Invasion of the Jellyfish Pelagia noctiluca in the Northern Adriatic: a non-success story

Over the last two centuries, massive outbreaks of Pelagia noctiluca were recorded on average at 12 years intervals in certain parts of the Mediterranean Sea. Population peaks of Pelagia noctiluca were sometimes accompanied by an expansion outside of its usual distribution range, and during 1977–1986 this jellyfish also populated the northern Adriatic Sea. After maintaining itself at rather high densities for several consecutive years, the Pelagia noctiluca population of the northern Adriatic collapsed rather abruptly and for no obvious reason.Environmental conditions before, during and after the invasion of Pelagia noctiluca to the northern Adriatic are described. A model using time series analysis (1984–1986) of its population size structure was utilized to simulate the abundance of this jellyfish in the northern Adriatic. Results of this modelling exercise indicate that the most important effect on Pelagia population density was maturation at an early age (thus, at smaller size). The incorporation of demographic parameters such as shrinkage (regression and recovery to sexual maturity) into the model was the second most important factor for simulating its population abundance.