Monitoring the abundance and distribution of taxa is essential to assess their contribution to ecosystem
processes. For marine taxa that are difficult to study or have long been perceived of little ecological
importance, quantitative information is often lacking. 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, A. aurita medusae
were heavier in stratified than in mixed waters, which we hypothesize may be linked to differences in
timing and yield of primary and secondary productions between water masses. These results show the
vulnerability of time-series from bycatch datasets to phenological changes and highlight the importance
of taking species- and population-specific distribution patterns into account when integrating jellyfish
into ecosystem models.
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