this is not true if changes in fish migrations or productivity occur suddenly and on a major scale as certain threshold values of environmental variables are exceeded such changes may be difficult to predict and all the more so since they might occur even if the underlying change in ocean circulation and temperature is gradual ocean conditions might suddenly reach a point where certain fish stocks can no longer survive or radically change their migratory habits the only certainty on what to expect would be if similar things had occurred in the past andor if one had a strong understanding of the mechanisms and interactions underlying climate change and its impacts on oceans and ecosystems it is useful to distinguish between two main types of changes that could occur i changes in the productivity of the ocean in a given location and ii changes in fish migrations or the location of their habitats