Effects of density-dependence and sea surface temperature on interannual variation in length-at-age of chub mackerel (Scomber japonicus) in the Kuroshio-Oyashio area during 1970–1997

Annual mean fork length (FL) of the Pacific stock of chub mackerel (Scomber japonicus) was examined for the period of 1970–97. Fork length at age 0 (6 months old) was negatively correlated with year-class strength which fluctuated between 0.2 and 14 billion in number for age-0 fish. Total stock biomass was correlated with FL at age but was not a significant factor. Sea surface temperature (SST) between 38–40°N and 141–143°E during April–June was also negatively correlated with FL at age 0. A modified von Bertalanffy growth model that incorporated the effects of population density and SST on growth was well fitted to the observed FL at ages. The relative FL at age 0 for any given year class was maintained throughout the life span. The variability in size at age in the Pacific stock of chub mackerel is largely attributable to growth during the first six months after hatching

Age and growth of the neon flying squid, Ommastrephes bartrami, in the North Pacific Ocean

Longitudinal sections of statoliths were observed by light microscopy on 237 individuals (158–510 mm mantle length, ML) of Ommastrephes bartrami, obtained from the North Pacific (26 °–46 °N, 143 °E–149 °W) during 1991–1994. The width of each increment sharply decreased from 5–7 μm to about 1–2 μm between the 80th and 100th increment, with a transition zone where increments were inconspicuous. ML-age relationships were linear, and slope and intercept values differed by geographic area, hatch season and sex. Overall growth rates (mm day−1) were generally higher in females (1.1–2.5) than in males (1.1–2.1). Since these values were similar to the growth rates of four individuals which were tagged and recaptured (0.8–2.4), the increments were assumed to be formed daily. The growth rate was highest for individuals hatched in summer, followed by those hatched in spring. Growth rate obtained by length-based methods may be underestimated. The hatch dates of neon flying squid are almost year-round, and the life span is likely 1 year since mature individuals were 7–10 months old

Occurrence of jumbo flying squid Dosidicus gigas aggregations associated with the countercurrent ridge off the Costa Rica Dome during 1997 El Niño and 1999 La Niña

Factors responsible for aggregations of jumbo flying squid Dosidicus gigas, an important component of the marine food web and target of commercial fisheries off the Costa Rica Dome in the Eastern Tropical Pacific Ocean (ETP), were examined during 2 years of different extreme oceanographic conditions: fall 1997 El Niño and fall 1999 La Niña. A high abundance of squid occurred in association with the well-developed countercurrent ridge (upwelling) off the Costa Rica Dome during fall 1997, but not during fall 1999, when the countercurrent ridge was less developed. Two features of the well-developed countercurrent ridge were considered important for the occurrence of high jumbo flying squid concentrations. Firstly, subsurface chlorophyll a (chl a) maxima were formed along the countercurrent ridge, resulting in integrated chl a concentrations in the upper 100 m being relatively high considering the generally low productivity of the ETP during an El Niño event. Secondly, a strong salinity front formed along the North Equatorial Countercurrent, which is possibly responsible for retention of jumbo flying squid in the ridge. Large yellowfin tuna Thunnus albacares, which mainly feed on micronekton (small fishes, cephalopods and swimming crabs), as do jumbo flying squid, were also more highly concentrated along the countercurrent ridge during 1997 than during 1999. It was noted that skipjack tuna Katsuwonus pelamis and small yellowfin tuna, which mainly feed on zooplankton, were associated with the equatorial ridge in the ETP, indicating that prey faunal components may also play an important part in the close association of jumbo flying squid with the countercurrent ridge