Study on acoustic estimation of myctophid fishes biomass

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1.a soft-X ray system was used to observe swimbladders of 14 species of myctophids. The shape and size of the swimbladder varied with developmental stage. In most species the sswimbladders varied with developmental stage. In most species the swimbladders did not grow in proportion with body length, and in a few species the swimbladders began to degenerate at a certain life stage.2.the body density ration (g) ranged from 1.003 to 1.020 for 5 species examined except Myctophum asperum. Especially, two species of Stenobranchius had a low density ratio (1.004) similar to sea water density. The sound speed ratio (h) of fish body to sea water changed depending on temperature. Therefore, considering temperature in habitat of fish, the sound speed rations were estimated 1.032-1.039 for Stenobranchius leucopsarus, 1.024-1.036 for Notoscopelus japonicus, and 1.012-1.024 for Myctophum asperum.3.based on the morphological measurement and values of g and h obtained above, the theoretical target strength (TS) was calculated using PSM and DCM scattering models. For fish with swimbladder, though there was a 2-5 dB difference in TS between the two models, measured TS in a tank was in accord with PSM. On the other hand, TS patterns of swimbladder-less fish calculated by PSM and DCM were similar, and accord with measured TS patterns in the broadside region.4.TS was less dependent on the tilt angle at frequencies of 38-200kHz, because the swimbladders of myctophids were spherical and very small. Normalized TS (TScm) was consistently below -70dB in this frequency range. TS of myctophids was not in proportion to square of body length because of irregular growth of swimbladders.5,8 million tons with density of 50-100gm^<-2>. Similarly, biomass of Diaphus theta was estimated 75 thousand tons with mean density of 35gm^<-2>.