Jellyfish medusae are key predators in many of the
world’s pelagic systems (Mills 1995) and can affect the
abundance of zooplankton (Purcell 2003), fish larvae, and
eggs (Purcell et al. 1994) and thus recruitment of fish
populations (Lynam et al. 2005). When in large numbers,
they have been implicated in the disruption of pelagic
ecosystems (Mills 2001; Stibor et al. 2004), particularly
where blooms have formed. Gelatinous plankton are rarely
the preferred prey item of other species (Moline et al. 2004)
and so have been considered ‘‘trophic dead-ends’’, yet have
become the dominant top predator symptomatic of some
overfished systems (Brodeur et al. 2002; Hughes et al.
2005). Any large-scale increase in the occurrence of jellyfish
could therefore have major consequences for pelagic
ecosystems and fisheries. Recent increases in abundance
of native (Mills 1995; Brodeur et al. 2002) and introduced
(Mills 1995) species have been reported for a range of
systems, although decreases have also been noted (Mills
1995; Boldt 2004).