Feeding habits of albacore Thunnus alalunga in the transition region of the central North Pacific

ABSTRACT:   The feeding habits of albacore Thunnus alalunga (fork length: 48.9–76.2 cm, n  = 132) were examined from late spring to early autumn in relation to its northward migration in the transition region between the subtropical and subarctic fronts in the central North Pacific. Samples were collected at night using surface gill nets or during daytime pole-and-line surveys in 2001 and 2002. During May and June, albacore fed mainly on Japanese anchovy Engraulis japonicus , which accounted for 27.2%, 67.0%, and 45.5% of the total stomach contents by number ( Cn ), wet weight ( WW ), and frequency of occurrence ( F ), respectively, and secondarily on the subarctic gonatid squid Gonatopsis borealis ( Cn , 15.8%; WW , 10.8%; F , 28.8%). From July to September, albacore continued to depend on Japanese anchovy ( Cn , 48.2–52.8%; WW , 79.9–95.2%; F , 27.8–85.4%). These results corresponded well with the remarkable rebound of the Japanese anchovy stock since the 1990s. Gonatopsis borealis , the main squid prey from May to June, almost disappeared from the stomachs of albacore from July to September, probably due to the northward migration of this squid to subarctic waters in summer. The feeding impact of albacore on the Japanese anchovy stock in the transition region was conservatively estimated to be from 1400 to 2100 tons per day from late spring to early autumn.     

Feeding habits of Pacific pomfret Brama japonica inthe transition zone of the central North Pacific

ABSTRACT:   We examined the diets of three size groups of the Pacific pomfret Bramajaponica collected in April and May 2000 in the transition zoneof the central North Pacific. Amphipods, cephalopods and fish werethree of the most common prey groups of pomfret. Of the three size groups,small pomfret (11.0–19.9 cm standard length (SL))preyed mainly on amphipods and occasionally on cephalopods (mainlytransitional species Onychoteuthis borealijaponica ) and fish.(These prey amphipods and fish could not be identified to lowertaxonomic levels because they were too digested.) Medium and largepomfret (20.0–30.9 and 31.0–40.9 cm SL,respectively) preyed mainly on cephalopods (mainly O. borealijaponica ,but also on two subarctic species, namely Berryteuthis anonychus and Gonatopsisborealis ), suggesting that these cephalopod species ensure energyfor the northward migration of these two size groups in the transitionzone in the spring. The next most common prey for medium pomfretwas amphipods (mainly subtropical species Phronima sedentaria and Platyscelusovoides ), followed by fish (mainly Bathylagus sp.). However,in the case of large pomfret, fish (mainly Paralepididae sp. 'A')were the next most common prey, followed by amphipods (mainly P.sedentaria and P. ovoides ), indicating that the importanceof fish as prey increases with growth for medium and large pomfret.