Pronounced interannual variability in the abundance of medusae of the jellyfish species Aurelia aurita, Cyanea lamarckii, and Cyanea capillata (Phylum Cnidaria, Class Scyphozoa) in the North Sea was evident in data arising from the International Council for the Exploration of the Seas International 0-group Gadoid Surveys between 1971 and 1986. Possible climatic forcing of jellyfish abundance, via the North Atlantic Oscillation (NAO), was investigated with data on medusae from four areas of the North Sea (east of Scotland, north of Scotland, east of Shetland, and west of northern Denmark). There were significant inverse relationships between medusa abundance and the NAO Index (December-March) in two regions: west of northern Denmark (A. aurita r 2 ϭ 0.70, P ϭ 0.003, n ϭ 10; C. lamarckii r 2 ϭ 0.74, P ϭ 0.002, n ϭ 10) and east of Scotland (A. aurita r 2 ϭ 0.53, P ϭ 0.008, n ϭ 12). Fluctuations in the abundance of A. aurita and C. lamarckii medusae might be linked to hydroclimatic changes induced through atmospheric effects (as encapsulated in the NAO Index) on wind stress, temperature, and currents. These fundamental hydroclimatic changes alter the timing of spring phytoplankton blooms and zooplankton community composition. Predation by an abundance of medusae on zooplankton and ichthyoplankton could affect the North Sea ecosystem through top-down and bottom-up mechanisms. Because the NAO is presently in a high phase, climatic conditions could be serving to depress the abundance of medusae: a future reversal of the NAO might favor jellyfish and weaken the persistence or recovery of fisheries.

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Marine ecosystems are influenced by multiple stressors in both linear and non-linear ways. Using generalized additive models (GAMs) fitted to outputs from a multi-ecosystem, multi-model simulation experiment, we investigated 14 major ecological indicators across ten marine ecosystems about their responses to fishing pressure under: (i) three different fishing strategies (focusing on low-, high-, or all-trophic-level taxa); and (ii) four different scenarios of directional or random primary productivity change, a proxy for environmental change. From this work, we draw four major conclusions: (i) responses of indicators to fishing mortality in shapes, directions, and thresholds depend on the fishing strategies considered; (ii) most of the indicators demonstrate decreasing trends with increasing fishing mortality, with a few exceptions depending on the type of fishing strategy; (iii) most of the indicators respond to fishing mortality in a linear way, particularly for community and biomass-based indicators; and (iv) occurrence of threshold for non-linear-mixed type (i.e. non-linear with inflection points) is not prevalent within the fishing mortality rates explored. The conclusions drawn from the present study provide a knowledge base in indicators’ dynamics under different fishing and primary productivity levels, thereby facilitating the application of ecosystem-based fisheries management worldwide.

Although an ecosystem approach to fisheries has been recognized as a means of progressing toward sustainable fishing, successful implementation of this approach has been limited. However, one way in which progress has been made is through the use of suites of indicators. Decision tree frameworks can be used to incorporate trends in ecological, fishing, and environmental indicators into ecosystem assessments. A relatively generic decision tree framework has been developed and successfully applied to multiple ecosystems. This framework incorporates trends in indicators, as well as the impacts of fishing pressure and environmental variability on ecological indicators in order to assess the state of each ecosystem. The inclusion of ecosystem expert knowledge from the outset ensures trends are correctly interpreted and allows analyses to contribute to global comparisons in a robust and meaningful manner. Although ecological and environmental indicators are well developed, those addressing the human dimensions of marine ecosystems are less so. This framework holds the potential to incorporate such indicators in order to fully assess marine ecosystems in a comparative context. Such assessments could help ensure food security from marine resources into the future as well as ensuring the well-being of coastal communities. Here critical review of the potential value of this framework has been conducted, with its usefulness emphasized in the similarities it holds, and in the contribution it could make, to current global methods of ecosystem assessments.