Life history and vertical distribution of the mesopelagic fish Cyclothone alba (family Gonostomatidae) in Sagami Bay, Central Japan

Abstract

Life history and vertical distribution of the mesopelagic fish Cyclothone alba (family Gonostomatidae) are described on the basis of over 4000 specimens taken during a series of 15 cruises from December 1982 to December 1984 at a station near the center of Sagami Bay, Central Japan. C. alba does not undertake diel vertical migrations, being concentrated in the mesopelagic zone between 300 and 500 m, with peak abundance at 350 m both day and night. Spawning occurs mainly during the late spring and summer months in Sagami Bay. C. alba is semelparous, releasing about 200–650 eggs at the end of its life. Duration of the egg and larval stages is estimated to be about 2–3 months. Many males and some females mature at 1 year, and all individuals mature by 2 years of age. Sexual dimorphism in smaller males and larger females results from an earlier decline of growth rate in males: on the average, males reach 17.5 mm SL (standard length) in 1 year and 21 mm SL in 2 years, whereas females reach 19 mm SL in 1 year and 26 mm SL in 2 years. It is suggested that such precocious maturation, together with its small larvalized form, is attained through progenesis.