Ecology of moon jellyfish Aurelia Sp. in southern Tasmania in relation to Atlantic salmon farming

• Main Author: Willcox, ST
• Format: Thesis
• Language: English
• Published: 2006
• Subjects: Jellyfishes; Atlantic salmon fisheries; Huon; Soi; Atlantic salmon
• Online Access: https://eprints.utas.edu.au/22181/; https://eprints.utas.edu.au/22181/1/whole_WillcoxSimonTrevor2006_thesis.pdf

The pattern of occurrence of medusae blooms in south east Tasmania was linked to both local and global scale environmental conditions. On average, summer water temperature was over one degree warmer, local autumn rainfall was less than half, winter salinity was 0.7%o lower, and Southern Oscillation Index (SOI) values in winter, spring, and summer were 10 - 12 points higher (and positive) in years where blooms occurred compared to those where they did not. The amount of local rainfall in autumn and the mean SOI value in winter were identified as the most useful environmental variables for predicting which summers will have medusae blooms. Blooming Aurelia sp. medusae were studied in the Huon Estuary from early December 2002 to late January 2003. Medusae grew exponentially and reached a maximum mean diameter of over 150mm in two months. Maximum mean growth rates of 7.3% body weight day-1 were measured before the pattern of growth broke and all medusae disappeared at the end of January. The total number of medusae in the Huon Estuary was estimated to be 169 million, with a total biomass of over 28 000 tons prior to the population senescing. Medusae formed into dense aggregations with densities up to 270 individuals m-3. Aggregations occurred in an environment with strong horizontal current sheer where surface and bottom waters often flowed in opposing directions and had velocities as high as 105 mm sec-1, yet were able to maintain their integrity. Observations with underwater cameras and by SCUBA diving revealed a complex structure with coordinated swimming of individuals within aggregations responsible for aggregation maintenance. Scyphistomae colony dynamics were examined in situ in south east Tasmania. Colonies were perennial and persisted for at least three years. Strobilation was observed every year in spring, however subsequent blooms of medusae did not always develop. The density of scyphistomae in colonies was a function of both the proportion of the substrate covered by the colony and the density of individuals within discreet colony patches. These variables were negatively correlated with competition from other encrusting organisms and local rainfall, and positively correlated with water temperature. Laboratory experiments showed that temperature and salinity affected rates of asexual reproduction. These factors resulted in numerical increases in colonies up to 150% over a 32 day period. These experiments also showed there is a trade off between increasing population size through budding at high temperatures, and increasing body size, possibly in preparation for strobilation, at low temperatures.