Interannual variation in Neocalanus biomass in the Oyashio waters of the western North Pacific

We examined the interannual variation in Neocalanus copepod biomass in the Oyashio waters in spring and summer from 1972 to 1999. In the mid‐1970s, mesozooplankton biomass in spring was high; however, it decreased significantly in the late 1970s. The timing of the decrease in mesozooplankton biomass corresponded to the 1976/77 climatic regime shift. The biomass of N. flemingeri, which dominated the Neocalanus community, was roughly constant from 1980 to 1999. Although species‐level estimates of Neocalanus biomass were not available for the 1970s, a previous study reported that Neocalanus copepods were the predominant mesozooplankton in the Oyashio waters in spring during the 1970s. Neocalanus copepods dominated the mesozooplankton community throughout the 1970s, and their biomass decreased in the late 1970s. Springtime net community production, an index of new production, also decreased in the late 1970s. We suggest that the reduction in new production negatively affected Neocalanus food availability, resulting decreased copepod biomass. New production may have been limited by a combination of subsurface iron supplies, increased vertical density gradient, and reduced vertical water mixing in winter, which resulted in diminished iron entrainment in winter. In summer, mesozooplankton biomass significantly decreased and increased synchronously with the 1976/77 and 1988/89 climatic regime shifts. The biomass of N. plumchrus, which dominated the Neocalanus community, was low in the 1980s and increased in the early 1990s. The biomass of the second‐most dominant copepod, N. cristatus, also increased in the early 1990s. Neocalanus copepods were reported to be a dominant component of the mesozooplankton community in the 1970s; Neocalanus biomass was high in the mid‐1970s and decreased in the late 1970s. Japanese sardine (Sardinops melanostictus), an important predator of Neocalanus copepods, exhibited interannual variation in standing stock that was inversely related to mesozooplankton biomass. At their peak in 1984, sardines consumed 32–138% of the daily Neocalanus production during summer. Therefore, predation pressure on Neocalanus by Japanese sardine is likely to affect interannual variation in mesozooplankton biomass during the summer.     

Environmental variability and growth histories of larval Japanese sardine (Sardinops melanostictus) and Japanese anchovy (Engraulis japonicus) near the frontal area of the Kuroshio

Environmental variability and growth‐rate histories from hatching to capture were investigated for larval Japanese sardine (Sardinops melanostictus) and Japanese anchovy (Engraulis japonicus). Larvae collected around the front of the Kuroshio Current were examined using otolith microstructure analysis, and their movement was estimated from numerical particle‐tracking experiments. Sardine larvae collected inshore of the Kuroshio front originated from a coastal area near the sampling site, while those collected in the offshore area originated from an area 500–800 km west‐southwest of the sampling site. Anchovy larvae collected both inshore and offshore had been transported from widely distributed spawning areas located west of the sampling area. At the age of 13–14 days for sardine and 19–20 days for anchovy, the offshore group exhibited significantly higher mean growth rates than did the inshore group. Although the offshore area was generally warmer than the inshore area, temporal variations in growth rate are not attributable solely to fluctuations in environmental temperature. While previous studies have examined the relationship between larval growth rates and environment based solely on data at capture, the methods used in the present study, combining otolith analysis and numerical particle‐tracking experiments, utilize data up until hatching. Although the relationship between growth rate and environment was not fully confirmed, this approach will greatly advance our understanding of fish population dynamics.     

Are tidal fronts good recruitment areas for herbivorous copepods?

Horizontal distributions of phytoplankton biomass and numerical abundance of copepod nauplii were investigated across a tidal front in the northern part of Bungo Channel, the Inland Sea of Japan, in May and June 1988. The frontal area was characterized by abrupt changes in subsurface (3 m deep) temperature and salinity, and by an increased standing stock of phytoplankton. The density of copepod nauplii was lower in the frontal area than in adjacent stratified and mixed areas. The population egg production rate of a predominant copepod species (Paracakmus sp.) was higher due to elevated fecundity on an individual basis in response to increased chlorophyll concentrations, in the frontal zone. However, the frontal population was subjected to higher mortality, probably due to predation by carnivores that were mainly accumulated passively within the area. From these facts, we conclude that tidal fronts are not always good areas for recruitment of herbivorous copepods, although fronts give rise to high standing stocks of phytoplankton.     

North–south contrasts in decadal scale variations in lower trophic-level ecosystems in the Japan Sea

Regional comparisons of interannual variations in springtime lower trophic-level ecosystems were made for northern subarctic regions, and for southern Tsushima Current regions of the Japan Sea, based on archival hydrographic and biological data sets collected from the mid-1960s to the early 1990s. Variations related to the Pacific Decadal Oscillation were detected for plankton biomass in both northern and southern regions, although there were regional differences with respect to mechanisms and timing. Springtime stratification increased after the late 1970s in the north, roughly coinciding with the northern Pacific regime shift in 1976/77. Stratification also increased due to warming in the south in the early 1980s, several years after the 1976/77 regime shift. Responding to the increase in stratification, springtime biomass of phytoplankton and zooplankton increased in the north and decreased in the south. Principal component analysis revealed that hydrographic conditions during spring, rather than winter, determined springtime phytoplankton biomass. In northern regions, spring phytoplankton production may be enhanced by increased light availability, due to mixed layer stabilization. In the south, where background nutrient concentration within the water column was low, increases in stratification were likely to limit nutrient supply to the surface layer, resulting in decreases in phytoplankton production. A positive relationship between phytoplankton and zooplankton biomass suggested bottom-up control of secondary production in northern regions. The nature of the links between phytoplankton and zooplankton production was not clear in southern regions, where hydrographic conditions during winter seemed to be responsible for variations in springtime secondary production.