The term jellyfish is taxonomically broad, referring to gelatinous plankton in the phyla Ctenophora (comb jellies) and Cnidaria (all other jellyfish). While jellyfish have been components of pristine marine ecosystems for millennia, recent worldwide increases in the abundance of some jellyfish have been associated with anthropogenic perturbations such as eutrophication (Arai 2001), overfishing (Lynam et al. 2006), climate warming (Mills 2001, Lynam et al. 2004, Purcell 2005), and coastal development (Richardson et al. 2009). Because many jellyfish have a complex life history that includes free-living sexual and asexual phases, populations can increase rapidly when environmental conditions change to favor them.

Jellyfish blooms can disrupt human activities such as fishing, recreational beach use, and power plant operations (Purcell et al. 2007, Richardson et al. 2009). Moreover, jellyfish blooms can substantially alter food webs (e.g., Ruzicka et al. 2007, Pauly et al. 2009) by decreasing energy flow to higher trophic levels (Richardson et al. 2009) and by altering community composition of lower trophic levels through selective feeding (Purcell et al. 2007). Notably, the high degree of diet overlap between jellyfish and forage fish such as herring (Purcell and Arai 2001, Brodeur et al. 2008) is thought to be a driver of observed increases in jellyfish abundances in systems where forage fish are removed (Lynam et al. 2006). After such removals, fish recovery can be impeded by jellyfish predation on eggs and juvenile phases of their fish competitors (Purcell and Arai 2001), effectively preventing the reestablishment of fish populations (Lynam et al. 2006). Chum salmon (Oncorhynchus keta) are one of the few reported predators of jellyfish that occur in Puget Sound (Purcell and Arai 2001, Rice 2007)


Source: Puget Sound Science Review

Moon jellyfish (Aurelia aurita). Photo by Hans Hillewaert, courtesy of USGS.



Recent worldwide increases in the abundance of some jellyfish have been associated with human-caused disturbances to the environment such as eutrophication, overfishing and climate warming.