No abstract

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. KEY WORDS: Jumbo flying squid · Dosidicus gigas · El Niño · La Niña · Countercurrent ridge · Equatorial ridge · Yellowfin tuna · Skipjack tuna Resale or republication not permitted without written consent of the publisherMar Ecol Prog Ser 231: [151][152][153][154][155][156][157][158][159][160][161][162][163][164][165][166] 2002 and are the target of commercial fisheries off the Costa Rica Dome in the ETP.The ETP is also a region of significant interannual variability associated with the El Niño Southern Oscillation (ENSO). The effects of El Niño/La Niña events have been frequently discussed in both the scientific literature and news media. Shifts in distribution and changes in abundance of marine animals due to these events receive much attention. It is still unknown, however, how El Niño/La Niña events affect the distribution of jumbo flying squid and even those of abundant tunas, such as yellowfin Thunnus albacares and skipjack Katsuwonus pelamis, in the ETP.Japanese commercial vessels intermittently harvest jumbo flying squid by jigging off the Costa Rica Dome. Good catches were obtained in this area during fall 1997 El Niño, but poor ones during fall 1999 La Niña. To identify factors influencing aggregations of this species, we studied their distribution using the fishery and survey data obtained during that 2 yr period. Satellitederived...

Interannual changes in abundance of the autumn cohort of neon flying squid Ommastraphes bartramii during 1979 to 2006 were examined in relation to the large-scale climate shift in the central North Pacific Ocean. Catch per unit of effort (CPUE) of driftnet surveys, which is assumed to reflect the squid stock level, was very low during 1999 to 2002, which was an anomalous climate period. Given that the autumn cohort uses productive areas near and north of the transition zone chlorophyll front (TZCF) as favorable nursery and feeding grounds, we examined 3 covariates -(1) the winter position of the TZCF in the neon flying squid nursery ground, (2) the summer position of the TZCF in the feeding ground and (3) fishing mortality -to estimate which covariates were significantly correlated with low CPUE time periods. Multiple linear regression analysis suggested that only the effect of winter position of the TZCF in the nursery ground best predicted the change in the CPUE. The anomalous climate conditions caused a significant reduction in net primary production over an extended area within the subtropical frontal zone (STFZ), which is the main nursery ground for the autumn cohort. Hence, we assume that food availability for juvenile neon flying squid could be critical for the level of stock recruitment of this cohort. Furthermore, the sea level anomaly (SLA), which is considered to be a useful index of variability in the upper ocean structure, significantly correlated to the CPUE during spawning and nursery periods in the STFZ. Our study supports the hypothesis that the bottom-up effect of the climate shift can be manifested rapidly in neon flying squid, indicating that squid abundance can act as a productivity integrator and a real-time ecosystem indicator that respond to large-scale environment changes.

The neon flying squid (Ommastrephes bartramii), which is the target of an important North Pacific fishery, is comprised of an autumn and winter-spring cohort. During summer, there is a clear separation of mantle length (ML) between the autumn (ML range: 38-46 cm) and the winter-spring cohorts (ML range: 16-28 cm) despite their apparently contiguous hatching periods. We examined oceanic conditions associated with spawning/nursery and northward migration habitats of the two different-sized cohorts. The seasonal meridional movement of the sea surface temperature (SST) range at which spawning is thought to occur (21-25°C) indicates that the spawning ground occurs farther north during autumn (28-34°N) than winterspring (20-28°N). The autumn spawning ground coincides with the Subtropical Frontal Zone (STFZ), characterized by enhanced productivity in winter because of its close proximity to the Transition Zone Chlorophyll Front (TZCF), which move south to the STFZ from the Subarctic Boundary. Hence this area is thought to become a food-rich nursery ground in winter. The winter-spring spawning ground, on the other hand, coincides with the Subtropical Domain, which is less productive throughout the year. Furthermore, as the TZCF and SST front migrate northward in spring and summer, the autumn cohort has the advantage of being in the SST front and productive area north of the chlorophyll front, whereas the winter-spring cohort remains to the south in a less productive area. Thus, the autumn cohort can utilize a food-rich habitat from winter through summer, which, we hypothesize, causes its members to grow larger than those in the winter-spring cohort in summer.