Reconsideration of Parapatric Distribution Between Pacific Saury (Cololabis saira) and Japanese Sardine (Sardinops melanostictus) in the Western North Pacific Ocean: Comparisons of Two Long-Term Field Survey Results

Pacific saury (Cololabis saira) and the Pacific stock of Japanese sardine (Sardinops melanostictus) are dominant small pelagic fishes and are targeted by international fisheries in the western North Pacific (WNP). Using monitoring results of a pelagic trawl survey during 2003–2019 in WNP and central North Pacific, a previous study detected a shift in the center of distribution along sea surface temperature gradient (GT) of saury only in WNP, together with “parapatric” horizontal distribution between the two species during the 2010s, when saury biomass declined and sardine biomass increased. Thus, it suggested biological interactions between the two species. To corroborate this “parapatric” distribution, we applied basically the identical statistical analyses to driftnet survey results along longitudinal transects at 155°E, 170°E, and 175.5°E during 1979–1999, when saury stock was abundant and sardine stock declined since 1989. Our results indicated GT of saury slightly shifted after 1989 at 155°E, and spatial distribution of the two species largely overlapped before 1989, which do not support the previous study. While saury biomass drastically declined in the western areas of their overall distribution since 2003 and age-0 sardine dominated in the pelagic trawl survey, age-0 sardine occupied the southern area of overall distribution in the driftnet survey. These observations suggest that interspecies interactions between saury and sardine, if existed, are unlikely to take a form of parapatry. The apparent parapatric distribution can be explained by different age compositions of sardine between the two surveys and biomass-derived eastward shrinkage of saury distribution since 2003.