ALBERT

Beschreibung: We describe the feeding habits of 70 blue sharks (Prionace glauca) and 39 salmon sharks (Lamna ditropis) caught at 0–7 m depth at night by research drift gillnets in the transition region of the western North Pacific during April–May of 1999 and 2000. Blue sharks of 50–175 cm total length fed on a large variety of prey species, consisting of 24 species of cephalopods and 16 species of fishes. Salmon sharks of 69–157 cm total length fed on a few prey species, consisting of 10 species of cephalopods and one species of fish. Important prey for the blue sharks were large, non-active, gelatinous, meso- to bathypelagic cephalopods (e.g., Chiroteuthis calyx, Haliphron atlanticus, Histioteuthis dofleini and Belonella borealis) and small myctophid fishes. Important prey for the salmon sharks were mid-sized, active, muscular, epi- to mesopelagic squids (e.g. Gonatopsis borealis, Onychoteuthis borealijaponica and Berryteuthis anonychus). Our results suggest that blue sharks feed on cephalopods mainly during the daytime when they descend to deep water. Salmon sharks may feed opportunistically with no apparent diurnal feeding period. Blue sharks and salmon sharks have sympatric distribution in the transition region in spring; they have different feeding habits and strategies that reduce competition for food resources.

Beschreibung: We examined the feeding habits of the neon flying squid Ommastrephes bartramii from late spring to mid-summer in relation to its northward migration in the transitional waters of the central North Pacific. The winter-spring cohort (ca. 15 to 25 cm in May and 20 to 35 cm in July) and the autumn cohort (ca. 30 to 45 cm in May and 35 to 50 cm in July) were identified by their dorsal mantle lengths. In May and July, the winter-spring cohort was distributed only in the transition zone (TZ) south of the subarctic boundary. This cohort preyed primarily on crustaceans such as euphausiids and amphipods in May, but in July, their primary prey shifted to the sternoptychid fish Maurolicus imperatorius. In May, the larger-sized autumn cohort was also distributed only in the TZ, but in July, these individuals migrated to the transitional domain (TD) north of the subarctic boundary. The main prey of the autumn cohort were micronektonic animals that dominated the TZ in May: the transitional-water myctophid Symbolophorus californiensis, and 2 subtropical myctophids, Ceratoscopelus warmingi and Electrona risso. Secondary important prey items included the transitional-water squid Onychoteuthis borealijaponica and subarctic gonatid squids such as Gonatus berryi and Berryteuthis anonychus. In July, the main prey species in the TD were S. californiensis and O. borealijaponica, both of which also migrated from the TZ into the TD, crossing the subarctic boundary in summer. We estimated the feeding impact of the autumn squid cohort on myctophids in the TD during summer.

Beschreibung: We examined the diel vertical migration of squid (dorsal mantle length 21 to 490 mm) in warm core ring (WCR) and cold water mass (CW) areas in the transition region of the western North Pacific. A total of 28 squid species belonging to 23 genera, 2 octopus species from 2 genera, and 1 vampyrmorph Vampyroteuthis infernalis were captured from depths of 20 to 700 m by day and night sampling using a commercial otter trawl. Three patterns of diel vertical migration were recognized for 11 of the squid species. (1) Migrant, in which day and night habitats are clearly separated with peak abundance deeper than 300 m during the day and shallower than 300 m at night: Gonatopsis borealis, Gonatus berryi, Gonatus onyx, Eucleoteuthis luminosa, Onychoteuthis banksii, Enoploteuthis chunii, and Watasenia scintillans. (2) Semi-migrant, in which part of the population migrates to the upper 100 m at night from its daytime habitat of 500 to 600 m, while the remainder of the population mainly remains in the daytime habitat: O. borealijaponica. (3) Non-migrant, in which the habitat is consistently distributed below 400 m: Histioteuthis dofleini, Belonella borealis, and H. inermis. Horizontally, E. luminosa, E. chunii, and H. inermis were restricted to the WCR, while other species were distributed in both the WCR and CW areas in the transition region. Among the vertically migratory and semi-migratory species, nighttime distribution depth was similar between WCR and CW for O. banksii, but was deepened by upper layers of warm subtropical waters in the WCR for G. borealis, G. berryi, W. scintillans, and O. borealijaponica. We also examined day–night habitat temperature ranges for these 11 species.