Jellyfish are not as charismatic as some marine species and consequently they have not received much research attention. Recently though, there has been increasing interest in them because their numbers appear to be on the rise worldwide. There are a few hypotheses about why this might be the case floating around in the literature.
|The beautiful and under appreciated jellyfish, Cyanea capillata. Perhaps the World's biggest (photo Wikipedia).|
Overfishing is argued to increase jellyfish numbers by reducing predation and competition for food, particularly of young jellyfish. While agricultural runoff stimulates phytoplankton blooms that directly or indirectly provide increased amounts of food to jellyfish. Some others argue that changes to marine communities that are occurring as a result of climate change are tipping the ecological balance in favour of jellyfish. But there isn't much agreement, even among experts, about whether jellyfish have actually increased globally.
|Nomura jellyfish, Nemopilema nomurai, causing problems for Japanese fishermen (Photo Shin-ichi Uye)|
During the last population minimum, which occurred in 1993, jellyfish numbers were higher relative to previous population minimums. This resulted in a weak, but statistically significant trend towards increasing jellyfish abundance in the last 40 years. The authors caution that the trend is too weak, given the limitations of the data set, to conclude that jellyfish populations really are on the increase. Data collected in the next few years should be able to determine, with confidence, whether the upwards trend is real.
Although they found no strong evidence that jellyfish numbers are increasing worldwide, there was good evidence that numbers are increasing in some regions. These regions included the Sea of Japan, North Atlantic shelf regions, the Barents Sea, and parts of the Mediterranean Sea. All of these regions exhibited the 20 year oscillation, but local factors seem to have acted in concert with the global population fluctuations. Notably, fishing is heavy in many, if not all, of those regions.