Aurelia aggregations

I had been told of sightings of large (8m x 3m, approx) aggregations of Aurelia (moon jellyfish) occurring in the lower Swan estuary in mid to late December 2011. Heading down past Nedlands and Dalkeith on 28 December I saw at least 3 of these aggregations. While I have been aware for years that this jelly and Phyllorhiza punctata bloom annually in late summer, I had never known of this behaviour to form aggregations (for spawning?).