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Using the Hydroacoustic Method with a 38 Khz Scientific Echo Sounding

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Using the Hydroacoustic Method with a 38 Khz Scientific Echo Sounding Fisheries Science 65(5), 694-699 (1999) Diel Vertical Movement of the Deep Scattering Layer on the Continental Slope of I-Lan Bay, Taiwan Shih-Chin Chou, Ming-Anne Lee,t and Kou-Tien Lee Department of Fisheries Sciences, National Taiwan Ocean University, Keelung 20224, Taiwan (Received December 4, 1998) risc Usingthe hydroacousticmethod with a 38 kHz scientificecho sounding system,the 51el vertical movementof the deep scatteringlayer (DSL)in I-Lan Bay, northeasternTaiwan, was studied on April 13 15and June 12-14,1997. The DSLstarted to descendabout 30 minutesbefore sunOat a speedof 1.4-1.7m/min and residedat 180to 280m in the daytime.It started to ascend ca. 30min before sun is t a speedof 1.5-1.9m/min and stayedat 10 to 150mat night. Plankton sampleswere simultane ously collected by an IKMT sampler to determine the speciescomposition. The dominant species (ca.> 1.5cm) were Sergialucens and Pasiphaea japonica, which made up 89.3% of the catch. The mean packing density during the daytimewas about 3 times higher than that at night. Key words: deep scattering layer, zooplankton, micronekton, diel vertical movement, acoustic monitoring Marine organisms tend to aggregate at specific depths in 2.773•~109m3) from Lan-Yang River and mix with the ocean, and they scatter acoustic waves as a sound scat Kuroshio and continental shelf water by tidal current in tering layer on echogram.1) When the sound scattering lay side the bay.11) From April to October, the shelf water at er occurs at depths between 180 and 900 m, it is referred to northeast of Taiwan might flow into I-Lan Bay along the as a deep scattering layer (DSL).2) Generally, DSL appears coast of Taiwan and then merge into the Kuroshio,11) with at depth of a few hundred meters in the daytime and the occurrence of upwelling of cold water nearby Cape ascend to shallow depth in the night. Most of the organ San-Diao.12) As a consequence of these hydrographic con isms in DSL consist of zooplankton and micronekton, ditions, I-Lan Bay is one of the most important fishing and which play important roles in the food chain dynamics of nursery grounds of marine fishes in Taiwan.13) Larvae and the marine pelagic ecosystems" as they are the major food juveniles of commercially important fish species, e.g. rib source for juveniles of many commercially important fish bonfish, carangids, bonito, mackerel, and squid, migrate species.4-6) Although the basic concept of a food chain to the bay for feeding. Former oceanographic and fishery formed by phytoplankton-zooplankton-fish has been well investigations conducted in this bay usually focused on established,7) it still remains difficult for us to assess food fishing methods and physical oceanography, 14-16) and rare requirements of marine fish based on zooplankton density ly on the ecological aspects of pelagic communities. alone,5) this may be due to the fact that the relationship be Sampling with a net has been commonly used in the stu tween fish population and zooplankton density has not dies of zooplankton and micronekton. This approach pro been clearly understood, and the effects of zooplankton vides detailed descriptions of species and development density on food chain dynamics still remain obscure. Fur stages of plankton, but does not survey over extended time thermore, the density and distribution of zooplankton are and space scales.17) On the other hand, the acoustic often influenced by tidal movement and also by their diet method has an ability to survey over a large area rapidly vertical migration. For these most of the studies in the past with a low operational cost, and the provision of a good es concerned with zooplankton as indicator species of water timate of the population.17,18) The objective of this study is mass in relation to tidal movement, and with specific differ to investigate the diel vertical migration of zooplankton ence in the diel vertical migration of zooplankton and its and micronekton particularly shrimps in this bay by detect causative factors.6) Although many hypotheses"') have re ing DSL. cently been developed to explain the phenomena, it ap pears that there is no universal mechanism governing the Materials and Methods diel vertical migration of all species of zooplankton.7) I-Lan Bay lies on the coast of northeastern Taiwan (24•‹ Acoustic monitoring 36'-25•‹N and 121•‹48'-122•‹12'E), where the offshore con The acoustic monitoring was conducted in the central tinental shelf is narrow and the continental slope is steeply part of I-Lan Bay (see Fig. 1) on April 13-15 (Cruise 319) declined to the Okinawa Trough (Fig. 1). The Kuroshio and June 12-14 (Cruise 341), 1997, on board the research Current flows northward off the bay, and there are great vessel "Ocean Research No.2" of National Taiwan Ocean amounts of fresh water supply (average annual runoff: University. Since the average bottom depth was about 300 I Address correspondence to : Professor M. A. Lee, Department of Fisheries Sciences, National Taiwan Ocean University, Keelung 20224, Taiwan, R.O.C. (Tel: 886-2-2463-4419, FAX: 886-2-2463-5941, E-mail: [email protected]). Diet Vertical Movement of DSL 695 Fig. 1. Sampling sites (the trawl sampling starting positions and net numbers denoted by solid squares and numbers) and approximately drogue range for acoustic monitoring in I-Lan Bay. Table 1. The percentage composition of species in the catches of IKMT sampler, April 13 -15, 1997 m. The drogue method was thus used to monitor the move Trawl sampling ment of DSL. Acoustic data were collected with a quantita Eight trawl samplings were conducted. Their sites, time tive echo-sounding system (SIMRAD EK-500), using a 38 and depth range are given in Fig. 1 and Table 1. During kHz split-beam transducer. EF500 post-processing the acoustic monitoring, the Isaac-Kidd Midwater Trawl software was used on-line to archive the acoustic signals (IKMT) with a net mouth of 1.43ml.54m, a total length and monitor the diel vertical distribution of DSL. The of 7.6m with mesh size of 1 to 5 cm, and a cod-end di parameters of the echo sounder were set as follows; absorp ameter of 0.52m with mesh size of 333 ƒÊm was used to tion coefficient: 10 dB/km, pulse repetition rate: 0.5 ping/ sample animals in DSL.211 IKMT was towed horizontally s, sampling distance: 10 cm, pulse duration: 1.0ms, half for 20-60 min at a speed of 3 knots at depths of DSL beam angle (-3 dB): 7.1•‹, target strength threshold: -90 shown on the echograms, and monitored by a SCANMAR dB, and bandwidth: 3.8 kHz. In the laboratory, the DSL acoustic net condition monitoring system (Model HC 1200). signals were post processed so that each data point The sensor was attached at the mouth of the IKMT sam represented an average of 5 m of vertical thickness and pler. The towing tracks were displayed on the computer sampled over 2min (30 pings). The mean volume backscat monitor and the depth data were saved on a floppy disk at tering strength (MVBS) data provided a convenient meas 2-second intervals. For estimating the volume of water ure of the density index of the zooplankton and micronek filtered, we attached the flowmeter (Model 2030 Series, ton,19,20) as the target strength was uncertain. The MVBS GENERAL OCEANICS INC.) at the center of the cod echograms of the water column for 25 hr (Cruise 319) and end. The water volume at the horizontal part in the trawl 36 hr (Cruise 341) were plotted by the software package of ing was estimated by reducing the volume when IKMT Suffer, Version 6.04, and used as a biomass index of DSL. towing in oblique at the beginning and the end of the oper 696 Chou et al . ation from the total operating time. In order to reduce the time were about 6.2 and 1.9 inds./m3, respectively. The possible contamination of organisms from depths other former was about 3 times higher than the latter. than DSL, the dropping and lifting of IKMT were made as quickly as possible, so that the net mouth and body shape Diel vertical movement was not opened completely. The samples collected were Figure 3 shows the MVBS echograms monitored at the preserved in a 5% buffered formalin/sea water solution. depth between 0m and 250m in April. There were 2 dis Because zooplankton and small unidentified shrimp larvae tinct layers: 1) the surface sound scattering layer (SSL) less than 1.5 cm and commercial fish larvae occupied less with MVBS of -65 - 55 dB and 2) DSL with MVBS than 1% of the total catch, they were ignored in this study. larger than -60 dB. SSL resided at a depth of 5 •` 80m For each trawl sample, the total individuals of adults and throughout the day, while DSL showed a remarkable diet juveniles of shrimps and fishes (the body length ca. >1.5 vertical movement (Fig. 4). In April, it resided at the depth cm) were counted, and these were identified to species. of 220•`250m during the period from 06:30 am to 05:00 pm and at 10 •` 150m from 06:45 pm to 04:47 am. In June, Oceanographic conditions it resided at 180 •` 280m from 07:00 am to 06:00pm and at A Seabird CTD system (Model SBE 911 plus) was used 10 •` 145m from 07:00pm to 03:57 am. Except for the to obtain temperature, salinity and density profiles (depth periods of descending and ascending, the vertical thickness range from 0 m to 300 m) at 4-hour intervals during the of DSL in the daytime and nighttime was about 25 30 m trawl sampling periods.
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