By the way the reduction in fish size in reaction to warmer ocean temperatures is a perfect example of Natural Selection.
No individual fish "adapts" to climate change. Rather those individuals who are genetically disposed to be smaller and therefore consume less oxygen will be better suited to the change in climate. On average they will produce more offspring who survive long enough to reproduce and therefore the genes for smaller phenotypes will prosper in the gene-pool.
Evolution = the change in frequency of alleles in a local population.