Fisheries Science 65(5), 694-699 (1999)

Diel Vertical Movement of the 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 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. 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, , 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 scat Kuroshio and 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 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 , migrate species.4-6) Although the basic concept of a food chain to the bay for feeding. Former oceanographic and fishery formed by -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 , 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 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 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 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/ 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 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. The raw data were archived on a and 60•`140m, respectively (Figs. 3 and 4). The maximum floppy disk, and isopleths of each factor were subsequent thickness of DSL was also observed around midnight. The ly plotted using the software package of Suffer, Version starting times of the ascending and descending and their 6.04. moving speeds are shown in Table 2.

Results Conclusions and Discussion The results obtained in this study were similar to those Oceanographic conditions described by Omori22,23)for the diet vertical movement of Temperature and salinity profiles from 0 m to 300 m in S. lucens in the coastal of Tung-Kong in April and June are plotted in Fig. 2. The salinity ranges Southwestern Taiwan. The ascending speed was fairly simi were fairly stable in both months, but temperature in June lar to that of descending, about 1.5m/min and 1.8m/ was slightly higher with larger standard deviations than min, respectively.23,24)Lath and Parsons7) found that the that in April. ascending and descending speeds of euphausiids were in a range of 100-200m/hr (1.67-3.33m/min). In this Species composition and packing density study the moving speeds were expressed by average speeds Four species, Systellaspis pellucids, Sergia lucens, of all species (Table 1) in DSL. Pasiphaea japonica and Benthosema pterotum were found In this study the starting time of ascending in April was in the catches of IKMT sampler. The catch composition va found to be earlier than that in June by about an hour, in ried with time and depth (Table 1). S. lucens and P. japoni dicating that diet vertical movement of DSL is controlled ca were dominant, and occupied 89.3% of the total catch. by light/dark cycle, and the vertical movement of DSL The packing density in DSL ranged between 0.7 and crossed both and pycnocline (Fig. 4). This 25.7 inds./m3 with the mean•}standard deviation of phenomenon was also observed by Rudstam et al.25) and 5.1 •} 8.4 inds./m3. The mean density in daytime and night Ackefors26) that Mysis mixta was not affected by strong

Fig. 2. Temperature and salinity profiles at depth range of 0 m to 300 m in I-Lan Bay, April 13•`15 (a) and June 12-14 (b), 1997. Diel Vertical Movement of DSL 697

Fig. 3. MVBS echograms in I-Lan Bay, April 13 -15.

Fig. 4. Moving tracks of DSL and the isopleths of the density and temperature monitored in 1-Lan Bay, April 13-15 (Cruise 319) and June 12-14 (Cruise 341), 1997. 698 Chou et al.

Table 2. Starting time and ascent and descent speeds in the diel vertical movement of DSL

temperature and salinity gradients and pressure changes, Acknowledgments Acknowledgments We are grateful to the National and showed nocturnal vertical migration by ascending to Science Council of the Republic of China (ROC) for financial support the surface during the nighttime and descending to the bot (NSC87-2611-M-019-002), and to Captain C. F. Liu and the members of tom in daytime. Rudstam et al.25) also found the starting Ocean Research No. 2 vessel for their assistance in the study. Thanks are time of nocturnal migration of M, mixta has seasonal also due to Mr. D. A. Lee of the Taiwan Fisheries Research Institute who assisted in identifying the shrimp samples and Dr. C. F. Tsai for review changes, controlling by the surface light and water trans ing the manuscript. parency. They suggested that light may be more important than temperature in determining the extent of the vertical References migration. However, it remains unknown whether DSL in

different geographical locaions react similarly to a given 1) D. D. Sameoto: Zooplankton and Micronekton abundance in photic stimulus, and whether there is a specific difference acoustic scattering layers on the Nova Scotian slope. Can. J. Fish. in such response. Aquat. Sci., 39, 760-777 (1982). The MVBS of DSL in daytime and nighttime ranged be 2) R. J. Urick: Principles of underwater sound, ISBN 0-07-066086-7, tween -50 dB and -60 dB and between -65 dB and 1975, 384pp. -55 dB 3) GLOBEC: GLOBEC Workshop on Acoustical Technology and the , respectively. Thus the mean strengths of DSL Integration of Acoustical and Optical Sampling Methods, Report measured in daytime was 5 dB larger than that in night Number 4, Joint Oceanogr. Inst., Inc., Washington, DC, 1991, 46 time. The mean packing density D monitored by the acous PP. tic system in the daytime was about 3 times higher than 4) C. Jeng, S. J. Lee, and J. K. Sheu: Marine Plankton, Fisheries that in the nighttime, according to the equation D Press, Keelung, Taiwan, 1991, 661 pp. (in Chinese). =10(_??_) and the hypothesis of the constant of TS (target 5) G. Hempel: Introduction: Food requirements for fish production, strength) in this study.20) This result was in accordance in "Marine food chains" (eds. J. H. Steele, Oliver and Boyd), Edin burgh, 1970, pp. 255-260. with the result from trawl samplings. Therefore, DSL was 6) T. R. Parsons and R. J. LeBrasseur: The availability of food to more compressed in daytime than at night (Table 1; Figs. 3 different trophic levels in the marine food chains, in "Marine food and 4a, c), a similar phenomenon noted by Demer and chains" (ed. by J. H. Steele, Oliver and Boyd), Edinburgh, 1970, Hewitt27) and Omori23) for krill and sergestid shrimp. They pp. 325-343. interpreted this as an evidence that these organisms dis 7) C. M. Lalli and T. R. Parsons: Biological oceanography: An in perse in shallow water and aggregate in patches in deep troduction, Pergamon Press, New York, 1993, 301 pp. water. This phenomenon may be attributed to an evolved 8) J. H. S. Blaxter: The role of light in the vertical migration of fish a review, in "Light as an Ecological factor ‡U" (ed. by G. C. Evans, strategy of foraging in the phytoplankton-rich shallow R. Bainbridge and O. Rackham), British Ecological Society 16th water during darkness and avoiding the visually oriented Symposium, Blackwell, Oxford, 1974, pp. 189-210. predators that are active during daytime.28) However, the 9) M. Z. Gliwicz: Predation and evolution of vertical migration in predators with high swimming speed (such as ribbonfish, zooplankton. Nature, 320, 746-748 (1986). carangids, bonito, etc.) were not sampled by the IKMT in 10) T. M. Zaret and J. S. Suffern: Vertical migration in zooplankton as the present study. a predator avoidance mechanism. Limn. Oceanogr., 21: 804-813 (1976). The dominant 4 species, S. pellucids, S. lucens, P. japon 11) I. 1. Lee and J. H. Hu: The relation between the I-Lan Bight water ica and B. pterotum in DSL varied with depth and time of and the shelf water northeast of Taiwan. Acta Oceanogr. Taiwani day (Table 1). For triple deep tows around noon ca, 37, 89-103 (1998).

(10:20•`13:35) at the depths of 180m to 270m (Table 1), S. 12) K. L. Fan: On upwelling off northeastern shore of Taiwan. Acta lucens made up 53.0% 82.2% of the catch, while in after Oceanogr. Taiwanica, 11, 105-117 (1980). 13) T. S. Chin: Variation of ichthoplankton density across the noon (15:15 •` 17:30) at the deeper depths of 230m to 330 Kuroshio edge exchange area with implication as to the water mass. m, P. japonica made up 43.7% 68.5% of the catch. In TAO, 2(2), 147-162 (1991). contrast, two night tows (02:20-03:30) at two depth 14) G. T. F, Wong, S. C. Pai, K. K. Liu, C. T. Liu, and C. T. A. Chen: ranges (10•`25m and 70•`80m), the dominant species Variability of the chemical at the frontal region be was P. japonica (Table 1). They were similar to those of tween the East China Sea and the Kuroshio Northeast of Taiwan. Omori et al.24) who state the small euphausiid shrimp, Eu Estuarine, Coastal and Shelf Science, 33, 105-120 (1991). 15) K. K. Liu, G. C. Gong, S. Lin, C. Y. Yang, C. L. Wei, S. C. Pai, phausia similis, and myctophid fishes such as Diaphus and C. K. Wu: The year-round upwelling at the shelf break near the coeruleus and D. grandulifer often constitute a major com Northern tip of Taiwan as evidenced by chemical hydrographY. ponent of the deep scattering layer. However it is difficult TAO, 3(3), 243-276 (1992). in identifying the echo of S. lucens from those of other or 16) T. Y. Tang and W. Z. Tang: Current on the edge of the continental ganisms, though the echo sounder can estimate the diel ver shelf northeast of Taiwan. TAO, 5(2), 335-348 (1994). tical movement and biomass index of DSL. 17) C. F. Greenlaw: Acoustical estimation of zooplankton populations. Limnol. Oceanogr., 24, 226-242 (1979). Diel Vertical Movement of DSL 699

18) R. E. Throne, R. J. Trumble, N. A. Lemberg, and D. 23) M. Omori: The biology of a sergestid shrimp Sergetes lucens Han Blankenbeckler: Hydroacoustic assessment and management of her sen. Bull. Ocean. Res. Inst. Univ., Tokyo, 4, 1-83 (1969). ring fisheries in Washington and Southeastern Alaska. FAO Fish, 24) M. Omori and O. Suguru: The use of underwater camera in studies Rep., 300, 217-222 (1983). of vertical distribution and swimming behaviour of sergestid 19) L. J. Wu, K. T. Lee, S. R. Leu, and C. H. Liao: Estimation of abun shrimp, Sergia lucens. J. Plank. Res., 3, 107-121 (1981). dance of carangids near the water intake of Nuclear Power Plant I 25) L. G. Rudstam, K. Danielsson, S. Hansson, and S. Johansson: Diel by dual-beam hydroacoustic method. J. Fish. Soc. Taiwan, 16, vertical migration and feeding patterns of Mysis mixta (Crustacea, 175-188 (1989). Mysidacea) in the Baltic Sea. Mar. Biol., 101, 43-52 (1989). 20) R. E. Throne: Investigations into the relation between integrated 26) H. Ackefors: Ecological zooplankton investigations in the Baltic echo voltage and fish density. J. Fish. Res. Bd. Can., 28, 1269-1273 proper 1963-1965. Rep. Inst. Mar. Res Lysekil (Ser. Biol.), 18, 1 (1971). 139 (1969). 21) H. J Lu, K. T. Lee, and C. H. Liao: Quantitative estimation of 27) D. A. Demer and R. P. Hewitt : Bias in acoustic biomass estimates zooplankton and micronekton biomass by a high- acous of Euphausia superba due to diel vertical migration. Deep-Sea Res. tic method. J. Fish. Soc. Taiwan, 21, 139-155 (1995). 1., 42, 455-475 (1995). 22) M. Omori: Fishery of sergestid shrimp Sergia lucens (Hansen) at 28) M. Omori, T. Konagaya, and K. Noya: History and present status Tung-Kong, Taiwan. Bull. Jap. Soc. Fish. Oceanogr., 53, 108-110 of the fishery of Sergestes lucens (Penaeidea, Decapoda, Crustacea) (1989). in Suruga Bay, Japan. J. Cons. Int. Explor. Mer., 35, 61-77 (1973).