Climatically induced changes in distribution of fish populationens. Example: anchovies and sardines:
European anchovy (Engraulis encrasicolus) and sardine (Sardina pilchardus) are southern Lusitanian species needing warmer temperatures than boreal ones. After about 40 years of absence, they were observed again in increasing quantities in the North and Baltic Sea, apparently driven by climate variability.
Sardines re-invaded the North Sea around 1990, probably as a response to warmer temperatures associated with the strengthening of the North Atlantic Oscillation (NAO) in the late 1980s.
However, surprisingly, increasing numbers of anchovy eggs, larvae, juveniles and adults were recorded only since the mid-1990s, indicating that the temperature rise in the winter months due to the NAO was not sufficient for triggering the re-appearance and spawning of this species in more northern waters.
Presumably, changes in current structures and in summer temperatures since the mid-1990s, in association with the contraction of the subpolar gyre, were responsible for the expansion of the anchovy distributional range into the North Sea.
We discuss, which atmospheric (e.g., AMO, NAO) and oceanographic (e.g. contraction of subpolar gyre) drivers might be responsible for the occurrence of anchovies and sardines in North and Baltic Seas and other changes observed in fish populations at the same time.