A study on the target strength of living juvenile yellowfin tuna (Thunnus albacores)
| Autor: | Hsing-Han Huang, 黃星翰 |
|---|---|
| Rok vydání: | 2011 |
| Druh dokumentu: | 學位論文 ; thesis |
| Popis: | 99 To provide target strength (TS) information for estimating the body length of yellowfin tuna (Thunnus albacores, YFT) and its abundance around fish aggregating devices, ex situ TS and in situ TS were measured for reducing the by-catch of juvenile YFT. In ex situ TS measurement, two cameras synchronized with a 200 kHz echosounder were used to obtain precise TS, orientation and fork length (FL; cm) of YFT under free swimming conditions. The orientation and TS of YFT were combined for finding out how the orientation affected to the TS. On the other hand, the swim bladder volume (VSB; ml) of YFT were measured to establish the relationship between the VSB and FL. The X-ray images were taken for examining the position of swim bladder in YFT’s body. In in situ TS measurement, one hour of stationary FAD survey was conducted for analyzing the fish around FAD. After TS measurement, the troll line was used to biological sample the fish around FAD for examining the species of aggregated fish around FAD. From ex situ and in situ TS measurement, the sound scattering characteristic of living juvenile YFT were summarized as the followings: 1. The TS of juvenile yellowfin tunas with length around 25 ~ 69 cm were positive significantly related to logarithmic fork length when the 200 kHz frequency transducer was used. Furthermore, the TS of yellowfin tuna with fork length in 69 cm was 13 dB more than in 25 cm. 2. The TS of yellowfin tuna changed significantly with the orientation of yellowfin tuna, and the larger TS always expressed with the negative orientation. From the interception of TS-Log(FL) regression for the grouped orientation analysis, the largest TS of yellowfin tuna was found in the group of [−15° to −20°]. 3. It’s a significant trend that the swim bladder of yellowfin tuna grew synchronized with the fork length. The development of swim bladder volume in the stage of juvenile yellowfin tuna was faster than in the stage of adult. The largest TS expressed with the orientation around -15° to -20° because the interior angle between the swim bladder and vertebra was 25° approximately in the yellowfin tuna’s body. 4. We segregated single target echoes collected in the FAD waters with condition of increasing TS while conducting downward swimming. The echoes were transformed into FL by the TS-FL equation established from the tank experiment. The mean FL (47.53 cm) was very close to those measured from yellowfin tuna sampled by troll lines (39 ~ 46 cm). The segregating method developed in this study may be useful for discrimination of young yellowfin echoes. |
| Databáze: | Networked Digital Library of Theses & Dissertations |
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