Quantifying ecological aspects of the seasonally abundant box jellyfish Chironex fleckeri within coastal and estuarine waters of Far North Queensland

The occurrence of Chironex fleckeri medusae in the near shore waters of tropical Australia represents a significant safety, financial and medical concern during the warmer months of the year, particularly along vast stretches of unprotected coastline. Although a number of theories relate to the seasonal occurrence of medusae, quantitative data documenting key aspects of C. fleckeri's ecology are currently lacking. As a result, management protocols tend to be based on anecdotal / opportunistic evidence or generalisations extrapolated between geographic regions. While considerable improvements have been made in reducing the risk of an envenomation occurring along confined stretches of coastline, such as within stinger nets, the capacity to model the occurrence and distribution of medusae, both spatially and temporally, would allow for more effective management of the broader tropical coastline. This study represents the first contribution to the development of such models by quantifying some of the long held theories relating to the ecology of C. fleckeri.\ud \ud The ability to predict when the medusae season will commence would be of considerable benefit to risk management models. Currently, the timing of stinger net installation is largely based on the generalisation that medusae are common inhabitants of the coastline during the warmer months of the year, their arrival being associated with changes in a suite of physical parameters including water temperature, estuary flows and salinity. In contrast, this study has shown that the shift from the polyp to medusa phase was temporally constrained across six seasons in Weipa, with the onset of medusae production varying by only one week in late August / early September. While variables such as water temperature, rainfall and tidal height were not correlated with the onset of medusae production, photoperiod could provide a higher degree of temporal consistency in the onset of the medusae season. Whether photoperiod acts as the cue for metamorphosis in this species is worthy of further investigation.\ud \ud Once within the medusa phase of the lifecycle, juvenile medusae are thought to undertake a downstream migration from the polyp habitat to the coastline where they grow in size rapidly, accumulating as the season progresses. Population structure data collected across seven seasons does not suggest that this is the case for Weipa sites, however. Instead, larger, older and sexually mature medusae were found in abundance within Weipa's estuarine habitats, while a high proportion of juvenile medusae were represented within the coastal habitat. Given that the oldest medusa collected was estimated to be 78 d in age and was from the estuarine habitat, medusae do not appear to accumulate along the coastline as the season progresses either. Not only were medusae growth rates rapid, increasing at up to ~3 mm d⁻¹ IPD (inter pedalia distance), tentacles were added more rapidly (up to a maximum of 13 per pedalium) as size increased. At this growth rate, medusae would undergo an ontogenetic shift in their cnidome and venom complement after ~50 - 60 d, at which time they become potentially lethal to humans. Given that sexual maturity would also be realised after ~45 - 50 d, the potential for several generations to occur within a single season has also been raised. Determining whether ecological patterns observed in Weipa are consistent between geographic locations is an aspect of C. fleckeri ecology into which future research should extend, allowing predictive models of regional rather than local context to be developed.\ud \ud Many references are made to the exceptional swimming abilities of C. fleckeri medusae, who, unlike other species of cnidaria, possess the ability to actively move from one location to another, presumably in response to the development of unfavourable conditions. Recent advances in acoustic telemetry allowed the movement patterns of 12 adult medusae to be documented for between 10 and 38 h, within both the coastal and estuarine habitats using manual telemetry. The rates at which medusae travelled varied from approximately 52 ± 26 SE m h⁻¹ to 828 ± 306 SE m h⁻¹, with both tidal state and time of day found to influence rate of travel. This effect was habitat specific, however, whereby medusae within the coastal habitat travelled at relatively consistent rates both day and night, while estuarine medusae travelled at significantly faster rates at night. At increased current speeds, estuarine medusae also appeared to travel with the current rather than across or against it, as they did at slower current speeds. With acoustic telemetry now demonstrated to be an effective method by which medusae movement data can be collected, future studies can focus on quantifying how medusae movements are influenced by changes in physical parameters such as wind speed and direction (wave action).\ud \ud Manual acoustic telemetry possess a number of logistical constraints that limit the timeframe over which movement data can be collected, however. To this end, this study has also demonstrated the novel application of automated acoustic arrays to C. fleckeri medusae. Combined with a revised method of tag attachment, the timeframe over which medusae were tracked increased from a mean of 16 ± 8 SE h and maximum of 38 h for manual acoustic telemetry to a mean of 37 ± 16 SE h and maximum of 84 h for acoustic arrays. A trade off in the resolution of position estimates derived from acoustic arrays was evident, however, particularly in those cases where position estimates were based on data from a single receiver (839 ± 89 SE m). Error was significantly less where multiple receivers contributed to a position estimate, with a minimum error of 157 ± 10 SE m observed when position estimates were based on four receivers. This was still significantly greater than the 10 m associated with manual telemetry, however. For two medusae tracked using both manual and array techniques, rate of travel estimates from manual telemetry (170 ± 24 SE m h⁻¹) varied significantly from those calculated from array data (487 ± 55 SE m h⁻¹). Similarly, diurnal and tide related movement patterns were inconsistent between manual telemetry and acoustic array analysis.\ud \ud When array data was considered in terms of presence / absence, however, repeated longshore movements of individual medusae became evident, the direction of which corresponded with tidally generated currents. Movements on several occasions that were restricted to relatively narrow stretches of beachfront (~200 m) indicate that medusae are not simply passive drifters, however. As such, automated acoustic telemetry provides a method by which either presence / absence data or low resolution position estimates can be collected over extended periods of time.\ud \ud Collectively then, this study has quantified some fundamental aspects of C. fleckeri's ecology, validating some long held theories, while bringing others into question. A contribution to the broader understanding of jellyfish ecology has also been made, with growth and development rates along with population structure and demographics now quantified. The development of a predictive model remains subject to further quantitative comparisons, particularly across geographic locations, however