Winter mixed layer depth and spring bloom along the Kuroshio front: implications for the Japanese sardine stock

Abstract

Recruitment of Japanese sardine Sardinops melanostictus is related to interannual variability in the winter mixed layer depth (MLD) near the Kuroshio axis (line of maximum current), possible because MLD may influence the feeding environment of sardine larvae through phytoplankton productivity. However, a relationship between the winter MLD and phytoplankton productivity has not been shown. Particle release experiments with quasi-observed current fields from ocean reanalysis products and satellite-observed phytoplankton (chlorophyll a) density from 1998 to 2006 showed that deeper waters of the winter mixed layer flowing 0° to 0.5° north of the Kuroshio axis led to a greater bloom in the subsequent spring, although such a relationship was not detected south of the Kuroshio axis. By using the output of a 3-dimensional, high-resolution lower trophic level ecosystem model that reproduced the MLD−phytoplankton relationship, we found that entrainment of deeper nutrient-rich subsurface water leads to abundant nutrients in the early spring and enhances the subsequent spring bloom along the northern side of the Kuroshio axis. On the southern side, where mode water develops in winter, the deeper winter mixed layer does not necessarily contain higher nutrient contents, because nutrient vertical profiles often have inversions. These results support the hypothesis that sardine larvae that are distributed in the deeper winter mixed layer north of the Kuroshio axis (called the Kuroshio frontal zone) encounter a higher phytoplankton density, which yields favorable feeding conditions, resulting in recruitment success.

Seasonal and spatial contrast in the surface layer nutrient content around the Kuroshio along 138°E, observed between 2002 and 2013

“…7); the concentration in the slope water was higher than that in the Kuroshio and the gyre water, as well as in the model study (Nishikawa et al 2013). Seasonal contrasts of nutrient concentrations are reported for the upstream of the Kuroshio and the East China Sea.…”

“…These seasonal variations of the mixed layer depth (Z m ) and sea surface Chl a concentration indicate that nutrients are supplied from the deeper layer associated with the development of the mixed layer during winter (Longhurst 2007). In fact, the reported nitrate concentration at the surface generally increases to C 1 lM during winter and spring, while it is depleted to the nanomolar level during summer (Limsakul et al 2001(Limsakul et al , 2002Furuya et al 2003;Chen 2008;Sugisaki et al 2010;Kodama et al 2011;Shiozaki et al 2011;Nishikawa et al 2013). The nutrient supply during winter supports the high biological production of the Kuroshio (Sugisaki et al 2010), and thus it is considered to control the survival rate of the Japanese sardine (Nishikawa et al 2011).…”

“…An eddy-resolving, coupled physical-biological model study shows a large spatial difference of nutrient concentrations during winter; that is, the surface nitrate concentration is several micromolars in the northern part of the Kuroshio axis and \ 1 lM in the southern part of the Kuroshio axis (Nishikawa et al 2013). This spatial nutrient variation of the Kuroshio is different from that of the Gulf Stream, which is the western boundary current in the North Atlantic Subtropical Gyre.…”

Scales and mechanisms of marine hotspot formation

“…In good years, increased retention lead to hotspots of larval haddock on the bank but additional factors played a role in the magnitude of haddock recruitment in a given year. Nishikawa et al (2013) examined how water column characteristics lead to phytoplankton blooms in the western North Pacific that develop into important spawning habitat for Japanese sardine Sardinops melanostictus. A deeper mixed layer and higher phytoplankton density resulted in increased spawning habitat north of the Kuroshio.…”

Recent Advances in Japanese Fisheries Science in the Kuroshio-Oyashio Region through Development of the FRA-ROMS Ocean Forecast System: Overview of the Reproducibility of Reanalysis Products

“…Assimilated data have been analyzed instead of observed data to clarify the linkage between fish abundance and oceanographic conditions [18], and they have been used for stock management applications [19]. Survival, growth, migration, and recruitment of important commercial fish have been examined by using particle-tracking models and individual-based models (IBMs) with assimilated oceanographic conditions (e.g., [20] [21] [22] [23]). Several studies have developed high-resolution nested ocean models with data assimilation products (e.g., [24]) in an effort to understand and predict the sudden occurrence of harmful marine organisms such as jellyfish, toxic algae, and viruses.…”