This is the case for jellyfish (medusae and other
gelatinous plankton). In the present work, 4 years of scyphomedusae by-catch data from the 2007e2010
Irish Sea juvenile gadoid fish survey were analysed with three main objectives: (1) to provide quantitative
and spatially-explicit species-specific biomass data, for a region known to have an increasing trend
in jellyfish abundance; (2) to investigate whether year-to-year changes in catch-biomass are due to
changes in the numbers or in the size of medusa (assessed as the mean mass per individual), and (3) to
determine whether inter-annual variation patterns are consistent between species and water masses.
Scyphomedusae were present in 97% of samples (N ¼ 306). Their overall annual median catch-biomass
ranged from 0.19 to 0.92 g m3 (or 8.6 to 42.4 g m2
). Aurelia aurita and Cyanea spp. (Cyanea lamarckii
and Cyanea capillata) made up 77.7% and 21.5% of the total catch-biomass respectively, but species
contributions varied greatly between sub-regions and years. No consistent pattern was detected between
the distribution and inter-annual variations of the two genera, and contrasting inter-annual patterns
emerged when considering abundance either as biomass or as density. Significantly,