Size and characteristics of aggregations of moon jellyfish (Aurelia sp) in Tasmania, Australia

Dense aggregations of moon jellyfish, Aurelia sp., occurred in four of eight summers from 1978/79 to 2004/05 in the Huon Estuary, southern Tasmania, causing significant mortality of farmed Atlantic salmon. This study investigated the biological characteristics of this jellyfish as part of a larger study examining factors influencing these aggregation events. Aggregations of medusae in the Huon Estuary were intensively sampled over 50 days in 2004/05. They typically had a strongly delineated structure throughout the water column, with a relatively high density of medusae at maturity, then the aggregations rapidly disappeared (size at maturity for 50% of the population was 101 mm bell diameter for females, 90 mm bell diameter for males). The discrete structure of the aggregations enabled relatively accurate estimates of the biomass of the Aurelia sp. population when growth rate was at its maximum. The number of aggregations in the estuary, their sizes and density of medusae were estimated from aerial photographs, by divers, and underwater video records. Mean density in the aggregations was 71 medusae m-3. They contained approximate totals of 174 million Aurelia sp. medusae, wet weight of 28,600 t, and 39.2 t of carbon. Our observations suggest that the main advantage of forming these complex aggregations is to enhance fertilisation success.