Multi-species regime shifts reflected in spawning temperature optima of small pelagic fish in the western North Pacific

Abstract

Small pelagic fish have exhibited cyclic population dynamics in complex marine ecosys- tems worldwide. These fish regime shifts have been associated with ocean regime shifts; however, the biological mechanisms have remained unresolved and have been considered to depend mainly on multi-step processes through food webs. In the present study, we focus on species-specific temper- ature optima and show that similarities and differences in spawning temperature optima reflect those in the long-term population dynamics among multi-species pelagic fish in the western North Pacific. Spawning temperature optima were examined for anchovy, sardine, mackerel and jack mackerel, based on the occurrence of eggs and larvae, using a long-term (1978 to 2004) data set of egg and lar- val surveys off the Pacific coast of Japan. Anchovy exhibited a plateau-like spawning temperature pattern with a peak at ca. 22°C. In contrast, sardine and mackerel exhibited steeply peaked patterns with marked peaks at ca. 16 and 18°C, respectively. On the contrary, jack mackerel showed a very similar spawning temperature pattern to anchovy. These relationships were consistent with their flourish and collapse in long-term population dynamics. Furthermore, sardine and mackerel with stenothermal spawning patterns showed dramatic fluctuations relative to anchovy and jack mackerel with more eurythermal features. The multi-species comparisons extracted a simple and direct path- way: if viewed at large scales, direct temperature impacts on vital parameters provide a plausible explanation of multi-species regime shifts of small pelagic fish in the western North Pacific.