Geographical And Seasonal Changes Of Prey Species And Prey Consumption In The Western North Pacific Minke Whales

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The forestomach contents of 426 minke whales Balaenoptera acutorostrata sampled in the western North Pacific from May to September during 1994 – 1999 JARPN (the Japanese Whale Research Program under Special Permit in the Western North Pacific) surveys, were analyzed. Sixteen prey species consisting of 1 copepod, 4 euphausiids, 1 squid and 10 fishes, were identified. Minke whales in this region pursue single prey species aggregations. Results showed geographical and seasonal changes of prey species. In the western North Pacific (from the Japanese coast to 170oE), Japanese anchovy was the most important prey species in May and June, while Pacific saury was the most important one in July and August. Pacific saury and krill were the important prey species in September. In the southern Okhotsk Sea, krill was the most important prey species in July and August. Estimates of the daily prey consumption rate for all prey species combined were 2.6 – 5.7 % (2.6 – 3.9 % (S = 0.69), 3.9 – 5.7 % (S = 0.82)) and 1.8 – 5.2 % of body weight by two independent methods, respectively. The total prey consumption by minke whales in the western North Pacific in August and September was estimated at 85 – 122 thousand tons (S = 0.69, 0.82) and 88 thousand tons by the two methods, respectively. There included 45 – 62 thousand tons (S = 0.69, 0.82) and 28 thousand tons of Pacific saury, respectively. Total consumption of krill in southern Okhotsk Sea in August and September was 15 – 21 thousand tons (S = 0.69, 0.82) and 27 thousand tons by the two methods, respectively.