Recent years' evidence indicates that Cnidaria have increased in abundance throughout the world's oceans and blooms (outbreaks of tens to hundreds of medusae per cubic metre) now occur more frequently in many seas (Purcell et al. 2007); these outbreaks have been attributed variously to alterations in the trophic structure of marine ecosystems owing to overfishing, and to hydroclimatic effects, since sea temperature can influence jellyfish life cycles and reproductive output (Purcell et al. 2007; Boero et al. 2008). The socio-economic effects of cnidarian outbreaks are not solely confined to pelagic fisheries, however. Since all Cnidaria possess stinging nematocysts, they are toxic and so can be detrimental to coastal aquaculture through damage to caged fish, and to tourism by curtailing bathing activities (Purcell et al. 2007).

Analysis of data collected by the continuous plankton recorder (CPR), the most temporally and spatially extensive plankton survey in the world, has revealed significant changes in northeast Atlantic plankton communities over the last two decades that appear to be related to hydroclimatic variability (Beaugrand 2004). While it has been suggested that hydroclimatic forcing has an important influence on the abundance of some North Sea cnidarian species (Lynam et al. 2005), the taxa in northeast Atlantic CPR samples are unknown, as their morphological identification is impossible.

Genetic analysis of selected northeastern Atlantic CPR samples collected between 2007 and 2008 revealed four different species of siphonophore and the scyphomedusa P. noctiluca, which was the dominant Cnidaria (table S2, electronic supplementary material). Distributional data from the CPR and genetic analysis indicated outbreaks of P. noctiluca during 2007 at latitudes of 45° N, 54° N and 58° N in September, October and early December, respectively (figure 2b). In 2008, outbreaks of P. noctiluca occurred during summer and autumn at 54° N in the northeast Atlantic and at 45° N in the Bay of Biscay in November (figure 2c).

Pelagia noctiluca is a warm-temperate holoplanktonic scyphozoan and it is distributed widely from coastal to oceanic waters as far north as the northern North Sea (Hay et al. 1990). Pelagia noctiluca can acclimate to a wide range of temperatures (from less than 8°C to greater than 22°C in the Mediterranean, (Sandrini & Avian 1991), varying its metabolism to enhance the recruitment of young medusae (Morand et al. 1992), features that enable it to reproduce rapidly under favourable conditions to reach high densities the whole year round. Long-term records from the Mediterranean since the late nineteenth century reveal that outbreaks of this species, that tended to occur only once every 12 years and with 4 years duration before 1998, are now more frequent (Daly Yahia et al. 2010). As outbreaks of P. noctiluca appear to be associated with warm winters (Goy et al. 1989), the recent increase in western Mediterranean SST (up to 0.5°C increase since 2002 (figure 2d)) may explain their increasing frequency in this region. Warmer waters in the northeast Atlantic (up to 1°C increase in winter SST since 2002 (figure 2d)) may have influenced P. noctiluca similarly.

The seasonal occurrence of high densities of P. noctiluca in the western Mediterranean and northeast Atlantic also appears to be influenced by surface hydrography. In the western Mediterranean, the occurrence of P. noctiluca swarms follows the progression of the Atlantic surface water stream, which flows eastwards from the Atlantic through the Strait of Gibraltar along the North African coast (close to the Tunisian coast in winter) before circulating anticlockwise around the western Mediterranean basin (Pinardi & Masetti 2000) (figure 2b). In the northeast Atlantic, outbreaks of P. noctiluca appear to follow the progression of the North Atlantic Current (NAC) and the surface continental slope current (CSC), a northward branch of the Azores Current that flows along the eastern slope boundary of the European basin (Garcia-Soto et al. 2002; Pingree 2002) (figure 2b; figure S1, electronic supplementary material). Large-scale atmospheric patterns, i.e. the North Atlantic Oscillation (NAO), influence the strength of the NAC and CSC. In particular, during 2007–2008, the NAO was positive during winter and negative during summer, giving climatic conditions associated usually with an enhanced northward penetration of the NAC and CSC around Scotland into the North Sea (Garcia-Soto et al. 2002; Pingree 2002).