Spatial distribution, size structure, and prey of Craspedacusta sowerbyi Lankester in a shallow New Zealand lake

The spatial distribution and size structure of the invasive freshwater jellyfish, Craspedacusta sowerbyi Lankester was measured in Lake Kainui, a shallow lake in the Waikato region of New Zealand. Population size structure conformed to a normal distribution with bell diameters ranging from 2-18 mm. Tentacle numbers were significantly correlated with medusa diameter and circumference. Similarly gonad length and volume increased with diameter, but the greatest contribution to reproductive effort was estimated to be in the 11-15 mm size range. Craspedacusta sowerbyi was present with a mean density of 12.7 per 100 l in the surface waters with highest densities at the lake surface. The distribution of C. sowerbyi along two transects showed an irregular contagious distribution with patches of C. sowerbyi recorded in open water away from the immediate littoral zones. The stomach contents of C. sowerbyi medusae included most of the taxa abundant in the zooplankton, with the exception of nauplii and small rotifers. There appeared to be a positive selectivity for the active copepod Ceriodaphnia with an estimated daily mortality rates of 3.41% of prey population compared to less than 0.5% per day for most zooplankton taxa. It seems unlikely that C. sowerbyi predation was a very significant component of zooplankton mortality in Lake Kainui but may shape the zooplankton community and availability of zooplankton prey for other predators such as fish.