| Summary: | 碩士 === 國立臺灣海洋大學 === 環境生物與漁業科學學系 === 97 === Recent decades, bycatch of small yellowfin and bigeye tunas caused by using fish aggregation device (FAD) during tuna purse seine operation had become a controversial issue for Regional Tuna Management Organizations. Evaluation prior to net setting by echosounder to avoid catching schools with high aggregation of small tunas is believed to be helpful for both tuna sustainable utilization and management. In this study, target strength (TS) was measured from 31 dead fishes sampled from moored FAD off Shiao-Liu-Chiu islet, including small yellowfin tuna (Thunnus albacore), skipjack (Katsuwonus pelamis), rainbow runner (Elagatis bipinnulate), bullet tuna (Auxis rochei rochei) with body length of 28~55 cm. There were 6 live small yellowfin tunas and 1 skipjack tuna for TS measurement in tank. With these TSs, we analyzed the characteristics among fishes and figure out measures on reducing bycatch of small yellowfin tuna. Results are summarized as the followings:
(1) The TS of small tunas with length around 28~55 cm is not significantly related to fork length. While, dorsal and lateral TS of yellowfin larger than 35 cm are higher than those of skipjack, because swimbladder in yellowfin tuna starts to grow since 35 cm fork length but skipjack has no swimbladder at all.
(2) Fishing (operation) should be carefully evaluated if TS are at the ranges of -34~38 dB where small yellowfin tunas and matured skipjack tuna co-existed by using 50 kHz echo sounder. Fishing action should be forbidden if TS are under -38 dB since both yellowfin and skipjack tunas are at young stage (juvenile).
(3) Variations of TS measurement between 200 KHz and 50 KHz echo sounder, i.e. △TS, are helpful to distinguish small yellowfin tunas. Mean △TS for all directions is 1.36 dB and the variance of △TS for yellowfin tunas caused by swimming orientation are larger than skipjack tunas. The mean △TS from dorsal aspect of yellowfin and skipjack tuna reached 1.53 dB is the most differentiable among others.
The samples of small yellowfin tuna obtained in this study are at very young stage (juvenile). As the size of swimbladders of yellowfin and bigeye tunas increase dramatically while growing, TS and △TS varied with diving and rising behavior would be much more significantly rom skipjack without swimbladder. Due to the measurements of TS in this study under contained environment and be different from natural environment, further practice and adjustment are necessary in future application.
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