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Summer Spatial Distribution of Copepods and Fish Larvae in Relation to Hydrography in the Northern Taiwan Strait

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Chih-Hao Hsieh and Tai-Sheng Chiu (2002) Summer spatial distribution of copepods and fish larvae in relation

to hydrography in the northern Taiwan Strait. Zoological Studies 41(1): 85-98. This study analyzed the spatial dis-tribution of copepods and fish larvae in relation to hydrographic conditions in the northern Taiwan Strait during summer as the prevailing southwestern monsoon drives the surface warm water from the South China Sea into the Strait and causes subsurface water upwelling in the west. Cluster analysis based on copepod and fish larvae assemblages resulted in recognition of 4 groups, of which 3 major ones conformed to the hydrography. The west-ern Taiwan Strait group was characterized by coastal and neritic species, the East China Sea group was dominat-ed by oceanic species, and the eastern Taiwan Strait group consistdominat-ed of both neritic and oceanic species. Analysis of dominant species also supports this result. Copepod abundance was positively related to water tem-perature and dissolved oxygen. The area of high copepod abundance did not correspond to the area of high pri-mary productivity in the upwelling area, but occurred at a stable area downstream of the upwelling. Fish larva abundance was positively related to copepod abundance. http://www.sinica.edu.tw/zool/zoolstud/41.1/85.pdf

Key words: Species composition, Southwestern monsoon, South China Sea, East China Sea, Upwelling.

Summer Spatial Distribution of Copepods and Fish Larvae in Relation

to Hydrography in the Northern Taiwan Strait

Chih-Hao Hsieh and Tai-Sheng Chiu*

Department of Zoology, National Taiwan University, Taipei, Taiwan 106, R.O.C. (Accepted November 1, 2001)

T

he Taiwan Strait, a shallow channel bound-ed by the southeastern China coast on the west and by Taiwan on the east, and connecting the East China Sea and South China Sea, serves as a pathway for migratory fishes between the 2 waters, and therefore it is an important fishing ground. The hydrographic conditions of the Strait are generally influenced by 3 current systems with seasonal variations, i.e., the Kuroshio, China coastal current, and South China Sea current, as well as river runoff from both mainland China and the island of Taiwan. When the southwestern monsoon prevails during summer, the Kuroshio branch does not enter the Strait, while warm water from the South China Sea penetrates through the Penghu Channel into the northern Taiwan Strait (Fang and Yu 1981, Fang 1982). Impeded by the Changyun Ridge, the surface and subsurface waters from the Penghu Channel flow in different directions. The light and warm surface water flows

over the Ridge into the northeastern Taiwan Strait (hereafter the eastern Strait), while the heavy cold subsurface water is blocked by the Ridge and turns northwestward along local isobaths into the northwestern Taiwan Strait (hereafter the western Strait). Because of surface Ekman transport dri-ven by the southwestern monsoon along the China coast, subsurface water originating from the Penghu Channel wells up and mixes with China coastal water in the western Strait (Xiao 1988, Jan et al. 1994). On the other side, surface water from the Channel flows into the eastern Strait and mixes with Taiwan coastal water. A detailed description of the summer hydrography of the Taiwan Strait was given by Jan et al. (1995). As oceanic physical processes influence the distribu-tion of marine plankton (Boucher 1984, Boucher et al. 1987), variations in hydrographic conditions of the Taiwan Strait provide an opportunity to study the effects of physical processes on the spatial

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distribution of copepods and fish larvae.

Although the Taiwan Strait is ecologically important for faunal exchange between the 2 mar-ginal seas in the western Pacific, studies on the controlling factors of the distribution and abundance of fish larvae and zooplankton in the Strait are scarce. Tzeng and Wang (1992 1993) studied the seasonal dynamics of fish larvae and their relation-ship to the Tanshui River discharge. Chiu and Chang (1994) reported on the fish larvae fauna of the eastern Strait during winter and spring. Chiu and Chang (1995) studied diurnal cycling of fish lar-vae in the Tanshui River plume. Copepod species in the northern Taiwan Strait were reported by Tan (1967), Zheng et al. (1982) and Chen (1992), and in the Tanshui River estuary by Hsieh and Chiu (1998). Some information about the distribution and seasonal variation of phytoplankton, zooplank-ton, and fish larvae was also compiled (Zhu et al. 1988); however these studies were confined to the western Strait and did not discuss the influence of hydrographic patterns on these organisms.

Survivorship of fish larvae affects the recruit-ment success of fishes. Food availability is crucial for larval survival, especially at very early life stages when the yolk is exhausted. Copepods are the major component of marine zooplankton and thus are the main food source of fish larvae (Poulet and Williams 1991). It has been shown that salinity, temperature, and primary production have immense impacts on zooplankton distribution and abundance (Tremblay and Roff 1983, Thomas and Emery 1986, Fernandez et al. 1993). In this study, we examined the hydrographic influence on the trophic relationship between fish larvae and copepods, and tested the spatial overlap of copepods and fish lar-vae in the northern Taiwan Strait in summer. This work is a part of Taiwan Strait Marine Ecosystem project in compliance with the GLOBEC (Global Ocean Ecosystem Dynamic Program).

MATERIALS AND METHODS Sampling

A cruise was carried out on 16-18 August 1999 on board the Ocean Research II with 11 stations (stations 5-15) in the northern Taiwan Strait and 4 stations (stations 1-4) in waters off northern Taiwan (Fig. 1). Fish larvae were collected using a round-mouthed ichthyoplankton net with a mouth diameter of 130 cm and mesh size of 1 mm, while zooplank-ton were collected using a standard North Pacific

zooplankton net with a mouth diameter of 45 cm and mesh size of 150 µm. A flowmeter was mount-ed at the center of the mouth of each net. The 2 nets were towed simultaneously and obliquely from near the bottom to the surface for surveying an average of total copepod and ichthyoplankton fauna. Ichthyoplankton samples were preserved with 95% alcohol, and zooplankton samples were preserved in seawater with 5% formalin. Vertical profiles of temperature, salinity, fluorescence and dissolved oxygen were obtained at each station from bottom to surface using a conductivity-temper-ature-depth (CTD) profiler (Sea-Bird Electronic) equipped with a fluorometer and an oxygen sensor. The fluorescence value was not calibrated but was used as an index of relative primary productivity. The current velocity was recorded using a ship-board ADCP (acoustic Doppler current profiler).

In the laboratory, each of the fish larvae in the ichthyoplankton samples was identified to species whenever possible. The number of individuals was then counted for each species. Fish larvae in zooplankton net samples were counted for esti-mating total abundance, but were not incorporated into cluster analysis in order to keep the data stan-dardized. Abundance of fish larvae was expressed as the number of individuals per 1000 m3. Copepods in zooplankton net samples were sub-sampled using a Folsom splitter until the sam-ple size was reduced to 250-400 specimens.

Fig. 1. Sampling stations for hydrographic data, fish larvae, and

zooplankton in the northern Taiwan Strait, August 1999. (PH, Penghu Islands; PHC, Penghu Channel; CYR, Changyun Ridge).

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Copepods were identified to species when possi-ble, and the number of individuals of each species was recorded. Abundance of copepods was expressed as the number of individuals per m3.

Data analysis

Principal component analysis was used to characterize the hydrographic regions based on station data of bottom depth, surface water temper-ature, salinity, fluorescence, and dissolved oxygen (Pielou 1984). Cluster analysis was used to exam-ine the association among sampling stations based on species percentage composition data (Pielou 1984). For copepods, only species which account-ed for 2% or more of the total number of speci-mens in at least 1 station were included in the analysis. Species comprising less than 2% are considered rare species and might lead to deriva-tion of erroneous reladeriva-tionships in the community analysis (Gauch 1983). Likewise for fish larvae, only species which accounted for 5% or more in at least 1 station were included. For normalizing the patchiness of zooplankton, data were square-root transformed (Krebs 1989). Normalized Euclidean distance was applied to measure the dissimilarity among stations, and Ward's method of grouping was used to obtain the correspondence dendro-gram (Pielou 1984). Based on the abundance of dominant species of copepods (> 3%) and fish lar-vae (> 5%) of each region defined by principal

component analysis, canonical correspondence analysis was used to determine the relationship between dominant species and oceanographic conditions (Braak 1994). A generalized linear model (McCulliagh and Nelder 1983) was applied to examine the relationship between copepod abundance and the 5 oceanographic variables of bottom depth, surface water temperature, salinity, fluorescence, and dissolved oxygen; as well as the relationship between combined fish and copepod abundance and the 5 oceanographic variables. The best model was achieved by stepwise multiple regression analysis, and the variables were stan-dardized before analysis. A t-test was used to examine the significance of the coefficients of the variables in the models and the α value set to 5%.

RESULTS Hydrography

The depth-averaged current vectors in the northern Taiwan Strait during the survey period are plotted at flood and ebb stages respectively (Fig. 2). The water moves northeastward from the Strait to the East China Sea at ebb, and south-westward from the East China Sea to the Strait at flood tides. The surface water temperature and salinity contour plots of the northern Taiwan Strait indicate that the water is divided along the middle

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into 2 parts, with a relatively low salinity and high temperature in the eastern Strait, and a relatively high salinity and low temperature in the western Strait, except that an extremely low salinity was found off the Chinese coast (Fig. 3a, b). Evidence

of river runoff was obvious near shore (around sta-tions 9 and 14-15) where 2 lenses of low salinity were found. The low temperature and high salinity of the western Strait reveals the occurrence of upwelling. During this study, the upwelling area was widespread (stations 7-8 and 10-13), indicat-ing that the southwestern monsoon was drivindicat-ing water from the South China Sea along the coast of mainland China, forming an Ekman transport phe-nomenon. Surface water fluorescence and dis-solved oxygen contour plots show that higher val-ues were found in the western than that in the eastern Strait (Fig. 3c, d). According to the princi-pal component analysis based on the 5 hydro-graphic variables, 15 stations were categorized into 3 regions: western Taiwan Strait - stations 7, 8, and 10-13; eastern Taiwan Strait - stations 5, 6, 14, and 15; and East China Sea - stations 1-4 (Fig. 4). The names are given according to the geographic regions where the stations are located. Station 9 is distinct from other stations because of its extremely low salinity as shown in figure 3a. For simplicity, station 9 is assigned to the western Taiwan Strait region due to its locality. The eigen-values and component loadings of principal com-ponent analysis indicated that variables of bottom depth and temperature contrasted with those of fluorescence and dissolved oxygen, while salinity was less distinctive (Table 1).

Fig. 3. Contour plots of surface salinity (a), temperature (b),

fluores-cence (c), and dissolved oxygen (d) in the northern Taiwan Strait.

Fig. 4. Principal component analysis biplot based on bottom

depth, surface water temperature, salinity, fluorescence, and dissolved oxygen, of 15 sampling stations indicating 3 hydro-graphic regions (WTS, western Taiwan Strait; ETS, eastern Taiwan Strait; ECS, East China Sea).

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Species composition and distribution

In this study, 122 copepod species in 26 fami-lies were identified, with 116 to species and 6 to genus level (Table 2), and a total of 96 fish species in 46 families were identified, with 76 to species, 9 to genus, and 11 only to family (Table 3). Cluster analysis of stations based on the copepod composi-tion shows 4 groups A1-A4 (Fig. 5a). The clusters of A1 (stations 8-12), A2 (stations 5 and 15), and A4 (1-4, 6, and 14) represent the western Strait, the eastern Strait, and the East China Sea groups respectively, but an extra cluster of A3 (7 and 13) was formed by interim stations. The clusters show 73.3% of stations matching the hydrographic regions; only stations 6, 7, 13, and 14 are contradic-tary to the hydrographic categories (Fig. 4). The dendrogram derived from the species composition of fish larvae also shows a fairly similar result (Fig. 5b), with B1 (stations 7-11), B3 (stations 4-6 and 14), and B4 (stations 1-3) representing the western Strait, the eastern Strait, and the East China Sea groups respectively. The fish larvae dendrogram also shows 73.3% stations matching the hydrographic regions with 4 stations (4, 12, 13 and 15) differing from the hydrographic categories. However, the dendrograms of copepods and fish larvae show 46.7% of similarity to each other, while stations 4-7 and 12-15 are categorized differently (Fig. 5).

Species compositions of copepods and fish larvae of the 3 hydrographic regions are listed in tables 2 and 3, respectively, and dominant species are highlighted in boldface. A canonical corres-pondence ordination biplot of dominant species of copepods and fish larvae shows the relationships between the species and oceanographic variables (Fig. 6). Of copepods, the dominant species can be separated into 3 groups. Group A, located in western Taiwan Strait with high fluorescence and

dissolved oxygen contrasting with low tempera-ture, salinity, and depth, is composed of Acartia

pacifica, Corycaeus lubbocki, Euterpina acutifrons,

and Oithona brevicornis. However, among the rest of the species, Clausocalanus forcatus,

Oithona plumifera, and Oncaea conifera were

located in deeper waters, preferred high tempera-ture and salinity, and are assigned to group C. The species group of Acrocalanus gibber,

Canthocalanus pauper, Corycaeus dahli, Oithona attenuata, Oncaea venusta, Paracalanus pavus, Paracalanus serrulus, Parvocalanus crassirostris,

and Temora turbinata, located at a hydrographic intermediate, was assigned to group B (Fig. 6a). The first 2 canonical axes explain 36.5% of the hydrographic variance among copepod species groups. Among dominant fish larvae, species group A, composed of Priacanthus macracanthus and Diaphus pacificus, prefers high temperature at deeper water where fluorescence and dissolved oxygen are low. However, species group C, com-posed of Leiognathus rivulatus, Nibea sp.,

Pseudosciaena sp., and Saurida elongata, prefers

high fluorescence and dissolved oxygen with low temperature at shallower depth. The remaining species forming group B, including Apogon

endekataenia, Auxis rochei, Ceratoscopelus warmingi, Diaphus theta, Encrasicholina heterolo-ba, and Trichiurus lepturus, are associated with

intermediate hydrographic factors (Fig. 6b). The first 2 canonical axes explain 38.19% of the hydro-graphic variance among fish larvae groups, and it is worth noting that both axes are feeble due to salinity variations.

Density of copepods and larval fish

Area contour plots of copepod and fish larvae abundances are shown in figure 7. Lenses of

rela-Table 1. Eigenvalues and component loadings of the principal components

based on correlation of hydrographic factors

Principal component 1 2 3 4 5 Eigenvalue 3.254 1.095 0.369 0.282 0 Percentage 65.087 21.894 7.387 5.63 0.001 Cum. percentage 65.087 86.981 94.368 99.999 100 Loading Depth 0.446 0.288 0.828 0.18 -0.002 Temperature 0.529 -0.185 -0.13 -0.408 0.709 Salinity -0.037 0.935 -0.258 -0.218 0.099 Fluorescence -0.484 -0.069 0.448 -0.749 -0.006 Dissolved oxygen -0.535 0.056 0.175 0.44 0.698

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Table 2. Relative abundance (%) of copepods collected from the northern Taiwan

Strait and southern East China Sea. Assemblages were categorized according to hydrographic regions. Dominant species of each region are marked in boldface

Order: Calanoida

Acartiidae Acartia negligence 0.04 1.53 1.57

Acartia longiremis 0.00 0.00 0.00

Acartia omori 0.00 0.00 0.06

Acartia pacifica 3.30 0.46 0.06

Acartia sp. 0.00 0.18 0.00

Arietellidae Metacalanus aurivillii 0.05 0.00 0.00 Calanidae Calanoides carinatus 0.04 0.00 0.00

Calanus sinicus 0.00 0.17 0.13 Canthocalanus pauper 4.16 6.24 6.25 Cosmocalanus darwinii 0.00 0.18 0.23 Mesocalanus tenuicornis 0.00 0.00 0.08 Nannocalanus minor 0.18 0.40 0.00 Neocalanus gracilis 0.04 0.24 0.19 Undinula vulgaris 0.58 1.41 1.97

Cadaciidae Candacia bipinnata 0.00 0.00 0.07

Candacia catula 0.00 0.09 0.00

Candacia discaudata 0.14 0.00 0.00

Candacia elongata 0.05 0.00 0.00

Candacia copepodid 0.37 0.40 0.80

Paracandacia truncata 0.03 0.00 0.00

Centropagidae Centropages orsinii 0.22 0.55 0.07

Centropages furcatus 0.04 0.00 0.26

Assemblage categories

Family Species West Strait East Strait East China Sea

tive high densities of copepods and fish larvae are located in the northwest and southeast of the study area. However, higher fluorescence indices were located in the western Strait in agreement with the upwelling region, and with an area of higher dis-solved oxygen as well (Fig. 3c, d). Apparently, higher abundances of copepods and fish larvae were located in vicinities of the upwelling region. Multiple regression analysis indicates that copepod abundance (C) was positively related to water tem-perature (T) and dissolved oxygen (D), but nega-tively related to fluorescence (FL) (C = 1.05 T + 1.65 DO - 0.55 FL, R2= 0.717, p = 0.0024, df = 11), and fish larvae abundances (F) were positively related to copepod abundance (F = 0.52, r = 0.521,

p = 0.0463, df = 13).

DISCUSSSION

In this study, we found that the current veloci-ty is higher in the eastern than in the western Taiwan Strait (Fig. 2; Jan et al. 1995). The water

movement is primarily affected by western Pacific tides and causes the water from the East China Sea to mix with water in the Strait, especially on the eastern side. However, the mean flow in the Strait is generally northward during summer (Jan et al. 1995). Therefore, the diurnal tidal movement and the prevailing northward current may be the 2 forces that drive variations of copepod and larval fish fauna in the northern Strait.

The distribution of copepods and fish larvae was found to be linked to the hydrography in the northern Taiwan Strait during our examination of assemblages. Both copepod and fish larvae clus-ters showed 73.3% matching hydrographic cate-gories (Fig. 6). However, only as low as 46.7% of similarity was found corresponding to copepod and fish larvae assemblages. In further analysis, we found that station groups of 1-3 and 8-10 were never mis-classified in the 3 comparisons --hydrography vs. copepod, --hydrography vs. fish lar-vae, and copepod vs. fish larlar-vae, owing to their attributes of purity having been located at 2 ends of examined area. We may therefore define the

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Centropages gracilis 0.00 0.07 0.00 Clausocalanidae Clausocalanus arcuicornis 0.04 0.07 0.07

Clausocalanus brevipes 0.05 0.00 0.00 Clausocalanus forcatus 0.80 1.88 4.04 Clausocalanus ingens 0.00 0.00 0.19 Clausocalanus minor 1.21 0.08 1.22 Clausocalanus mastigophorus 0.09 0.09 0.26 Clausocalanus parapergens 0.04 0.00 0.00 Clausocalanus pergens 0.16 0.00 0.00 Ctenocalanus vanus 0.11 0.00 0.00

Eucalanidae Subeucalanus crassus 0.53 0.08 0.08

Subeucalanus longiceps 0.00 0.00 0.13

Subeucalanus pileatus 1.24 0.38 0.34

Subeucalanus subcrassus 0.27 0.15 0.07

Subeucalanus subtenuis 0.00 0.09 0.00

Subeucalanus copepodid 1.14 1.08 0.00

Euchaetidae Euchaeta cocinna 0.05 0.00 0.00

Euchaeta indica 0.00 0.00 0.08

Euchaeta rimana 0.09 0.15 0.00

Euchaetidae copepodid 2.36 0.09 0.66

Heterorhabdidae Heterohabdus papilliger 0.00 0.00 0.06 Lucicutiidae Lucicutia flavicornis 0.10 0.00 0.00

Lucicutia gausiae 0.04 0.00 0.00

Lucicutia ovalis 0.00 0.00 0.06

Mecynoceridae Mecynocera clausi 0.14 0.08 0.08

Metridicidae Pleuromamma borealis 0.04 0.00 0.00

Metridia copepodid 0.00 0.00 0.22

Paracalanidae Acrocalanus gibber 2.93 6.44 4.36

Acrocalanus gracilis 0.56 1.57 0.42 Acrocalanus monachus 0.00 0.50 0.00 Calocalanus contractus 0.00 0.26 0.07 Calocalanus gracilis 0.05 0.09 0.33 Calocalanus monospinus 0.00 0.00 0.13 Calocalanus pavo 0.00 0.00 0.14 Calocalanus pavoninus 0.04 1.30 1.10 Calocalanus plumulosus 0.00 0.66 0.13 Calocalanus styliremis 0.04 0.82 0.79 Paracalanus aculeatus 0.20 0.48 1.62 Paracalanus pavus 15.19 8.22 12.72 Paracalanus serrulus 3.41 3.10 1.83 Parvocalanus crassirostris 8.37 21.73 11.04

Pontellidae Calanopia elliptica 0.00 0.08 0.34

Calanopia minor 0.09 0.18 0.13

Lobidocera copepodid 0.00 0.00 0.59

Pontellopsis tenuicauda 0.00 0.08 0.00

Scolecithricidae Scolecithricella longispinosa 1.40 0.00 0.08

Scolecithrix danae 0.00 0.09 0.00

Temoridae Temora discaudata 0.13 0.08 0.06

Temora stylifera 0.04 0.24 0.06

Temora turbinata 4.71 6.71 7.71

Temoropia mayumbaensis 0.05 0.00 0.00

Tortanidae Tortanus forcipatus 0.05 0.00 0.00

Table 2. (Cont.)

Assemblage categories

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Order: Cyclopoida

Oithonidae Oithona atlantica 0.09 0.08 0.00

Oithona attenuata 2.03 4.92 3.76 Oithona brevicornis 3.22 0.46 0.55 Oithona decipiens 0.16 0.00 0.00 Oithona fragilis 0.28 0.00 0.00 Oithona longispina 0.32 0.07 0.00 Oithona similis 2.12 0.09 0.06 Oithona simplex 0.10 0.35 0.00 Oithona plumifera 2.52 1.45 3.76 Oithona fallax 0.59 0.30 1.37 Oithona rigida 0.35 0.47 0.35 Paroithona sp. 0.00 0.08 0.13 Order: Harpacticoida

Clytemnstridae Clytemnestra scutellata 0.40 0.08 0.00 Ectinosomatidae Microsetella norvegica 0.00 0.00 0.06

Microsetella rosea 0.46 0.08 0.25

Euterpinidae Euterpina acutifrons 10.39 2.12 0.20

Miraciidae Macrosetella gracilis 0.00 0.08 0.21

Order: Mormonilloida

Mormonillidae Mormonilla minor 0.00 0.00 0.14

Order: Poecilostomatoida

Corycaeidae Corycaeus flaccus 0.00 0.00 0.07

Corycaeus typicus 0.00 0.00 0.07 Corycaeus speciosus 0.00 0.00 0.13 Corycaeus affinis 0.36 0.08 0.00 Corycaeus andrewsi 0.14 0.43 0.25 Corycaeus asiaticus 0.00 0.00 0.06 Corycaeus dahli 2.79 3.83 0.41 Corycaeus erythraeus 0.00 0.00 0.14 Corycaeus lubbocki 4.94 0.00 0.35 Corycaeus subtilis 0.66 0.23 0.82 Corycaeus agilis 0.04 0.09 0.27 Corycaeus catus 0.04 0.45 1.00 Corycaeus giesbrechi 0.66 0.55 1.89 Corycaeus pacificus 0.13 0.34 0.13 Corycaeus lautus 0.05 0.00 0.00 Corycaeus longistylis 0.04 0.09 0.00 Corycaeus pumilus 0.00 0.14 0.00 Farranula concinna 0.00 0.09 0.00 Farranula gibbula 0.09 0.83 1.07 Farranula rostrata 0.00 0.00 0.13

Oncaeidae Oncaea conifera 2.52 2.59 6.31

Oncaea mediterranea 0.08 0.00 0.41

Oncaea minuta 0.08 0.09 0.39

Oncaea similis 0.04 0.16 0.13

Oncaea venusta 9.09 10.34 11.98

Oncaea sp. 0.00 0.08 0.00

Sapphirinidae Copila mirabilis 0.00 0.09 0.00

Corina granulose 0.00 0.00 0.06

Sapphirina iris 0.00 0.00 0.08

Sapphirina stellata 0.14 0.00 0.00

Table 2. (Cont.)

Assemblage categories

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Table 3. Relative abundance (%) of fish larvae collected from the northern Taiwan

Strait and southern East China Sea. Assemblages are categorized according to hydrographic regions. Dominant species of each region are marked in boldface

Order: Calanoida

Acanthuridae Ctenochaetus binotatus 0.00 0.00 0.68

Ambassidae Ambassis sp. 0.00 0.00 0.46

Ammodytidae Embolichthys mitsukurii 0.78 0.33 0.93

Apogonidae Apogon endekataenia 5.22 0.00 0.93

Apogon lineatus 0.55 0.00 0.27

Apogon pseudotaeniatus 0.00 0.45 0.27

Blenniddae Omobranchus elegans 0.00 0.33 0.00

Bothidae Engyprosopon multisquama 1.64 0.00 2.53

Laeops kitaharae 0.29 0.00 0.00

Psettina gigantea 1.69 0.00 1.60

Bregmacerotidae Bregmaceros arabicus 0.00 1.82 4.32

Bregmaceros nectabanus 1.07 0.00 0.00

Callionymidae Callionymus beniteguri 0.00 0.00 1.08

Carangidae Caranx sexfasciatus 0.60 0.00 0.54

Caranx sp. 0.00 0.00 0.46

Decapterus macrosoma 0.00 1.82 1.87

Scomberoides lysan 0.00 0.00 1.19

Selar crumenophthalmus 1.33 0.00 3.05

Chaetodontidae Chaetodontid 0.00 0.00 0.27

Champsodontidae Champsodon snyderi 0.00 0.00 1.75

Clupeidae Etrumeus teres 0.60 2.30 0.46

Coryphaenidae Coryphaena hippurus 0.00 0.00 0.27

Creediidae Limnichthys fasciatus 0.00 0.45 0.00

Cynoglossidae Cynoglossus joyneri 1.10 0.00 0.68

Cynoglossus sp. 0.00 0.45 0.00

Paraplagusia japonica 0.26 0.00 0.00

Symphurus orientalis 1.69 2.50 0.00

Emmelichthyidae Erythrocles schlegelii 0.00 0.00 0.81 Engraulidae Encrasicholina heteroloba 7.51 12.00 2.78

Setipinna tenuifilis 1.58 0.00 0.00

Gerreidae Gerres oyena 0.00 0.99 0.00

Gobbidae Amblychaeturichthys hexanema 0.00 0.33 0.00

Bathygobius cotticeps 0.78 0.00 0.00

Parachaeturichthys polynema 0.00 0.33 0.00

Gonostomatidae Cyclothone atraria 0.00 0.33 0.00

Gonostoma atlanticum 0.49 0.00 0.00

Maurolicus muelleri 0.00 0.00 0.27

Assemblage categories

Family Species West Strait East Strait East China Sea

water from stations 1-3 as the end-member water of the East China Sea, and stations 8-10 as the end-member water of the western Strait. The remaining stations of 4-7 and 12-15 were condi-tionally modified from the 2 end-members showing various joining topographies in our cluster analysis, because they are influenced either by the south-western monsoon or the south-western Pacific tides. This is evidenced by the southern monsoon which

drives the South China Sea water northward during summer (Jan et al. 1995), and the Pacific tides which agitate the Strait water back and forth diur-nally (Fig. 2). Accordingly, the oceanic species of southern East China Sea (stations 1-3) were trans-ported to the eastern Strait (Fig. 2), mixed with the local neritic species and thus formed an intermedi-ate assemblage in the eastern Strait (stations 4-6 and 14-15). On the west side, due to huge

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vo-Haemulidae Hapalogenys nitens 0.00 0.00 0.68

Labridae Xyrichthys sp. 0.55 0.00 0.93

Leiognathidae Leiognathus rivulatus 5.26 2.76 0.00

Secutor insidiator 1.71 2.78 2.78

Lethrinidae Lethrinus nematacanthus 0.00 0.00 0.93

Lutjanidae Lutjanus bohar 0.00 0.00 3.26

Monacanthidae Rudarius ercodes 0.60 0.00 0.46

Mugillidae Liza affinis 0.00 0.33 0.00

Mugiloididae Parapercis pulchella 0.00 0.00 0.27

Mullidae Upeneus bensasi 0.00 1.82 1.39

Myctophidae Benthosema fibulatum 0.00 0.00 0.93

Benthosema pterotum 2.04 0.00 0.93 Bolinichthys pyrsobolus 0.00 1.91 0.00 Ceratoscopelus warmingi 0.00 5.78 0.00 Diaphus pacificus 0.00 1.25 5.20 Diaphus sp. 0.00 0.45 0.00 Diaphus theta 0.60 5.00 0.73 Lampadena luminosa 0.00 1.25 0.00 Myctophum asperum 0.00 0.00 0.27 Nemipteridae Scolopsis sp. 2.84 0.00 0.00

Ophidiidae Sirembo imberbis 0.00 0.00 0.27

Percichthyidae Acropoma japonicum 0.00 0.00 0.93

Synagrops philippinensis 0.00 0.00 3.01

Percophidae Percophid 0.00 0.00 5.86

Pomacentridae Chromis sp. 0.00 0.00 0.93

Priacanthidae Priacanthus macracanthus 0.00 0.33 12.54

Scaridae Scarus sp. 0.00 0.33 1.66

Scatophagidae Scatophagus argus 0.57 1.11 0.68

Sciaenidae Argyrosomus argentatus 1.14 1.36 0.00

Nibea japonica 0.49 0.00 0.00

Nibea sp. 17.90 0.00 1.35

Pseudosciaena sp. 6.62 0.00 0.00

Scombridae Auxis rochei 0.00 4.50 5.44

Auxis thazard 0.00 2.61 0.93

Euthynnus affinis 0.00 1.36 0.00

Euthynnus pelamis 0.00 0.66 0.46

Scorpaenidae Scorpaenid 0.00 0.99 0.00

Serranidae Epinephelus akaara 0.00 0.00 0.27

Sacura margaritacea 0.00 0.00 0.93

Sillaginidae Sillago japonica 2.04 0.78 0.93

Sillago sihama 0.00 0.91 0.00

Soleidae Aseraggodes kobensis 0.29 0.00 0.00

Synodontidae Saurida elongata 9.26 0.00 0.00

Saurida wanieso 0.55 0.00 0.00

Synodus macrops 0.00 0.00 0.27

Trachinocephalus myops 0.00 2.43 0.73

Teraponidae Rhyncopelates oxyrhynchus 0.00 0.00 0.46 Trichiuridae Benthodesmus elongatus 0.00 0.00 0.46

Trichiurus lepturus 10.62 25.30 7.13

Uranoscopidae Gnathagnus elongatus 0.55 0.00 0.00

Uranoscopus japonicus 0.00 0.00 0.93

Others 9.18 9.57 7.67

Table 3. (Cont.)

Assemblage categories

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lumes of river runoff (Zhu et al. 1988) and subsur-face water upwelling to the sursubsur-face (Jan et al. 1995), both coastal and neritic species occur in the assemblage of the western Strait (stations 7 and 11-13). Topologies of dendrograms derived from copepods and fish larvae matched fairly well to the hydrographic regions (Figs. 4, 5). In the dendro-gram of copepods, cluster A2, consisting of sta-tions 5 and 15, was located in the Tanshui River plume and was thus grouped together with the highest similarity (Figs. 1 and 5). Stations 7 and 13 of cluster A3 were expected to join the cluster of the western Strait (A1), but were grouped into clus-ters A4 because of their location near the hydro-graphic boundary (Fig. 1). In the dendrogram of fish larvae, cluster B2 of stations 12 and 13 should be included in the western Strait according to the hydrographic categorization. As they were also located at the hydrographic boundary with compli-cated flood-ebb tides (Fig. 2), stations 12 and 13

depicted the highest similarity due to the relative scarcity of fish larvae (Fig. 7b), which was unex-pected to those of the other stations.

Among dominant species of copepods,

Clausocalanus forcatus, Oithona plumifera, and Oncaea conifera (group C, Fig. 6a) were abundant

in the East China Sea region (represented by sta-tions 1-3) corresponding to the water mass located

Fig. 6. Canonical correspondence ordination biplot of

domi-nant species of copepods and fish larvae in relation to hydro-graphic variables. (D, bottom depth; T, surface water tempera-ture; S, surface water salinity; F, surface water fluorescence; DO, surface water dissolved oxygen; the 3 circles represent the 3 groups of species associations).

Fig. 5. Dendrograms of station association derived from the

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Fig. 7. Contour plots of copepod abundance (a) and fish

lar-vae abundance (b) in waters of the northern Taiwan Strait.

where it is deeper, and characterized by high tem-perature and salinity with low fluorescence and dis-solved oxygen at the surface. Four species, Acartia

pacifica, Corycaeus lubbocki, Euterpina acutifrons,

and Oithona brevicornis (group A, Fig. 6a), were confined to shallower water of the Chinese coastal region in the western Strait. The region is charac-terized by relatively low temperature and salinity but high fluorescence and dissolved oxygen. Ten species, Acrocalanus gibber, Canthocalanus

pau-per, Corycaeus dahli, Oithona attenuata, Oithona plumifera, Oncaea venusta, Paracalanus pavus, Paracalanus serrulus, Parvocalanus crassirostris,

and Temora turbinate (group B, Fig. 6a), were rela-tively widely distributed in the study area so that their distribution pattern was not as pure as the species groups of the end-member waters men-tioned previously. According to Chen et al. (1965), Chen and Zhang (1974), and Noda et al. (1998),

Acartia pacifica, Corycaeus lubbockii, Corycaeus dahli, Euterpina acutifrons, Oithona brevicornis, Paracalanus pavus, Paracalanus serrulus, and Parvocalanus crassirostris are neritic species; while Acrocalanus gibber, Canthocalanus pauper, Clausocalanus furcatus, Oncaea conifera, Oncaea venusta, Oithona plumifera, and Temora turbinata

are oceanic species. With apparently higher toler-ance to salinity and temperature fluctuations,

Acrocalanus gibber, Canthocalanus pauper, Corycaeus dahli, Oithona attenuata, Oithona plumifera, Oncaea venusta, Paracalanus pavus, Paracalanus serrulus, Parvocalanus crassirostris,

and Temora turbinata occurred all over the northern Taiwan Strait. In particular, no oceanic species of Chen and Zhang (1974) and Noda et al. (1998) occurred in the Chinese coastal region, but some neritic species (Acartia pacifica, Corycaeus

lubbocki, Euterpina acutifrons, and Oithona brevi-cornis) invaded the Chinese coastal region. It is

therefore reasonable to propose that the southwest-ern monsoon brings neritic copepod species to the China coastal region, while the tidal movement brings oceanic copepod species from the East China Sea to the eastern Strait. In fish larva sam-ples, Diaphus pacificus and Priacanthus

macracan-thus (group A, Fig. 6b) were abundant in the deeper

water of the East China Sea region, characterized by high temperature but low fluorescence and dis-solved oxygen. The species of Leiognathus

rivula-tus, Nibea sp., Pseudosciaena sp., and Saurida elongata (group C, Fig. 6b) were abundant in the

shallow water of the western Strait, characterized by low surface temperature but high fluorescence and dissolved oxygen. There are 6 species

grouped in correspondence to the medians of mea-sured variables (group B, Fig. 6b), among which

Auxis rochei, Diaphus theta, and Ceratoscopelus warmingy were abundant in the eastern Strait; Apogon endekataenia in both the western Strait and

East China Sea; and Encrasicholina heteroloba and

Trichiurus lepturus were widely distributed

through-out the study area. The dominant fish larvae found in the East China Sea are consistent with those reported by Huang and Chiu (1998). Larvae of

Encrasicholina heteroloba were found abundantly in

both the eastern Strait and East China Sea, and are an important commercial species from May to September (Huang and Chiu 1998). The fish fau-nas in the northern Taiwan Strait are distinct in the 3 regions due to hydrographic variations.

The oceanographic data show that river runoff from mainland China and upwelling water along the Chinese coast bring nutrients into the western

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Strait region. In this study, the input of nutrients resulted in a phytoplankton bloom as represented by fluorescence indices (Fig. 3c). Copepod abun-dance was negatively related to fluorescence. Areas of peak copepod abundance did not corres-pond to areas of high primary production but were in the marginal zone outside the high production area (Fig. 7a). It has been shown that phytoplank-ton develop over time on stable sides of a front or mixing area, which provides a more suitable place for zooplankton (Lalli and Parsons 1993, Maravelias and Reid 1997). This is supported by our study, which reveals an up- and down-stream relationship between primary production and cope-pod abundance. Other factors appearing to have effects on copepod abundance as shown in this study were temperature and dissolved oxygen. Our results support those studies showing that copepod production is temperature dependent (Huntley and Lopez 1992 and references therein) and that copepod distribution is constrained by dis-solved oxygen (Roman et al. 1993).

This work has extended our knowledge of fish larvae and copepods in the northern Taiwan Strait. In our study, Trichiurus lepturus and Encrasicholina

heteroloba were dominant larval fish species, and Paracalanus pavus, Parvocalanus crossirostris, Oncaea venusta, and Temora turbinata were

domi-nant copepod species in the northern Strait in sum-mer with high abundance and wide distribution.

Trichiurus lepturus and Encrasicholina heteroloba are

also important fishery resources in this area. We tried to relate the abundance of the 2 dominant species of fish larvae with the 4 dominant copepod species, but the outcome from correlation analysis implied that the relationship was weak. Gut content analysis of the larvae of the Japanese anchovy

Engraulis japonicus revealed that their diet

composi-tion primarily consisted of copepodites of

Paracalanus and Oithona species (Hirakawa and

Ogawa 1996, Hirakawa et al. 1997). The abundance of Paracalanus and Oithona species was very high in our study area. Further studies on gut content analy-sis and more intensive sampling are necessary to address the trophic relationships among dominant species of fish larvae and copepods. Incorporation of biological/physical modeling synthesis is also needed for better understanding of the hydrographic effects on trophoecology in the Taiwan Strait.

Acknowledgments: We thank the crew of Ocean

Research II for their help in the collection of

plank-ton and oceanographic data. We especially thank Ms. K.Z. Chang and Ms. C.C. Chen of the

Economic Fish Laboratory, Department of Zoology, National Taiwan Univ. for their help with fish larva identification and Dr. C.T. Shih of the Taiwan Fisheries Research Institute for his advice on cope-pod identification. We benefited from discussions on the hydrography of the Taiwan Strait with Dr. S. Jan of the National Center for Oceanographic Research. Dr. Jefferson Turner provided a con-structive review and editorial assistance. Insightful opinions from 2 anonymous reviewers are highly appreciated. This study was financially supported by a grant from the National Science Council, ROC (NSC89-2611-M002-008) to TSC.

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Zhu CS, JZ Wu, YS Lin, M Su, CJ Huang, QB Hu. 1988. The species composition, distribution and abundance of plankton. In Fujian Institute of Oceanography, ed. A comprehensive oceanographic survey of the central and northern part of Taiwan Strait. Beijing: Science Press, pp. 259-305. (in Chinese) 本 研 究 之 目 的 在 於 分 析 橈 腳 類 及 仔 魚 對 應 於 臺 灣 海 峽 中 尺 度 之 水 文 狀 態 之 分 布 情 形。研 究 時 間 為 夏 季 西 南 季 風 盛 行 時。海 流 資 料 顯 示,漲 潮 時 東 海 水 流 入 海 峽 東 側,退 潮 時 海 峽 水 退 至 東 海 南 部。依 據 表 水 之 溫 度 及 鹽 度 等 值 分 布 圖 以 及 水 團 之 鹽 溫 線 圖 分 析,此 時 海 峽 可 依 中 線 區 隔 為 二,東 側 為 南 海 表 水 及 東 海 水 之 混 合 區,而 西 側 則 表 現 出 南 海 次 表 層 水 在 近 海 區 湧 升 之 現 象。此 研 究 在 海 峽 及 北 臺 灣 水 域 探 測 之 測 站 可 區 分 為 三 個 水 文 區:即 東 海 峽 區、西 海 峽 區 及 東 海 區。西 海 峽 之 表 水 具 相 對 之 低 溫 高 鹽,證 為 源 於 澎 湖 水 道 之 南 海 水 所 湧 升。東 海 峽 表 水 具 高 溫 低 鹽,然 而 東 海 水 則 為 高 溫 及 次 高 鹽 之 狀 態 (稍 低 於 西 海 峽 水 )。以 聚 類 分 析 法 針 對 橈 腳 類 及 仔 魚 群 集 進 行 分 析,結 果 各 顯 示 四 個 叢 集,其 中 有 三 個 叢 集 所 定 義 的 群 集 分 別 對 應 於 三 個 水 文 區,即 東 海 峽 群、西 海 峽 群 及 東 海 群;後 兩 者 為 主 要 群。西 海 峽 群 由 沿 岸 及 近 海 物 種 所 組 成,東 海 群 主 要 為 外 洋 物 種,東 海 峽 群 在 兩 主 要 群 之 中 間 地 帶 屬 混 合 群, 有 近 海 及 外 洋 物 種。典 型 對 應 分 析 顯 示,橈 腳 類 及 仔 魚 群 集 組 成 之 變 異 可 由 海 峽 北 部 之 水 文 因 子 所 解 釋。由 迴 歸 分 析 顯 示,橈 腳 類 之 密 度 與 水 溫 及 溶 氧 量 呈 正 相 關,但 密 度 最 高 之 橈 腳 類 出 現 區 並 不 與 最 高 之 初 級 生 產 區 域 相 吻 合,卻 出 現 在 湧 升 流 區 外 圍 之 穩 定 海 區。仔 魚 密 度 與 橈 腳 類 之 密 度 分 布 則 呈 統 計 上 之 一 致 性。 關鍵詞:物種組成,西南季風,南海,東海,湧升流。 國立臺灣大學動物學系

臺灣海峽北部橈腳類及仔魚對應於夏季水文之分布

謝 志 豪   丘 臺 生

數據

Fig. 1.  Sampling stations for hydrographic data, fish larvae, and zooplankton in the northern Taiwan Strait, August 1999
Fig. 2.  Scatter plot of current vectors showing the depth-averaged current velocities at flood and ebb tides, respectively.
Fig. 3. Contour plots of surface salinity (a), temperature (b), fluores- fluores-cence (c), and dissolved oxygen (d) in the northern Taiwan Strait.
Table 1.  Eigenvalues and component loadings of the principal components based on correlation of hydrographic factors
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