Background
The authors write that "a rich literature documents potential evolutionary and plastic responses to physical drivers in fish," but they say that "predicting actual responses in natural populations remains a core challenge because observed responses typically fail to match predictions based on theory or laboratory experiments," citing in this regard the study of Merila et al. (2001).

What was done
In order "to clarify what patterns have been observed," Crozier and Hutchings "undertook a specific review of the literature that detected phenotypic responses to climate change in wild fish populations, with a particular interest in the extent to which these responses could be attributed to evolutionary or plastic processes."

What was learned
The two researchers report there is "abundant evidence that many traits in fish can respond rapidly to changes in environmentally driven selection pressures and that these traits are strongly plastic," while noting that "changes in the timing of migration and reproduction, age at maturity, age at juvenile migration, growth, survival and fecundity were associated primarily with changes in temperature." And in light of the fact that "these traits can evolve rapidly," they found that "only two studies attributed phenotypic changes formally to evolutionary mechanisms."

What it means
As for the significance of these several observations, Crozier and Hutchings conclude that "ultimately, many species will likely adapt to long-term warming trends overlaid on natural climate oscillations," which would have to be considered a positive consequence.

Reference
Merila, J., Sheldon, B.C. and Kruuk, L.E.B. 2001. Explaining stasis: micro-evolutionary studies in natural populations. Genetica 112-113: 199-222.