| Summary: | 碩士 === 國立海洋大學 === 環境生物與漁業科學學系 === 91 === Abstract
Cutlassfish is one of the most important target species of Taiwan’s offshore/coastal fisheries and particularly important for sport angling, a major recreational fishery in Northeastern Taiwan. However, recreational fishery has never been brought into national fishery statistic figure and as a result the stock status of cutlassfish off the coastal waters in Northeastern Taiwan remains unclear. In this study, we conduct a three-year logbooks collection on recreational fishery and biological sampling in order to realize species composition, fishing season, geographical distribution, length-frequency fluctuation, spawning season, catch and abundance of cutlassfish.With regard to abundance estimation, we use Hydroacoustic method to observe schooling behaviour and to carry out in situ behaviour of cutlassfish as well as its packing density and abundance while under fishing light. The results are summarized as follow:
(1)Summers are major season for sport angling activities with a significant peak during June-September. Winters are relatively poor season because of bad weather during winter monsoon. However, Seasonal variation of CPUE is not significantly low down during winter season. This may reveal that cutlassfishes seems to stay all year round in the coastal waters in Northeastern Taiwan.
(2)About 80% of fishing sites located at regions from Bi-tou to Ye-liu where depth between 50-100m, and at the vicinity of Gue-San Islet where depth deeper than 200m. The two regions are independent temporarily with the former in summer and the later in winter.
(3)Species identification of the specimens collected indicate that all catch are Trichiurus lepturus. Female dominates the inshore regions while male dominates offshore regions. According to GSI of specimens, the spawning season are estimated around May-June. The juvenile recruit to the stock around June of next year as indicated by length-frequency distribution data.
(4)Target Strengths obtained by echosounder provide good estimates on length composition of cutlassfish under fishing light. The estimated packing densities along the drifting paths vary greatly which represent the schooling behaviour of cutlassfish are very unstable. Therefore, the “Instantaneous density” or “peviodic density” fluctate greatly as well. However, the average densities estimated for whole cruises have significant positive correlation with hook rates. ( Y=0.2364X+0.3091, r=0.58, p<0.01 ), which indicate that the average density under drifting path estimated by acoustic method is a good indicator of abundance.
(6) The schooling densities under fishing light estimated by acoustic method fluctuat within 0.01 - 1.53 (fish/m3) with average of 0.60 (fish/m3). If eliminate the influence from aggregating effect of fishing light, the school density without luring effect from fishing light are around 0.001 - 0.198 (fish/m3), with an average of 0.077 (fish/m3).
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