Inter-regional comparison of climate effects on marine fish populations facilitated through classification of mechanisms

Variations in climate strongly affect the structure and function of marine ecosystems, but a number of different mechanisms are at play and their relative importance varies between regions and with time. There are obvious semi-permanent regional differences in how marine populations respond to climate, but there may also be long-term trends either in climate itself or in the response pattern. In some cases single strong climate events may shift an ecosystem from one state to another (e.g., El Niño). To facilitate comparison between different large marine ecosystems we here give an overview of some of the manners in which one can classify how marine fish populations are affected by climate. Responses to climate fluctuations may be bottom-up, top-down or middle-out, immediate or temporally delayed, direct or via an intermediate population of predators, prey or competitors. Climate may invoke a linear or non-linear effect at the population or community level. Ecological effects of the NAO have been classified according to the four major classes: direct effects, indirect effects, integrated effects and translations, which also may be applied to other climate patterns and regions. By using classification schemes a more precise description of the particular properties of the various ecosystems may be possible. This approach enhances the possibility to compare between regions that may differ not only with regards to the relative importance of different climate factors for ecology, but also through dissimilarities in scientific tradition and terminology. Keywords: Climate, fish, population dynamics, mechanism, comparative approac