| Summary: | 碩士 === 國立臺灣海洋大學 === 環境生物與漁業科學學系 === 101 === Shih-men reservoir is an important source of drinking water for the northern Taiwan. To keep water clear and to increase fishery resources, the authority restocked fishes into the reservoir annually and the bighead carp (Aristichthys nobilis) is one of the main species. However, there is relatively less information about the fish abundance and distribution in the reservoir. Well understanding the standing crop of bighead carp is very important for water purification, fishery economics and ecological influence. In this study, Simrad EY60 scientific echosounder system was used to measure the target strength of bighead carp in tank. Six acoustic surveys were conducted in the Shih-men reservoir during 2010~2012 to estimate the distribution and abundance of the bighead carp. Results are summarized as below:
(1) No matter in rotations motions of pitching, rolling or yawing, the variation of target strengths was about 25 dB. The occurrence of the highest and lowest TS value were caused by both fish shape and orientation of swim bladder. Mean TS of pitching angle around -15º~+15º was chosen to build the TS-TL regression: TS=24.18 log(TL) – 77.11 (r=0.95), which were further used to transform TS of bighead carp to TL in acoustic surveys.
(2)The frequency distribution of TS presented normal distribution. However, after the TS was transformed into TL, it presented in steep allometric pattern. The mean estimated number of big fish (>60cm) accounts for 12.38%, and only 3.8% in May.
(3)Regarding horizontal distribution of fishes, most of them were located in near shore and rare were in offshore. The major habitat were between Da-Wan-Ping and Sing-An-Ping, followed by Ba-Ding and Chang-Xing. In terms of vertical distribution, fishes gathered near bottom and shallow areas with depth less than 20 meters. Generally, big fishes gathered in upstream, except for in dry season (e.g. May).
(4)The estimated fish abundance varied with seasons and showed a decreasing trend from spring to winter. Estimated average abundance (ind./m^3) for each survey were 0.00371(Aug.), 0.00207(Nov.), 0.00866(May), 0.00248(Aug.), 0.00159(Dec.) and 0.00131(Dec.)and the estimated total number in thousand (±95%) of big fishes (>60 cm) are 7.2~110.3, 37.0~118.5, 116.2~280.3, 49.9~106.6, 48.9~81.8 and 42.6~108.2, respectively. The variations of fish abundance were corresponding to the volume effect caused by capacity differences between dry and monsoon seasons. Substantial increase in dry season was observed, but the results should be overestimated.
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