Intraspecific variation in the swimbladder occurrence and possibility of acoustic discrimination of the dominant mesopelagic fish Diaphus garmaniioff western Kyushu, Japan

Abstract

Diaphus garmanii (Family, Mictophidae) is the most abundant mesopelagic fish in the continental slope off western Kyushu. For the application of acoustic monitoring, we have observed the swimbladder morphology of this species to estimate the target strength (TS). In this session, we show the intraspecific variation in the occurrences and shapes of swimbladder in D. garmanii, and propose the acoustic discrimination of the swimbladder morphology for accuracy improvement in biomass estimates. Acoustic data and biological samples were collected in the summer of 2012 to 2014. Both of fishes with (30% in all samples) and without (70%) swimbladder were observed by the soft X-ray method, and frequencies of the swimbladder occurrence were significantly different between male and female fishes. Theoretical scattering models showed that the TS difference between 38 kHz and 70 kHz was the most preferable parameter to discriminate the swimbladder occurrence of D. garmaniiin field acoustic data. Field acoustic data analyses implied different patterns of vertical distribution between swimbladder and swimbladderless (male and female) fishes.