Disentangling the effects of climate and density-dependent factors on spatiotemporal dynamics of Japanese sardine spawning

Changes in ocean conditions in response to climate variability and density-dependent factors are increasingly recognized as primary determinants of spatiotemporal dynamics of spawning in small pelagic fishes. However, the effects of climatic and density-dependent factors are complexly intertwined, and their respective effects remain controversial. We estimated climatic and density-dependent effects simultaneously using long-term egg survey data for Japanese sardine Sardinops melanostictus in the Tsushima Warm Current system. We found that in years with warmer sea surface temperature (SST) (climatic factor), egg abundance decreased in southern regions and increased in northern regions, and egg distribution expanded northward along the coast in the northernmost region. In years with colder SST, egg abundance decreased in northern regions, and egg distribution expanded offshore in the southernmost region. In contrast, spawning-stock biomass (SSB) (density-dependent factor) had little effect on the locations of main spawning grounds, whereas changes in SSB caused expansion or contraction of spawning grounds. The temporal peak of spawning shifted earlier in years with warmer SST and was delayed in years with colder SST, although the degree of the changes varied among regions. In contrast, spawning timing did not shift when SSB changed. These results indicate that climatic factors cause shifts and expansion or contraction of spawning grounds and also shifts in spawning timing, whereas density-dependent factors are related to expansion or contraction of spawning grounds but scarcely affect spawning timing. Our study shows that climatic and density-dependent factors differentially affect the spatiotemporal spawning patterns, with some overlap in their effects.