Spatio-temporal distribution and biomass of Benthosema pterotum (Pisces: Myctophidae) in the shelf region of the East China Sea

Abstract

We examined the spatio-temporal distribution and biomass of Benthosema pterotum, a dominant mesopelagic-boundary myctophid in the shelf region of the East China Sea, based on data from seasonal bottom-trawl surveys. They are a major prey item for commercially important demer- sal fishes in this area. A total of 980 000 individuals with a wet weight of 554 kg were collected from 694 bottom-trawl net samples. Dense distributions (>10 5 ind. km -2 ) of B. pterotum were observed mainly in the area south of Cheju Island in all seasons, where a persistent cyclonic eddy occurs. The distribution pattern indicated the presence of an independent population completing their life cycle in the area south of Cheju Island. Daytime habitat depth of B. pterotum (mainly 40 to 90 m depth) was a markedly shallower depth than previous reports for adult fishes for the family Myctophidae. The mechanism of population maintenance of this mesopelagic species in such a shallow area was hypothesized to be related to the presence of highly turbid waters in the benthopelagic layer, which produces dim light conditions during the daytime. The biomass of B. pterotum in the area south of Cheju Island was estimated to range from 3900 to 38 400 t.