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.