Characteristics of Fish Schools Aggregating Around a Surface Fish Aggregating Device Estimated by Echo Sounder

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1 J. Fish. Soc. Taiwan, 27(3): (2000) Characteristics of Fish Schools Aggregating Around a Surface Fish Aggregating Device Estimated by Echo Sounder Long-Jing WU, Jiun-Chern Lin, Cheng-Fang Lee and Wei-Cheng SU* (Received, June 10, 2000; Accepted, July 20, 2000) ABSTRACT An experimental surface fish aggregating device (FAD) was deployed in the coastal waters off southern Liuchiu Island, Taiwan. The acoustic survey and fishing experiments were carried out to investigate fish aggregation patterns around the surface FAD. The acoustic signals of six main species were obtained in this experiment. The results of echo integration, signal feature analysis and cluster analysis showed that butterfly fish (Chaetodon speculum) and Unicorn leatherjacket (Alutera monoceros) aggregated in the nearby area of the surface FAD and had no obvious diurnal change. Yellow-finned runner (Elagatis b 伊 innulatus)distributed themselves with a reach of 150 m from the surface FAD, at depths of m in daytime, and almost disappeared in nighttime. Skipjack tuna (Katsuwonus pelamis) and yellowfin tuna (Thunnus albacares) aggregated with a reach of 800 m from the surface FAD, and in the surface area of 50 m depth in daytime. During nighttime, Skipjack and yellowfin tunas seemed disappear from the FAD zone. Moonfish (Lampris regius) schools had an extensive range from the vicinity of the FAD. In the nearby area of the FAD, moonfish schools aggregated with a reach of 550 m from the FAD at depths of m, and disappeared in nighttime. The fish aggregation effect around the FAD was quite significant. And the results show that the application of acoustic methods used to investigate the distribution patterns and biomass of fish aggregating in the FAD zone should be feasible Key words: Fish aggrθ gating device (FAO), Acoustic methods, Fish aggregation effect INTRODUCTION The installation of FADs aims to attract more pelagic fish to aggregate and stay, for the purpose of successful resource conservation. Although the reasons that fish aggregate around FADs differ from one species to another, their surprising effect on fish aggregation is undeniable (Inoue, 1978 ;1985). In Indonesia, F ADs had rendered a positive effect on the skipjack and yellowfin tuna fisheries (Monintja 1989; Monintja et al., 1991), as well as on additional successful case revealed in the coastal waters of India (Ahmed 1996) In the early 1970s, the FAD technique was fi rst introduced to the U.S. with experimental areas covering the waters around Florida, South Carolina, the Gulf Coast, and the Virgin Islands. The installation of FADs in Hawaiian waters started later in the 1980s (Mya 泣, 1985). Studies on economic considerations in configuring fish aggregating device networks (Samples and Hollyer, 1989) and FAD enhancement of offshore fisheries (Buckley et al., 1989) had been achieved in In Taiwan, experiments on the use of FADs have been ongoing since 1981 (Lin and Su, 1992; Lin, 1994; 1995), with initial focus on FAD's effect on fish aggregation (Chen, 1989; Lin and Su, Taiwan Fisheries Research Institute Kaohsiung Branch *Corresponding author.

2 160 Long-Jing Wu, Jiun-Chern Lin, Cheng-Fang Lee and Wei-Cheng Su 1995) but later on designing durable FADs which will subsequently be use to establish the most suitable management model (Su and Lin, 1996). Aggregation behavior of fish associated with FADs has been extensively studied elsewhere, e.g., the possible effect of FADs on fish distribution (Ishida, 1981; Nonawa, 1981; Sawada, 1981, and Myatt, 1985) and the short-time behavior of fish aggregating around FADs (Brill et al., 1984; Hollandθ t al., 1990). In Taiwan, although FADs had been deployed since 1981, but very little basic research on the effect of fish aggregation, suitable installation locations, and the possible impact on surrounding fish populations have been conducted This study aims to investigate the effects of surface FADs and features of fish aggregation with the aid of acoustic technique. This in turn will help build up a basic database for biomass estimation in the ' E 20' FADs zone. MATERIALS AND METHODS An experimental surface FAD was deployed in the waters 2.7 nautical miles off the southern coast of Liuchiu Island, off southwestern Taiwan, in 1995 (Fig. 1). The fishing experiments and acoustic surveys around the FAD region were conducted from September 1996 to February (J) Fishing Experiment Fishing experiments were carried out around the surface FAD by research vessels, Haifu and Haichane, with hand lines and trolling lines to investigate the fish species aggregating during the acoustic survey. Additional catch data were collected from commercial fishing boats to analyze the species composition. 25' 120' 27' E 20 Liuchiu Island B C D E N E 二 - 二 =- O 2km Fig. 1. Arrangement of two acoustic surveying methods around the surface FAD. (1) The 5 parallel transects (dotted line A to E) were arranged regularly every 1 nautical mile, and (2) 4 cross transects (solid lines) were arranged around the surface FAD. The research vessel sailed along these two types of transects respectively. 口. location of the surface FAD deployed.

3 Characteristics of fish schools around a FAD 161 (11) Acoustic Survey The acoustic survey was carried out along a network of 5 parallel transects (Iine A to line E), spaced regularly every 1 nautical mile (Fig. 1). This type of acoustic transects was used to investigate the distribution patterns of fish schools in a wider range. The additional transects of 4 cross lines were arranged to investigate fish aggregation patterns in the range of 2000 m from the surface FAD. A JVC echo sounder and a SIMRAD EY-500 scientific echo sounder (70-kHz, 11 0 beam width) Iinked with GPS were employed to analyze the distribution of fish schools and their quantities.. Standard echo-integration theory (Forbes and Nakken, 1972) was used to convert measurements of scattering strength to estimates of fish density The target strengths needed in this conversion were estimated from in-situ exper 卜 ments by using SIMRAD EY-500. (111) Species Identification Fish species were identified by the fishing experiments during acoustic surveys But for the widely distributed moonfish schools, each of the 5 parallel transects was divided into ten sectors for integrating the acoustic signals. And the signal feature extraction techniques (Johnson and Wichern, 1988; Richards 剖訓, 1991; Lee θt al., 1993) were used to select the seven feature factors as variables from the 50 sectors. The seven factors include scattering strength value (SV), the standard deviation of SV, mean target strength (TS) value, the standard deviation of the TS value, the average peak number of school signals, the average band width of school signals, and the aggregating depth of fish schools. Then, the matrix composed of the 50 x 7 elements was processed by the statistical method of c1uster analysis to select the sub-areas with the same acoustic signal features (Rose and Leggett, 1988) that might be recognized as the same specles. RESUL TS AND DISCUSSION (1) Fishing Results The surface FAD zone had become the main fishing ground of trolling-lines and hand-lines fishing rafts of Liuchiu Island. The operating area was usually very near the surface FAD, and the farthest operating area was about mile away from the surface FAD. The main species caught by fishing rafts around the surface FAD included 10 species, and the percentage composition was shown in Fig. 2. Among them, five species, Skipjack tuna (Katsuwonus pelamis), yellowfin tuna (Thunnus albacares), dolphin fish (Corypha θna hippurus), yellow 寸 inned runner (Elagatis b 似的 nulatus) and moonfish (Lampris regius), comprised more than 85% of totallandings by weight. The species caught by the research vessel during acoustic surveys was shown in Table 1, and compared to the 5 species above, the acoustic signals of four important species had been derived but dolphin fish in this experiment According to the commercial fishing results, the aggregating species varied from one season to another in the surface FAD zone; for examp 悟, skipjack and yellowfin tunas dominated from July to February, while other species did so in other periods (Fig. 2). This phenomenon of pelagic fish has commonly occurs in various regions of the world ocean and resembles that repeatedly documented in the waters around Japan (Kawai, 1991). But, the mechanisms of species alternations among them are still not understood. (11) Monthly Changes in Aggregation Quantity of Fish Schools The monthly change of echo integration results within 2000 m from the surface FAD was shown in Fig. 3. The average scattering strength value (SV) was higher from September to December in 1996 and When comparing to test fishery catches (Table 1) and commercial catches (Fig. 2), the higher biomass indicator (i.e. SV) may be mainly caused by the signals of skipjack and yellowfin tunas. However, the peaks of SV in March 1997 and February 1998 should be caused by the signals

4 162 Long-Jing Wu, Jiun-Chern Lin, Cheng-Fang Lee and Wei-Cheng Su Moonfish (Feb. to Mar.) 37.4% Yellow-finned runner (Jan. to Sept.) Buttert1y fish (July to Sept.) Unicom leatherjacket (J an. 的 Feb.) Pacific sailfish (Feb. to Mar.) Layang scad (Jan.ωMay) Dolphin fish (Annual) 26.5% Fig. 2. Weight percentages and fishing periods of rnain species caught within 1. 5 rniles frorn the FAD by 4 sarnple fishing rafts in The total catches were about 7540 kg -65 旬國 70 ω -- 2 (u >..c... 0) 5 圓 吋圖, ω... 0) C ω 自 8 回 80 cf) 且 85 ~~~~~~~~~~~~~~~~~~ Month Fig. 3. Monthly changes in biornass of fish aggregating around the surface FAD by echo integration method between September 1996 and Feburary 1998.

5 Characteristics of fish schools around a FAD 163 Table 1. 8pecies, catch in number, average fork length and weight of fish caugth during acoustic surcey. Species Date Catch Fork length (cm) Weight Butterfly fish (Chaetodon speculum) Aug Yellow-finned runner (Elagatis bipinnulatus) Jun (kg) Feb Unicorn leatherjacket (Alutera monoceros) Jan Yellowfin tuna (Thunnus albacares) Dec Sept Nov Skipjack tuna (Katsuwonus pelamis) Dec Moonfish (Lampris regius) Sept Nov.1999.' Mar of other species. Ingeneral, the change of biomass indicator showed that the nearby area of the surface usually could attract fish schools to wander around. (1 11) Aggregating Characteristics (1) Fish Aggregating Effect of the Surface FAD Results from 15 acoustic surveys reveal that the area with the highest occurrence rate of fish schools (>80%) was located in the immediate nearby area of the F AD (Fig. 4). The incidence rate decreased gradually toward its lowest value further from the FAD in about 3000 m. From the stated results, the aggregating effect of the FAD was obvious. (2) Distribution of butterfly fish, yellowfinned runner and unicorn leatherjacket The catch species and integrated values of 8 acoustic surveys in the daytime were listed in Table 2. These average scattering strength values (8V) were integrated from acoustic signals in the range of 2000 m from the FAD. The results showed that butterfly fish, yellow-finned runner and unicorn leatherjacket were found to wander around the surface FAD. Butterfly fish and unicorn leatherjacket gathered in the range of 50 m from the surface FAD at depths of 4-16 m (Tabl~ 2). Yellow- finned runner gathered in the range of 150 m from the surface FAD at depths of about m. During the night (Table 3), the unicorn leatherjacket and butterfly fish still rested below the surface FAD, and showed no significant diurnal changes, but yellowfinned runner almost disappeared from the surface FAD zone. (3) Distribution of ski 叫 ack and yellowfin tunas In December 1996, 8eptember 1997 and November 1997, the catch species included skipjack and yellowfin tunas during the acoustic surveys. Because their body size is similar (Table 1), and the measured results of target strength (Fig. 5) showed that it's hard to distinguish these two species by the acoustic signals. The report of Ishida (1981) also pointed out skipjack and yellowfin tunas wandered in the same area around a FAD. 80 these two species were considered as a mixed group in the study. The results of 50m-interval integration (Table 2) showed most of the fish schools of skipjack and yellowfin tunas distributed around the surface FAD. Then, these 8V were converted to the numbers of fish by the sonar equation (Forbes and Nakken, 1972) with the average target

6 164 Long-Jing Wu, Jiun-Chern Lin, Cheng-Fang Lee and Wei-Cheng Su 120' 15' E 20' 25' ' E N行 Liuchiu Island E 二 - 二. o 1 2km ' N 亡 =:J <20% 1'.>1 <40% 國體 <60% _ <80% _ >80% Fig. 4. Occurrence frequency of fish appearance around the surface FAD by acoustic survey, with estimation on a yearly basis. 0: location of the deployed surface FAD strength of db and db (Fig. 5) for the data of September 1997 and November 1997 respectively (Table 4). The results showed that more than 80% of fish schools of skipjack and yellowfin tunas gathered in the range of 400 m away from the surface FAD at depths of 8-46 m, and the farthest distance from the surface F AD was about 800 m. The surface F AD showed very obviously aggregating effect for these two species. But during the night, most of the fish schools of skipjack and yellowfin tunas seemed to disappear from the acoustic surveying area (Table 3) Although the biomass of skipjack and yellowfin tunas around the F AD decreasing during the night, fishermen still caught skipjack and yellowfin tunas by trolling lines and surface drift nets around the surface FAD at night. This indicates that some fish moved close to the surface water or away beyond the survey region during the night and were unable to be located by the echo sounder in this study. Such phenomena of fish movement associated with FADs were also found in Hawaiian waters (Holland et al., 1990). This report pointed out that the juvenile yellowfin tuna trended to remain tightly associated with the FAD during the day, moving to the surface water and away at night. And it is thought that feeding actions maybe cause such phenomena. (4) Distribution of moonfish The distribution of moonfish within the range of 2000 m from surface F AD was wider than the other six main species (Table 2). In the nearby area of the surface FAD, moonfish schools seemed to aggregate within the range of 550 m at depths of m, and moonfish were caught by test fishery catch in this area.

7 Characteristics of fish schools around a FAD 165 Table m-interval echo integration and the depth for different catch species and the date of acoustic surveys in the range of 2000 m from FAD during daytime Catch species Date Range \ 0-50 m m m m m m m m m m m m m Butterfly Yellow-finned fish runner le~~~~j~~~et Skipjack and Yellowfin tunas Moonfish Aug. Jun. Feb. Dec. Sept. Nov. Mar. Jan Sv (db) Sv (db) Sv (db) Sv (db) Sv (db) Sv (db) Sv (db) Sv (db) (Depth) (Depth) (Depth) (Depth) (Depth) (Depth) (Depth) (Depth) 也 (4-16 m) (35-48m) (31-45 m) (3-24 m) (8-24 m) (50-60 m) (32-47m) (31-45 m) (10-42m) (7-24 m) (8-3 6 m) (55-63 m) (32-35m) (10-47m) (10-44m) (10-39m) (14-40m) (12-43m) ( (10-46m) -~ 一品占丑 / 過 (8-42 m) (10-38 m) (57-68 m) (10-30 m) (15-42 m) (53-78 m) (10-40 m) (58 千 8m) (10-33 m) (56-76 m) (52-77 m) (10-26 m) m m m m m i 己 J _4 電量皆可 2. " ~~ι (21-37m) (16-23 m) m m (51-73 m) (53-72 m) m m m (53-76 m)

8 F 166 Long-Jing Wu, Jiun-Chern Lin, Cheng 干 ang Lee and Wei-Cheng Su Table m-interval echo integration for the different catch species and the date of acoustic surveys in the range of 2000 m from FAD during night. Butterfly Yellow-finned Unicorn Catch species fish runner Bathe lacket Skipjack and Yel owfin tunas Moonfish Date Aug. Jun. Feb Dec Sept Nov Mar. Jan Range\Sv (db) Sv (db) Sv (db) Sv (db) Sv (db) Sv (db) Sv (db) Sv (db) 0-50 rn m m m m 250 > 300m m m m m m m m Table 4. Biomass estimation of 8kipjack and Yellowfin tunas round the FAD. The density value was calculated by sonar equation (8v= T8+1010g (density)), and the average T8 values were db and db for the data of 8ept and Nov respectively. Distance Average Density Surveying Estimated ' 司司 l Date from FAD Sv vojume biomass ~ ~ 一 (db) 一 (fish/m 3 ) 盯 1 3 (Individuls) Sept m 巨 一 - 一一 - 一 - 一一一一一一 - 一 一 -- 一 一 一一 -- 一 一 一一 Nov m E Nov m E 司 Nov m E Nov m E Nov m E Nov m E Nov m E Nov m E Nov m E

9 Characteristics of fish schools around a FAD 167 Sept Average TS = (db) 10 Target strength (db) 自 ε20. 3 司 ;25...c 法 30 日 Nov.1997 Average TS = (db) ε20. 立志 30 o Target strength (db) s..i. --. ti Fig. 5. The distributions of target strength measured by EY-500 in Sept and Nov But the highest 8V value appeared in the range of m. It means that more abundance of moonfish wandered in the farther area. Furthermore, the results of large-scale surveys showed many fish schools aggregated on the survey line B (Fig. 6) with an average scattering strength value of db at depths of m. In order to realize whether the fish schools were the same species or not. The 5 parallel transects were divided into 50 sectors. And c1uster analysis was applied to analyze the 50 survey sectors. The result showed that 50 acoustic survey sectors could be grouped into four c1usters (Table 5). The representation of these data on the first 2 main component is given in Fig. 7. The first c1uster included the result of acoustic surveys from 37 subdivided sectors (Table 5), having low value of average scattering strength, with no fish school signals. It was confirmed that there was no trace of aggregated fish schools indicated in the first c1uster. The second c1uster included information from seven subdivided sectors (Fig. 8) where moonfish were caught nearby the surface FAD. Therefore, signals of the second c1uster should belong to moonfish. The FAD seemed to have a good positive effect on aggregating, moonfish schools. In addition, the species of the third and fourth c1usters are unknown, because we caught no samples in these sea areas. 80 these subdivided sectors were ignored in the c1assification of moonfish. During the night, the average scattering strength value was lower at d 日 (Fig. 6) and was associ-

10 168 Long-Jing Wu, Jiun-Chern Lin, Cheng-Fang Lee and We 卜 Cheng Su ' E 20' 25' ' E N Liuchiu Island A C E G 20' O 2km B D F H J -1 15' ' E 20' 25' ' E N Liuchiu Island A C 20' 亡 - - B D o 1 2km F H J 15' <-80 db. -80~-77dB. -77~-74dB >-74dB Fig. 6. The distribution of moonfish schools in the acoustic survey area in daytime (upper) and nighttime (Iower), respectively in March The circles denote the mean value of scattering strength (Sv) in db calculated by echo integration method. 口 : location of the surface FAD.

11 Characteristics of fish schools around a FAD 169 Table 5. Cluster analysis classification by average 8v, the standard deviation of 8v value, average T8 value, the standard deviation of T8 value, the average peak number of school signals, and the average band width of school signals as well as the resting depth of fish schools of 50 sub-areas. Cluster Frequency percent IV Total Transect transect A transect B transect C transect D transect E Total % 4% 14% 20% 18% 74% O 5 2 O O 7 0% 10% 4% 0% 0% 14% 3 O O O 4 2% 6% 0% 0% 0% 8% O O O 2 0% 0% 2% 0% 2% 4% % 20% 20% 20% 20% 100% NZ < O -3 CAN 1 Fig. 7. Distribution diagram of 50 sub-areas classified by cluster analysis. CAN1 and CAN2 denote the first and secondary principal axes, respectively. The symbols in the diagram represent the locations of sub-areas.

12 170 Long-Jing Wu, Jiun-Chern Lin, Cheng-Fang Lee and Wei-Cheng Su ' E 20' 25' ' E N乃 A B 20' C D E ' l E 三 - 二 =- o 1 2km N Fig. 8. Distribution maps of cluster 11 in figure 7. In the test fishery catches, moonfish was caught in the nearby area of FAD..: location of the surface FAD. ated with the rapid decrease in biomass (T able 3) in the range of 2000 m from the surface FAD. Moonfish schools maybe scattered into the waters far beyond the current survey area at night. Features extracted from acoustic signals of fish schools enabled classification by species (Rose and Leggett, 1988). The classification of moonfish by school descriptors was successful in this study. It is possible to identify the species of schooling fishes aggregating around a FAD by using an array of discrirninators extracted from acoustic signals. General 旬, species composition is not very complex during one period, especially during the seasons with main species appearing around the surface FAD. 80 target fish identification during acoustic surveys could mostly be achieved by fishing experiments and signal feature extraction techniques for some species, and the distribution of fish schools in the surface FAD zone could be illustrated by the acoustic method. But for species of jack-mackerel, pacific sailfish, and dolphin fish, target fish identification was more difficult by echo signal processing because catch data and echo signals of fish schools did not be obtained at the same time in this study. Therefore, the distribution of these species is still not illustrated. The acoustic signals could clearly discriminate among fish species including butterflyfish, unicorn leatherjacket, moonfish, yellow-finned runner, skipjack and yellowfin tunas in this study. In the nearby area of the surface FAD, more than 80% of these fish schools distributed themselves within 600 m of FAD (Table 2). In Japan, 60% of fish schools wandered around the FADs ranging between 1 and 100 m, 20% between 100 and 200 m, 10% between 200 and 300 m, 3% between 300 and 400 m, and without any fish schools more than 400 m away (Nozawa, 1981). The effective radius of FADs for aggregating fish

13 Characteristics of fish schools around a FAD 171 Nov.1997 Thunnus ajbacares :? 30 0 ~ 20 ro glo ω (L 恤m N=51 M.L.=45.3 (cm) M.W.=2.2 (kg) Fork length (cm) VA (法)~ 20 ro 51O ~ 0 B U m 也mm N=29 M.L. =43.7 (cm) M.W.=1.8 (kg) Fork length (cm) Fig. 9. The body-iength composition of catches of commerical boats in the sub-surface FAD zone in Nov was estimated to be only within 300 m in Nozawa's report. Although the distribution of fish schools around different FADs varied from each other and they mainly aggregated within a radius of 50 m around the FAD, the farther away from the FAD, the less the aggregation of fish schools (8awada, 1981). Around large-sized FADs, the main species, including skipjack tuna, dolphin fish, and small yellowfin tuna, were horizontally distributed in a range from 300 to 500 m (Ishida, 1981). In Taiwan, the farthest distance for detecting fish was 2.8 km upstream from the buoy of the surface FAD (Lin and 8u, 1995). In Hawaii, fish schools stayed beyond 4 to 8 km from the FAD (Myatt, 1985) and the maximum abi 卜 ity of FADs to aggregate yellowfin tuna was approximately 5 nautical miles (Holland et al., 1990). The different extent in the influence of the FAD may result from different views of how to define the range of aggregation. Furthermore, it is difficult to confirm whether fish schools far beyond the FAD are aggregating by the effect of the FAD or not. For example, moonfish schools aggregating in the nearby area of the surface FAD in this study also appeared in a range of 2 km away from the buoys, even as far as about 3.2 km (Fig. 7). Furthermore, the estimation by scientific echo sounder (Table 6) showed that the biomass of moonfish aggregating within the 200 m horizontal area from the surface FAD accounted for 73 kg, while there was 279 kg from 250 to 2000 m. Particutarly, the biomass of moonfish was the highest at the distance of m. And there was no moonfish detected at the distance of m. 80 it is difficult to judge the aggregating range for moonfish schools. (IV) Future studies about FADs In this survey area, skipjack and yellowfin tunas that aggregated around the

14 172 Long-Jing Wu, Jiun-Chern Lin, Cheng-Fang Lee and Wei 司 Cheng Su Table 6. Acoustic estimation of biomass of moonfish in FAD zone. The estimated T8 value (-48.4 db) of EY -500 echo sounder was used to calculate the density data by sonar equation (8v= T8+1010g (density)). The estimated biomass in individuals was converted to weight with catch data (0.23 kg per individual). Range Average Density Sv (db) (fish/m 3 ) 0-50 rn E m 巨 m E m E m E m 巳 m E rn E m E m E-02 surface FAD were mainly smal 卜 sized fish measuring about 30 to 40 cm in fork length. The smaller sizes of skipjack and yellowfin tunas at FADs were exmined for possible size range bias in the test fishery catches (Buckley et al.,1989). But in this study, the maximum target strength was about -43 db (Fig. 5). It means that no large individuals were detected during acoustic survey. On the other hand, another subsurface FAD deployed in f arther and deeper waters southwest of the Liuchiu Island attracted larger skipjack and yellowfin tunas, some fishermen once caught skipjack and yellowfin tunas that were larger than 60 cm in length (Fig. 9). Therefore, although FADs possess a good effect on fish aggregation, the location of deployment also plays an important role. In addition, dolphin fish was one of the major species aggregating around the surface FAD (Fig. 2), and was usually caught within the distance of about 800 m from the surface F AD in test fishery catches. But the acoustic signals were not obtained at the same time in this region. The dolphin fish, however, usually aggregates on the surface, and has often been Surveying Estimated Estimated volume biomass biomass rn 3 Individuls (kg) the object of trolling lines and hand lines (8awada, 1981). As a result, the aggrega ing state of dolphin fish around a surface FAD needs further study to understand their aggregating behavior. The quantitative information in this study conclusively shows that the surface FAD is an effective method for enhancing hand lines and trolling lines fishery. Properly sited FADs could take advantage of attracting pelagic fish, thus providing a good opportunity for increasing catches, while decreasing the search times and operating costs for fishing vessels. Two types of FADs, i.e., surface FADs and subsurface FADs, have been set in the waters off southwestern Taiwan. Therefore, in the near future it is necessary to continue studying the aggregating characteristics of subsurface FADs which have brought great benefits to local fishermen ACKNOWLEDGEMENTS The authors wish to thank Dr. 1 Chiu Liao, director general of Taiwan Fisheries Research Institute (TFRI), for his support.

15 Characteristics of fish schools around a FAD 173 Thanks are also extended to the staff of Haifu and Haichane vesse 怡, Mr. James Chang of the Kaohsiung Branch, TFRI, for their help with the fishing experiment and acoustic survey. We also specialiy wish to thank Dr. Sin-Che Lee, researcher at the Institute of Zoology, Academia Sinica, for his kind reviewing of the manuscript. REFERENCES Ahmed, MK. (1996). FAD: A new fishing aid in India's horizon. Fish. Chimes, 15(12): Brill, R. W, K. N. Holland and J. S. Ferguson (1984) 回 Use of ultrasonic telemetry to determine the short-term movements and residence times of tunas around fish aggregating devices Proceedings of the Pacific Congress on Marine Technology, MRM1: 1-7. Buckley, R. M., Itano, D. G. and Buckley, T. W (1989). Fish aggregation device (FAD) enhancement of offshore fisheries in Amer 卜 can Samoa. Bull. Mar. Sci., 44(2): Chen, S. R. (19 89). Study on aggregation effect of artificial floating fish reef. J. Fish. Soc. Taiwan, 16(1): Forbes, S. T., and O. Nakken (1972). Manual of methods for fisheries resource survey and appraisal. FAO Man. Fish. Sci. 2: 138pp. Holland, K 瓜, R. W. Brill and R. K. C. Chang (1990). Horizontal and vertical movements of yellowfin and bigeye tuna associated with fish aggreg ating devices. Fis h. Bull., 88(3): 回 Inoue, M. (1978 ). The behavior and fishing methods of fish. Koseisha-Koseikaku, Tokyo, pp Inoue, M. (1985). Fishing gears and fish behavior. Koseisha-Koseikaku, Tokyo, pp Ishida, Y. (1981). Field experiments of fish aggregating devices (1). Ocean Age, 14: Johnson, R. A. and D. W. Wichern (1988). Applied multivariate statistical analysis. Prentice Hall International, Inc. 607pp Kawai, T. (1991). On the mechanism of long-term variability of pelagic fish population, in longterm variability of pelagic fish population and their environment. (Edited by Kawasaki, T., S. Tanaka, Y. Toba and A. Taniguchi), published by Pergamon Press, Oxford, Lee, K. 下, L. J. Wu, C. H. Liao and H. J. Lu (1993) Classification of schooling structures of Engraulis japonica by processing the hydroacoustic signal and discriminant analysis. J. Fish. Soc. Taiwan, 20(3): Lin, J. C. (1994). The technologies of designing and setting fish aggregation devices. China Fisheries Monthly, 496: Lin, J. C. (1995). Experience in the design and deployment of midwater fish aggregation device. China Fisheries Monthly,. 509: Lin, J. C. and W. C. Su (1992). Hydrodynamic characteristics of fish aggregation devices. The Proceedings of the First International Symposium on Marine Engineering, Lin, J. C. and W. C. Su (1994). Early phase of fish habitation around a new artificial reef off southwestern Taiwan. Bull. Mar. Sci., 55(2-3), P Lin, J. C. and W. C. Su (1995) 回 Li fe expectancy and effectiveness of fish aggregation devices in Taiwan. ECOSET' 95, 1: Monint 悶, D. R. (1989). The development of rumpon as a fish aggregation device for skipjack tuna fisheries in Indonesia. Proceedings of the 1988 World Symposium on Fishing Gear, Fishing Vessel Design, 1989, Monintja, D. R., A. Purbayanto and R. 1. Wahju, RI (1991). Study on the development prospect of fish aggregation device for tuna fisheries in Pelabuhan Ratu waters. MARITEK, 1(1): Myatt, D. O. (1985). Midwater fish attractors, in artificial reefs (Edited by Frank M. D'ltri), Lewis Publishers, USA, Nonawa, M. (1981). Field experiments of fish aggregating devices (2). Ocean Aglθ, 14: Richards, L. J., R. Kieser, T. J. Mulligan and J. R. Candy (1991). Classification of fish assemblages based on echo integration surveys. Can. J. Fish. Aquat. Sci., 48: Rose, G. A. and W. C. Leggett (1988) 回 Hydroacoustic signal classification of fish schools by species. Can. J. Fish. Aquat. Sci., 45: Sawada, T. (1981). Field experiments of fish aggregating devices (3). Ocean Age, 14: Samples, K. C. and J. R. Hollyer (1989). Economic considerations in configuring fish aggregation device networks. Bull. Mar. Sci. 44(2): 缸, W. C. and J. C. Lin (1996). Effectiveness of fish aggregating devices in fisheries. Proceedings of Symposium on Marine Ranching, Ser, 1, Marine Science Research Center of National Sun Yat-Sen University,

16 174 Long-Jing Wu, Jiun-Chern Lin, Cheng-Fang Lee and Wei-Cheng Su 利用魚 探機評估表層人工浮魚礁週邊海域 魚群聚集特性 吳龍靜.林俊辰.李政芳.蘇偉成 * ( 2000 年 6 月 10 日收件 2000 年 7 月 20 日接受 ) 本研究在小琉球外海投放一年且表層人工浮魚礁, 為了解主要聚集魚種的分布特性, 利用聲探技術及漁獲試驗在浮魚礁週邊海域進行計測 結果獲得六種主要魚種的分布情 形, 經聲探回訊積分法 信號特信篩選法及集群分析 ; 去等分析結果, 確認蝶魚及單棘純 ; 回游於礁體表層附近, 但沒有明顯的日夜變化情形 ; 雙帶穆白天酒游於礁體附近 100 公 尺以內, 水深 公尺的海域, 入夜後聚集魚群明顯減少 ; 體賄魚群白天分布於約 800 公尺以內水深 50 公尺以淺的海域, 入夜後有離開礁體的現象 ; 皮力魚群的分布則較 廣, 白天時, 在礁體附近的皮力魚群聚集於離礁體約 550 公尺以內水深 公尺的海 域, 但入夜後貝 IJ 消失不見 浮魚礁周邊海域的聲探計測結果, 顯示本組人工浮魚礁具有 相當良好的緊魚效果 此外, 利用聲探技術評估人工淨魚礁週邊海域的魚群群緊特性應 屬可行 關鍵詞 : 人工浮魚礁, 聲探技術, 緊魚效果 o 行政院農業委員會水產試驗所高雄分所 * 通訊作者