Variation of Ichthyoplankton Density Across the Kuroshio Edge Exchange Area with Implications as to the Water Ma sses

lchthyoplankton in a water column off north�eastern Taiwan, an area referred to as the Kuroshio edge exchange area, was sampled using a conical ichthyoplankton net on board R/V Ocean Research I. The purpose of this survey was to understand the ichthyoplankton community structure and its linkage with the Kuroshio edge exchange process. This preliminary analysis illustrates the spatial density distribu­ tion of fish eggs, ichthyoplankton and an incidental catch of zooplankton. Biological densities and their linkage with the hydrological variables were also analyzed. The principle findings include: 1) that the distribution of ichthyoplankton densities exhibited a consensus pattern of high density in a northwesterly direction near the East China Sea, low density in the southern area of the Kuroshio current proper and moderately high densities in the shore area and 2) that principal compo­ nent analysis based on hydrological and biological variables produces a clear picture that discriminates between the water mass of mid-shelf origin and those of oceanic origin. Station. specific variation of the hydrological variables is also discussed.


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In the offshore area of north-eastern Ta iwan:, the waters generally come from two origins; one from the East China Sea (mid-shelf origin) and the other from the Kuroshio (oceanic origin) (Chern and Wang, 1989). This is known to be one of the most productive neritic fishing grounds in Taiwan (Anon., 1988), however, basic biological and environmental information for this area is still very rare.
Ichthyoplankton surveys in the northwestern Pacific for the purpose of fishery development and management have been conducted routinely in Japan for many years (Anon., 1978(Anon., -1982. However, in Taiwan similar ichthyoplankton 2.
MAT ERIALS AND METHODS

The Area
The study area is demarcated by latitudes between 121° 30' E and 123°15' E and longitudes between 25° 30' N and 26° N. It consists of two types of marine topography, the continental shelf and the continental slope.

The Survey
The present ichthyoplankton survey was conducted during June 5-6, 1989, as part of cruise No. 212 of the R/V Ocean Research I. The sampling stations were designed to represent a distance 15 latitudinal or longitudinal minutes apart (Fig. 1). The sampling range roughly met with the pre-determined KEEP studying area with minor adjustments for specific sampling sites. lchthyoplank ton sampling was conducted 24-hours a day. Information on the location of each sampling station is presented in Table 1.
The sampling gear used in this study for collection of the fish eggs and ichthyoplankton was a 4-m conical net with a mouth opening of 1.3 m (details see Chiu and Liu, 1989). The mesh size of the ichthyoplankton net was 1.0 mm. During sampling, the net was first released at a rate of 40 m/min., thereafter the ship was kept at a speed of approximately 2.0 knots, and the net was retrieved at a rate of 20 m/min. The net was towed at about 200 min depth or within 10 m of the bottom. A hydrological fl.owmeter was mounted in the center of the mouth of the ichthyoplankton net to estimate the volume of water filtered. Hydrological profiles of temperature, salinity, dissolved oxygen and total fluorescence were measured by in situ sensors attached to a CTD (SBE 9/11, Sea-Bird Electronic, Inc.).

Data analysis
Samples taken were fixed on board with 10% formalin in sea water, and then sorted upon return. All larval or early juvenile fishes were removed for  an electric balance. The count and weight was converted to density according to the readings of the fiowmeter following the method described by Kendall and Dunn (1985).
Fifteen variables in this study were subjected to principal component anal ysis . Those variables were: 1) density of the number of fish eggs; 2) density of the number of ichthyoplankton; 3) density of fish eggs by weight; 4) density of ichthyoplankton by weight; 5) density of zooplankton by weight; 6) bottom depth; 7), 8) and 9) water temperature at surface, at -50 m, and at -100 m (or bottom temperature when shallower than 100 m ) , respectively; 10) and 11) water salinity at the surface and at -50 m; 12) and 13) apparent total flu orescence value (in relative units) at the surface and at -50 m; 14) and 15) apparent dissolve<;]. oxygen value {relative units) at -50 m and -100 m respec tively. These item numbers shall be referred to as variable numbers hereafter.
Among these variables, variables 1 and 3 were removed during selection of the final model due to their low correlation with the other variables.

Bottom Depth
The bottom depth of the sample sites ranged from 72 m to 1, 091 m. Stations 2, 3, and 4 were located beyond the continental shelf. A 200 m isobathic contour passed through the south-eastern corner of the rectangular study area  (Table 1).

Temperature and Salinity
The surface temperature of the sampling sites ranged from 22. 1° C to 27.3° C. The fluctuation of the surface temperature paralleled with solar ra diation with higher magnitudes found around noon (about 27° C) and a lower magnitudes from mid-night to dawn (lower than 24°C). Some temperature variation might also be due to the specific water mass, but in this case the pri mary contributing variable for surface temperature variation was diurnal. The water temperature at a depth of 50 m and a:t a depth of 100 m for the sample sites ranged from 19.6° C to 26.4° C and from 15.5° C to 24. 7° C, respectively.
The water temperature at greater depths was generally shown to have a par allel trend with the magnitude of the surface temperature, i.e . higher surface temperature indicated a higher sub-surface temperature except for minor vari ations at Stations 6, 7, 10, and 11. From Stations 1 to 4, thermal radiation from the surface down was quite homogenous but a thermocline developed at a depth from 50 ,... ., 100 m at Stations 6 and 7, while a reverse temperature distribution occurred at Stations 8 and 9. A shallower thermocline was found at a depth ranging from 0,.., , 50 mat Stations 10 and 11. At Stations 12-18, a homogeneous water temperature distribution was found.
The salinity at the surface to a 50 m depth, ranged from 33.8o/oo to 34.5o/oo and from 34.lo/oo to 34.7o/oo, respectively. Surface waters generally had lower salinity than sub-surface waters did, except at Station 4 which was located at the outer edge of the sampling area.   The T-S curves for stations on the second transet line are shown in Fig. 3. All stations on this trar_set line are located on the continental shelf. These curves were aligned in a sequence from the shelf toward the slope. Water from Stations 5, 8 and 9 has a pattern of T-S curves similar to that of Stations 3 and 4 when the water temperature is greater than 17° C. Water from Stations 5, 8 and 9 can be recognized as water that came from the Kuroshio. Water from the upper layer of Stations 12, 13 and 16 is similar and should be categorized as from the shelf. On the other hand, water on the lower layer of Station 12 had an intermediate salinity between that of the Kuroshio and the shelf. The phenomenon of water mass mixing on the lower water layer at Station 12 can be inferred.  (2) Density Distribution Patterns

Individual Densities
The density of the number of fish eggs from each sampling station ranged from 0 to 31, 765 e.gg.s/1, 000 m 3 ( Table 2). Two patches were found with fish eggs, and their centers located at Stations 2 (24, 133 e.ggs/1, 000 m 3 ) and 15 (31, 764 e.gg.s/1000 m 3 ) , respectively. No eggs were found at Stations 8, 10 and 12, suggesting a high patchiness. The general pattern found was that a high density distribution occurred in the waters of mid-shelf origin (on the north western side) and a low one in waters of oceanic origin. The density of ichthyoplankton from 18 sampling stations ranged between 242 and 19, 649 inds/1, 000 m 3 • The ichthyoplankton density indicated also a patchy distribution peaking at Station 15, where densely distributed fish eggs were also recorded. A concentric ichthyoplankton distribution around Sta tion 15 might indicate a centrifugal dispersion of fish. The ichthyoplankton density was significantly higher on the north-western side (mid-shelf origin) than the south-eastern side (oceanic origin) . When the distribution of ichthy oplankton was compared with that of fish eggs, it was found that the distribution of eggs was less even. In addition, no zero catches for ichthyoplankton were ever TAO Vol. 2, No.2 observed at any sampling station. Therefore, a drift of ichthyoplankton _ during their ontogenic process can be inferred:

Density of the Biomass
The total fish egg biomass in the specified sample sites ranged from 0 to 22.68 g/1, 000 m 3 (Table 2). Two patches centered around Station 2 (7.05 g/l, 000 m 3 ) and 15 (22.68 g/1,000 m 3 ) , were found that were composed of eggs from two different origins as described by density measured by indi vidual counts. Additionally, those eggs from Station 15 had a higher average individual biomass.
The ichthyoplankton biomass f rom the 18 sampling stations ranged from 0.35 to 25.04 g/l, 000 m 3 • T h e ichthyoplankton distribution pattern indicated by its biomass was similar to that measured by the total fish count, i.e. a general distribution pattern with high density in the waters of mid-shelf origin and lower in those of oceanic origin was again indicated.
The station specific biomass for all zooplankton taken by the ichthy oplankton sampling gauge ranged from 0.62 to 2, 126. 92 g/1, 000 m 3 • The highest zooplankton density measured from the biomass depicted a patchy dis tribution at Station 15, where dense ichthy oplankton also occurred. This con centric zooplankton distribution around Station 15 might indicate a dispersion of zooplankton due to eddy convection.

(3) The Isopleth Density Diagram
The isopleth density diagrams of fish egg by weight are shown in Fig. 5. This pattern indicates that the steepest peak comes from the northwestern cor ner. The isopleth line relief axis bends in a northeasterly direction as the gra dient drops. The southern flushing from the Kuroshio caused the density drop. Another radiation axis pointed in a northerly direction. The dilution effect from this southern flushing resulted in sparsly distributed eggs of a density as low as 0.1 g /1, 000 m3 in the area between 25.5° N ,..., 26° N and 122° E ,..., 123° E. The second and the third minor concentrations were found at 25° N, 122.5° E and 26.5° N, 123.5° E, respectively. These minor centers were influenced by water masses from coastal zones in the neighborhood of small islands.  The isopleth diagrams of the number of larval fishes are shown in Fig. 6. A similar pattern is also seen for weight density. Its peak is in the northwestern corner and the relief axis points first toward the southeast, and then around 25.5° N and 122.5° E, the contour representing 500 inds/1, 000 m3 was inter-. sected by the Kuroshio current. The Kuroshio current pushed the contour line of 1, 000 ind8/l, 000 m 3 north-easterly and finally pulled the 500 inds/1, 000 m3 contour line away from the study area in the direction of the Japanese coastal zone. The isopleth diagram of incidentally caught zooplankton as indicated by the biomass is shown in Fig. 7. One major concentration occurred in the north western corner. The dilution effect of the Kuroshio current was apparent in the area of 25.5° N ,..., , 26° N and 122° E ,..., , 123° E. A high density on the eastern side of the study area was found to be as much as 100 g /1, 000 m 3 • Apparently the dilution effect of the Kuroshio current to the density of zooplankton was marginal when compared to the ichthyoplankton pattern. at -100 m (variable 9) and density scores of ichthyoplankton and zooplankton (variables 2, 4, and 5). The position of the stations for the first two principal component axes is shown in Fig. 9. This scatter plot of sampling stations exhibites a straight line alignment; which indicates a good relationship between the first two principal component scores� In this diagram the lower-left of the coordinate system implies attributes of the Kuroshio proper (Stations 1,2,3,4,5,6,7,8 and 9). In keeping with this major tendency, the distance away from the lower-left corresponds to a decreasing trend of the Kuroshio influence. Therefore, those stations scored law with the principal component system indicating that their elements (variables) came from the Kuroshio. On the other hand, the higher scores came from water of East China Sea origin (Stations 14,15,16,17 and 18) and these modest scores indicated a mixture of two type of waters (Stations 10, 11, 12 and 13). This combination of biological and environmental data can therefore be useful as a tool to define water origin and probably vice versa. 5) density of zooplankton in weight; 6) bottom depth; 7), 8), 9) water temperature at surface, -50 m, and -100 m respectively; 10), 11) water salinity at surface and -50 m; 12) and 13) apparent total fluorescence value at surface and _:_50 m; 14) and 15) apparent dissolved oxygen value at -50 m and 100 m. Among those variables variable 1 and variable 3 have been deleted due to lower correlation with the other variables. The first two principal components collected 35.8% and 29.4% variance. An apparent reverse tendency can be traced on the water temperature at -100 m (variable 9) and gathering of ichthyoplankton and zooplankton (variables 2, 4, and 5).

DISCUSSION
T he K uroshio edge exchange area has two types of marine topography, continental shelf and continental slope. The latter emerges from the shelf boundary leading to an abyss of several thousand meters in depth. The shallow shelf bottom and coastal water forces the. Kuroshic;> water off its straight course paralleling the eastern coast of Ta iwan thus forming a convergence with ma rine waters. Consequently due to this convergence there may occur a biological cosmopolitan area. In addition, the compression of sea water may. cause eddy currents forming a local upwelling (Fang, 1980). These two expectations still  remained to be thoroughly verified. On the other hand, the fact that this area is an important commercial fishing ground (Huang, 1986), makes it worthwhile to undertake further studies from the viewpoint of fishery management.
Comparison of results from this preliminary study with similar attempts in this area are not possible since some relevant data are still not available. Tseng (1970) reported a zooplankton survey for this area but the distribution was made through specific occurrence with no abundance estimation, which handicaps the contrast of oceanic and mid-shelf attributes. Irie and Yamaji (1970) described the distribution of the zooplankton biomass in the Kuroshio and adjacent regions. Their diagrams generally depict double concentric con tours; one for the East China Sea and the other for coastal areas of Honchu, Japan. Tzeng (1988) quoted Uda's (1960) generalized oceanographic features for interpretation of mackerel fishing grounds, where surface coastal waters from both the East China Sea and northeastern Taiwan went southeastward, a TAO Vol.2, No.2 sub-surface area of highly saline waters went northwestward and therefore an ocean front formed along the shelf-slope boundary. Fang (1980) and Chern and Wang (1989) have made thorough surveys of temperature distributions or T-S diagrams in the northeastern waters off Ta iwan and upwelling can be tenta tively detected. These temperature profiles were similar to density isoplethes of ichthyoplankton or zooplankton in our study indicating that a non-active mobile or limited mobile article had similar distribution patterns after strong flushing from the Kuroshio current. This preliminary result also agrees with the conclusions of Irie and Yamaji (1970) but with a higher resolution which may point to a local realization on water exchange processes sensu Uda (1960). In fa ct, the zooplankton distribution pattern of Irie and Yamaji (1970) can be treated as the "big" island supplier to the Kuroshio which distributes those or ganisms to a wider range of habitats. More examples of the island supplier effect have been illustrated by isopleth diagrams, such as that for ichthyoplankton. From the supplier-distributor point of view, the Kuroshio current has played an important role in the faunal transport of north eastern Pacific fish species.
Principal component analysis is a powerful tool for multiple separation of attributes from different water masses. The author used this method to try to discriminate between water masses of different origins. It appears that discrimination between two water types using principal analysis and T-S curves analysis are equally valid. In this study, those variables for the ichthyoplankton, zooplankton densities and apparent total fluorescence value, which were closely . related to primary productivity, can be positively loaded high in the first two principal components. Therefore, these variables were relevant candidates for further water mass identification. The fish eggs density variables were deleted in the scatter plot of the stations owing to their low resolution. The noise tied to the density of eggs is an additional subject for further study. More detailed results and higher resolutions for biological water mass identification still remain for continued elaboration.