Wei-Cheng Su

Influence of hydrographic features on larval fish distribution during the south-westerly monsoon in the waters of Taiwan, western North Pacific Ocean

The spatial distribution patterns of fish larvae and the relationships with hydrographic conditions in the waters surrounding Taiwan were studied in August 2004 when the south-westerly monsoon prevailed. A total of 6566 fish larvae were identified, belonging to 80 families, 129 genera and 230 species. Cluster analysis revealed two station groups, one associated with the South China Sea Surface Current (SCSSC) and the other with the Kuroshio Current (KC). The SCSSC group was characterized by mostly coastal and neritic species, and the KC group was predominated by oceanic species. Larger fish larvae were generally more abundant in the neritic waters west of Taiwan where the SCSSC prevails than in the waters east of Taiwan where the KC prevails. The combination of chlorophyll a, zooplankton and nitrite best explained the relationship of larval fish distribution and environmental variables, implying that the distribution of fish larvae in summer was closely linked to the food source. The changing of monsoons potentially affects the succession of water masses and the transport and assemblage of fish larvae in this study area.

Winter spatial distribution of fish larvae assemblages relative to the hydrography of the waters surrounding Taiwan

The aim of this study was to investigate the species composition and distribution of fish larvae in relation to hydrographic conditions in the waters surrounding Taiwan Island (TI) in February 2003. In total, 242 kinds of fish larvae belonging to 127 genera and 75 families were recognized. Among these, 109 taxa were identified to the family or genus level, others to the species level. The 12 predominant types, which constituted 71% of the total fish larvae, were Engraulis japonica, Scomber sp., Diaphus spp., Benthosema pterotum, Carangoides ferdau, Embolichthys mitsukurii, Maurolicus sp., unidentified Myctophidae, Gonostoma gracile, Trichiurus lepturus, unidentified Gobiidae, and Myctophum asperum. The distribution of fish larvae showed a clear association with water masses around TI, with higher abundances and lower species richness northwest of TI where the China Coastal Current prevails, and lower abundances and higher species diversity east of TI where the Kuroshio Current dominates. Cluster analysis distinguished three station groups and four species groups, and the distribution patterns of fish larvae also corresponded to hydrographic conditions. The total abundances of fish larvae and eight of the 12 predominant taxa showed significant and positive correlations with zooplankton abundance, which suggests that food source might be a key factor determining the abundance and distribution of fish larvae during the winter.

Spatio-temporal distributions of tuna species and potential habitats in the Western and Central Pacific Ocean derived from multi-satellite data

In this study, potential tuna habitats in the Western and Central Pacific Ocean (WCPO) area from 1998 to 2005 were identified using a combination of multi-satellite oceanographic data related to sea-surface temperature (SST), chlorophyll-a concentration (Chl-a) and sea-surface height anomaly (SSHA). Histogram analyses of tuna catch per unit effort (CPUE) by the Taiwanese purse seine fishery in relation to the satellite oceanographic data were used to determine tunas preferred ranges of the three satellite-derived oceanographic parameters. The highest CPUE corresponded to areas where SST ranged from 29.0–29.5°C, Chl-a ranged from 0.0–0.1 mg m−3 and SSHA ranged from 0.0–5.0 cm. Local areas within the WCPO with similar satellite-derived oceanographic parameters were assumed to be potential tuna habitat, and a spatially explicit model was developed to determine their locations. It was found that the potential tuna habitats were distributed in warm pools, in the vicinity of the western cold tongue in the equatorial Pacific. Consequently, by using these potential habitats, tuna-fishery management and protection of habitats of tuna species can be improved, and these can possibly form the basis of fishing-ground forecasting in the future.

The Kuroshio variations from satellite-derived sea surface temperature and Argos satellite-tracking Lagrangian drifters

This study aimed to identify the spatial and temporal variations of the Kuroshio using satellite-derived Sea Surface Temperature (SST) data and a total of 585 Argos satellite-tracking Lagrangian drifters during 1987–2006 in the northwestern Pacific Ocean. The threshold of high surface velocity, more than 75 cm s–1, was used to determine the location of the Kuroshio Current and found that its width ranged from 50 to 100 km. Moreover, this study first conducted a synoptic view for three predominant regions of the Kuroshio, including the area east of Taiwan, the area east of the East China Sea (ECS) and the area south of Japan. We found that the Kuroshio flows at a three-step increase in speed from upstream to downstream. The maximum mean surface velocity, 153 cm s–1, occurred to the south of Japan. Furthermore, the higher surface velocity of the Kuroshio was usually during the summertime (from April to September). In addition, the four-cruise CTD (Conductivity, Temperature, Depth)-transect data observed in January 2005 were collected to identify and validate the path of the Kuroshio axes. The results showed that the measure of 17.5°C at a depth of 200 m could be regarded as an indicator of the Kuroshio axis, corresponding with the maximum surface velocities of the Argos drifters. The monthly satellite-derived SST values of the Kuroshio were usually more than 10°C, so it was difficult to find a specific isotherm to express the location of the Kuroshio. However, extensive Kuroshio fronts derived from satellite SST data were found in this study. These Kuroshio fronts, associated with high SST gradients, were usually located on the boundary between the warm water of the Kuroshio and the colder water of the shelf, corresponding with the boundary of high surface velocity of the Argos drifters. Consequently, this study suggests that these Kuroshio fronts can be regarded as an effective indicator of the Kuroshio paths and its main axes.

Identification of the Relationship Between Environmental Features and Copepod Abundances in the Kuroshio Waters Adjacent to Eastern Taiwan Using Generalized Additive Models

Spatial distribution patterns of copepod abundances were analyzed in relation to environmental variables in the Kuroshio waters adjacent to eastern Taiwan. Generalized additive models (GAMs) were applied to examine the relative influence of environmental factors on copepod abundance collected from 2005 to 2008 in this region. To construct a GAM, relationships were analyzed for physical and biotic variables (season, chlorophyll-a concentration, temp at 5 m, temp at 100 m, temp differences between 5 m and 100 m, salinity at 5 m). Stepwise GAM building revealed the relative importance of the variables in explaining the variance in copepod abundance. The variables ranked (1) season, (2) chl-a concentration, (3) temp_100 m, (4) sal_5 m, (5) temp_5 m and (6) temp_diff in decreasing order. Time series of copepod abundance standardized for physical and biotic factors quantified in the GAM implied a seasonal trend, which was highest in summer-autumn period and lowest in winter. The spatial pattern of copepod abundance was characterized by noticeable high abundances associated with higher chlorophyll-a concentration, lower temperature at 5 m and lower salinity at 5 m in the northwestern (121.5-122°E, 23.5-24.5°N) and southwestern (121-121.5°E, 22-23°N) nearshore region of the study area.