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Study on target strength of tuna species aggregated by fish aggregating device
|Contributors: ||NTOU:Department of Environmental Biology and Fisheries Science|
Fidh Aggregating Devices;Tuna Purse Seine Fishery;Target Strength;Yellowfin tuna;Skipjack tuna
|Issue Date: ||2011-10-20T08:24:54Z
|Abstract: ||近十年來鰹鮪圍網大量使用人工集魚器（FAD）作業，已造成大目鮪及黃鰭鮪幼魚的嚴重混獲，因此FAD的禁用被熱烈討論及付諸實施。若下網作業前先以魚探加以判別，以減少混獲幼魚比率高的網次發生，必然有助於鮪資源的永續利用與管理。有鑑於此，本研究以小琉球外海錨碇式FAD聚集之魚類(黃鰭鮪、正鰹、雙帶鰺、圓花鰹)共計31尾，體長為28~55cm間之幼魚，於無反響水槽中以200及50 kHz魚探機測定懸垂魚體不同角度之標物反射強度(TS)，並計測於水槽自然游泳狀態之TS(黃鰭鮪6尾，正鰹1尾)，分析不同魚種之TS特性，提供避捕FAD附近聚集鮪類幼魚之參考，成果摘述如下：
28~50cm之鮪類幼魚，其TS與尾叉長較無相關，但尾叉長35 cm以上之黃鰭鮪開始具有膨發之泳鰾構造，大於此體長之黃鰭鮪不論背部或體側，於50 kHz魚探下所測得之TS較無泳鰾之正鰹大(約2 dB)。
本研究數據顯示：在50 kHz下，TS在-34~-38dB為黃鰭鮪幼魚及正鰹成魚混淆的區間，下網與否必須審慎判斷；當TS小於-38 dB時，黃鰭鮪及正鰹均為幼魚，此時建議停止下網捕撈作業。
200 kHz與50 kHz之TS差值(△TS)有助於黃鰭鮪幼魚之區別，全方位之平均△TS約1.36 dB，且黃鰭鮪隨運動方向改變之△TS變異較大於正鰹，其中正背部探測黃鰭鮪(算術平均數X ̅=-1.66, SD=1.54)與正鰹(算術平均數X ̅=-0.13, SD=1.50)之△TS達1.53 dB，為最易鑑別之角度。
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.
|Appears in Collections:||[環境生物與漁業科學學系] 演講及研討會|
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