Contrasting spawning temperature optima: Why are anchovy and sardine regime shifts synchronous across the North Pacific?

Anchovy and sardine are both abundant small pelagic fish that have exhibited out-of-phase population oscillations in both the western boundary current region off Japan and the eastern boundary current upwelling region off California. These species alternations could indicate approximately synchronous patterns (with some variations in timing between ecosystems) despite the reversed temperature regimes for certain periods across the North Pacific, at least until recently. Here we suggest a contrasting pattern of temperature optima for anchovy and sardine spawnings between the opposite sides of the North Pacific as a possible mechanism of the synchronous phases of species alternations. Spawning temperature optima were examined for Japanese anchovy (Engraulis japonicus) and Japanese sardine (Sardinops melanostictus), based on occurrence of eggs and larvae in relation to sea surface temperature. The analyses were based on an updated long-term data set of egg and larval surveys from 1978 to 2004 (102,660 net tows) in the western North Pacific. The ratio of relative frequencies of plankton net samples that are positive for eggs or larvae versus all samples was calculated for temperature intervals of 0.1 °C for both species. This spawning temperature index exceeded 1.0 (baseline for optimum) from ca. 15 to 28 °C with a mid-point at 22 °C for anchovy and ca. 13–20 °C with a marked peak at 16–17 °C for sardine. Patterns of spawning temperature clearly show “warm” and “eurythermal” Japanese anchovy and “cool” and “stenothermal” Japanese sardine in the western North Pacific. This relationship between anchovy and sardine showed a marked contrast to earlier published reports on the spawning temperature optima of northern anchovy (E. mordax) and California sardine (S. sagax) in the California Current system. The spawning temperature optimum appears to be species-specific rather than genus-specific. The reversed species-specific temperature optima of anchovy and sardine under the reversed temperature regimes could provide a possible theoretical solution to the synchronous anchovy and sardine regime shifts across the North Pacific.