in nearshore waters14. The fish sampled from Peron Islands have a parasite assemblage that is independent of the other populations, irrespective of the time of year. Although Taillebois et al.12 identified an exponential decay in the similarity of parasite assemblages as the distance between host populations increased (see Poulin & Kamiya9), the distance between Sampan Creek and the offshore locations is greater (approximately 240 km) compared to Peron Islands (approximately 120 km). Thus, distance alone is not sufficient to explain the distinctiveness of the parasite assemblages at Peron Islands. Previous research suggests that tidal flows and ocean currents might influence biotic assemblage composition, including parasites, in this region. Condie22 showed that tidal flows from the Mary River (Sampan Creek) tracked north-west towards the Tiwi Islands into the Timor Sea, whereas the tidal flows from the Daly River (Peron Islands) tracked west across the Joseph Bonaparte Gulf (JBG). Therefore, the similarity in parasite assemblages between fish populations from Sampan Creek, Mitchell Point and Caution Point in the late wet season may be more influenced by tidal flows. Additionally, the shallow water environment of the JBG with its warm, low salinity waters and increases in sediment discharge and resuspension could influence the composition of benthic organisms in this area of water, with previous surveys confirming that biota in the inner areas of JBG differ from the biota from the adjacent Timor Sea23,24.
The main parasite taxa driving the results of the analyses in our study were the ectoparasitic copepod Caligus sp. and the two digeneans Hemiuridae and Pleorchis sp., a finding which confirmed the need to study different components of parasite communities or assemblages of hosts, when conducting population studies24. Ectoparasites, such as copepods, and endoparasites, such as digeneans, have different routes of transmission to hosts, and, thus, reflect different aspects of the host’s biology and ecology. Ectoparasites, generally, have a direct lifecycle and, due to their positioning in the gills, are directly affected by environmental shifts and fish behaviours, making them ideal indicators of short-term influences25. Endoparasites, on the other hand, tend to utilise trophic transmission in aquatic systems, and are indirectly mediated by changes in the abundance and distribution of their hosts (especially any intermediate hosts). Therefore, any changes seen during different seasons of the year will often be indirect and reflect conditions experienced by earlier hosts than P. diacanthus, thus providing information on a longer time scale. Depending on the type of parasite, the local host environment can buffer parasites from environmental change to some degree. However, as host condition (including immunology and physiology) and host behaviour and distribution (which affects availability for infection) becomes affected by environmental change, these changes to the host can indirectly influence parasites26,27.
While our study has provided information on spatial and temporal variation in the parasite assemblages of P. diacanthus, it is important to consider the limitations of this study. Despite substantial sampling effort, uneven numbers of fish were caught across seasons, leading to data being combined across two years, with the wet season of 2019–2020 significantly drier compared to the 2020–2021 wet season13. Despite this, strong spatial structure was found, although we were unable to assess whether the strength of variation between seasons within a year would be stronger than the potential effect of variation of seasons between years. Additionally, the biology and ecology of the parasites of P. diacanthus remains largely unknown. Subsequently, we can only speculate on the life span of parasites and the possible influence of environmental variation on their abundance within a host population and their usefulness in assisting with host population structuring.
Of the parasitic taxa collected for the present study, not all were able to be identified to the lowest taxonomic unit possible. Despite a number of parasites not being described to the species level, we are confident that all taxa utilised in the analyses of this study are individual species. From the perspective of stock discrimination studies, the key to reliable analysis is not in the identification of a parasitic organism, but rather in the distinction of parasitic organisms28. Parasites are notoriously hard to identify and, although finer-scale differentiation of parasites would potentially increase inferential power28, the lack of species-level identification for some taxa did not prevent the identification of statistically significant parasite assemblage structure in this study.