Jellyfish populations in the Mediterranean Sea

The Mediterranean Sea is home to numerous species of jellyfish, both native and invasive. Some of these jellyfish, which here include pelagic cnidarians, ctenophores and salps, have been monitored for centuries, while others are rarely sampled or studied. In addition, particular species of jellyfish are of high public interest due to impacts on tourism and other human activities, yet others may be of little concern. While several populations appear to be increasing in recent decades, these increases are not uniform throughout the Mediterranean Sea. Some groups show differing or opposite trends, and these trends may vary widely depending on the region in question. Despite the variability of jellyfish population dynamics over time, space, and taxa, several conspicuous groups of jellyfish appear to exhibit sustained increases in the Mediterranean Sea. Anthropogenic impacts have been suggested as potential drivers of these increases, including global warming, eutrophication, overfishing, bottom-trawling, mariculture, and increased coastal development. Similar trends in jellyfish populations to those in the Mediterranean Sea have been observed on a global scale. While increases are not ubiquitous, they are both numerous and widespread in coastal ecosystems and seas around the world (BROTz et al., 2012). The relatively extensive knowledge of jellyfish populations in the Mediterranean Sea provides an ideal opportunity to discuss their abundance trends, along with suggested causes and management practices, in the context of overall ecological changes in that body of water (COLL et al., 2012). Knowledge of jellyfish populations in the Mediterranean Sea is rather extensive, due to a combination of long-term datasets and interest relating to impacts on human activities. The notorious jellyfish Pelagia noctiluca appears to be blooming with increased frequency in some areas, and may sting tens of thousands of sea bathers in a single year. In addition, a number of invasive species of jellyfish appear to be thriving in the Mediterranean, some of which have significant impacts on fishing and other industries. In contrast, other groups of jellyfish show variable trends over time, and may be of little interest to the public. Here, we summarize knowledge of jellyfish in the Mediterranean Sea, including temporal trends for a variety of species, and discuss possible anthropogenic causes of increased jellyfish abundance and management interventions in the face of uncertainty.