Ming-An Lee

Seasonal and spatial distribution of chlorophyll-a concentrations and water conditions in the Gulf of Tonkin, South China Sea

The Gulf of Tonkin is a semi-closed gulf northwest of the South China Sea, experiencing reversal seasonal monsoon. Previous studies of water conditions have been conducted in the western waters of the gulf, but very few studies of the Chlorophyll-a (Chl-a) distribution have been carried out for the entire gulf. The present study investigates seasonal and spatial distributions of Chl-a and water conditions in the Gulf of Tonkin by analyzing Sea-viewing Wide Field-of-View Scanner (SeaWiFS) derived Chlorophyll-a (Chl-a), in situ measurements, sea surface temperatures (SST), and other oceanographic data obtained in 1999 and 2000. The results show seasonality of Chl-a and SST variations in the Gulf of Tonkin, and reveal phytoplankton blooming events in the center part of the gulf during the northeast monsoon season. In summer, Chl-a concentrations were relatively low (<0.3 mg m 3 ) and distributed uniformly throughout most of the area, with a belt of higher Chl-a concentrations along the coast, particularly the coast of Qiongzhou Peninsula; in winter, Chl-a concentration increased (0.5 mg m 3 ) in the entire gulf, and phytoplankton blooms offshore-ward from the northeast coast to the center of the gulf, while Chl-a concentrations reached high levels (0.8–1 mg m 3 ) in the center of the blooms. One peak of Chl-a concentrations was observed during the northeast monsoon season in the year. SST were high (27–29 jC) and distributed uniformly in summer, but lower with a large gradient from northeast (17 jC) to southwest (25 jC) in winter, while strong northeast winds (8–10 m/s) were parallel to the east coast of the gulf. Comparison of Chl-a values shows that SeaWiFS derived Chl-a concentrations match well with in situ measurements in most parts of the gulf in May 1999, but SeaWiFS derived Chl-a are higher than in situ data in river mouth waters. The seasonal variation of Chl-a concentrations and SST distribution were associated with the seasonally reversing monsoon; the winter phytoplankton blooms were related to vertical mixing and upwelling nutrients drawn by the northeast wind. D 2003 Elsevier Science Inc. All rights reserved.

Evaluating the accuracy of morphological identification of larval fishes by applying DNA barcoding.

Due to insufficient morphological diagnostic characters in larval fishes, it is easy to misidentify them and difficult to key to the genus or species level. The identification results from different laboratories are often inconsistent. This experiment aims to find out, by applying DNA barcoding, how inconsistent the identifications can be among larval fish taxonomists. One hundred morphotypes of larval fishes were chosen as test specimens. The fishes were collected with either larval fish nets or light traps in the northern, southern and northwestern waters of Taiwan. After their body lengths (SL) were measured and specimen photos were taken, all specimens were delivered, in turn, to five laboratories (A–E) in Taiwan to be identified independently. When all the results were collected, these specimens were then identified using COI barcoding. Out of a total of 100 specimens, 87 were identified to the family level, 79 to the genus level and 69 to the species level, based on the COI database currently available. The average accuracy rates of the five laboratories were quite low: 80.1% for the family level, 41.1% for the genus level, and 13.5% for the species level. If the results marked as “unidentified” were excluded from calculations, the rates went up to 75.4% and 43.7% for the genus and species levels, respectively. Thus, we suggest that larval fish identification should be more conservative; i.e., when in doubt, it is better to key only to the family and not to the genus or species level. As to the most misidentified families in our experiment, they were Sparidae, Scorpaenidae, Scombridae, Serranidae and Malacanthidae. On the other hand, Mene maculata and Microcanthus strigatus were all correctly identified to the species level because their larvae have distinct morphology. Nevertheless, barcoding remains one of the best methods to confirm species identification.

Wintertime sea surface temperature fronts in the Taiwan Strait

[1] We present wintertime variations and distributions of sea surface temperature (SST) fronts in the Taiwan Strait by applying an entropy-based edge detection method to 10-year (1996-2005) satellite SST images with grid size of 0.01°. From climatological monthly mean maps of SST gradient magnitude in winter, we identify four significant SST fronts in the Taiwan Strait. The Mainland China Coastal Front is a long frontal band along the 50-m isobath near the Chinese coast. The sharp Peng-Chang Front appears along the Peng-Hu Channel and extends northward around the Chang-Yuen Ridge. The Taiwan Bank Front evolves in early winter. As the winter progresses, the front becomes broad and moves toward the Chinese coast, connecting to the Mainland China Coastal Front. The Kuroshio Front extends northeastward from the northeastern tip of Taiwan with a semicircle-shape curving along the 100-m isobath.

Nutrient supply in the Southern East China Sea after Typhoon Morakot

Author Posting.

Using remote-sensing data to detect habitat suitability for yellowfin tuna in the Western and Central Pacific Ocean

The empirical habitat suitability index (HSI) has been widely used to examine the habitat characteristics of terrestrial animals, though rarely used in highly migratory fish such as tuna. This study used the geographic information system technique to establish empirical models of HSI for yellowfin tuna (YFT) in the Western and Central Pacific Ocean (WCPO). Daily catch data from the Taiwanese purse seine fishery during 2003–2007 were aggregated monthly into sequential degrees before match processing the conducted data to obtain monthly remote-sensing data for multi-environmental factors, including sea surface temperature (SST), chlorophyll-a (chl-a), sea surface height (SSH) and sea surface salinity (SSS). According to the frequency distribution of each factor on which YFT were caught, this study transformed the values of the four factors into a suitability index (SI) ranging from low to high (0–1). These SI values were consequently combined into different empirical HSI models, and the optimum models were selected using the general linear model. The optimum empirical HSI for YFT in the study area was converted for SI (SST, SSH, chl-a and SSS) using the arithmetic mean model, of which the correct prediction rate was 71.9%. An agreement was present between the average HSI and total YFT catch. Furthermore, the high HSI area corresponds with the displacement of catch per unit effort (CPUE).

Fine-scale sea surface temperature fronts in wintertime in the northern South China Sea

This study presents finer structures and inter-seasonal evolutions of sea surface temperature (SST) fronts in wintertime in the northern South China Sea (SCS) by applying an entropy-based edge detection method to 7-year (2001–2007) satellite-derived SST images with a grid size of 1 km. From monthly mean maps of SST front, six significant SST fronts were defined in the northern SCS. This study not only reveals the earlier defined frontal bands, but also provides finer structures and gradient variability of the fronts. One front alongside Luzon Island (the Philippines), which was undetected by a gradient-based method in previous studies, is well defined by the entropy-based method used in the present study. This result suggests that the gradient-based method might not be appropriate for front detection in coastal waters or marginal seas, but using SST data of lower spatial resolution SST data might not be suitable to describe the detailed structure of short or small fronts through the sophisticated edge detection methods.

Wintertime high-resolution features of sea surface temperature and chlorophyll-a fields associated with oceanic fronts in the southern East China Sea

The southern East China Sea (ECS) and the sea north of Taiwan comprise an important transit area between the coastal waters of mainland China and the open water of the western North Pacific. Their wintertime dynamical features were examined using long‐term high‐resolution satellite‐derived sea surface temperature (SST) and chlorophyll‐a (Chl‐a) images. Along the Chinese coast, a cold‐water tongue with an SST of less than 15°C extended southwestwards from the ECS in December, intruded into the Taiwan Strait (TS) in January, and extended further southwestwards in February. Conforming with the cold SST development, the Chl‐a concentration starts to increase along the 50‐m isobath. However, in the sea north of Taiwan, the SST and the Chl‐a front between the shelf region and the eastern open ocean was semicircular in shape, left the shelf break and extended northeastwards. It was found that, along the semicircular Kuroshio front in the sea north of Taiwan, alternative cold and warm fronts appeared in the geographically fixed area in January and February. Based on the investigation of the snapshot SST images, their systematic appearance is attributed to an SST pattern similar to frontal eddies at the Kuroshio northern front. High concentrations of Chl‐a (>2.0 mg/m3) were found at the warm front and the offshore cold front patch.

INFLUENCE OF THE MARINE ENVIRONMENT VARIABILITY ON THE YELLOWFIN TUNA (THUNNUS ALBACARES) CATCH RATE BY THE TAIWANESE LONGLINE FISHERY IN THE ARABIAN SEA, WITH SPECIAL REFERENCE TO THE HIGH CATCH IN 2004

In this study, we collected Taiwanese longline (LL) fishery data and environment variables during the period of 19982004 to investigate the relationship between LL catch data of yellowfin tuna (YFT) and oceanic environmental factors using a principal component analysis (PCA). Results of the PCA showed that monthly variations in catch per unit effort (CPUE) values were significantly correlated with the sea surface temperature (SST), subsurface temperature at 105 m, thermocline depth (horizontal) gradient magnitude, chlorophyll-a concentration, and fish size. April and May were the warmest months of the year in terms of the SST, and the thermocline was generally deep. After July, a drop in the temperature below the preferred temperature range for YFT is probably the reason that the CPUE subsequently decreased in the period of 1998-2003. It was suggested that the CPUE by age at a given time was significantly affected by chlorophyll-a concentrations 1-3 months prior to that time. The lower thermocline depth gradient magnitude enhanced the aggregation density of YFT in 2004 which showed that the high catch and high CPUE of the YFT fishery increased from the western to the eastern Arabian Sea.

Feeding Ecology of Juvenile Yellowfin Tuna from Waters Southwest of Taiwan Inferred from Stomach Contents and Stable Isotope Analysis

Abstract The Yellowfin Tuna Thunnus albacares is one of the major fish species caught around subsurface fish aggregation devices (FADs) in the waters southwest of Taiwan. However, how it interacts with other organisms around FADs is poorly known. In this study, the diet and feeding habits of juvenile Yellowfin Tuna were estimated from the analysis of stomach contents from 1,477 specimens with FLs ranging from 24 to 108 cm and stable isotope analysis (202 specimens) collected around FADs in the waters southwest of Taiwan. The analysis of stomach contents indicated that juvenile Yellowfin Tuna with FL < 50 cm mainly feed on larval purpleback flying squid Sthenoteuthis oualaniensis, larval shrimps, and zooplanktonic organisms such as amphipods. Yellowfin Tuna with FL of ∼50 cm switch their diet to teleost fishes such as Japanese Barracudina Lestrolepis japonica, Skinnycheek Lanternfish Benthosema pterotum, and fishes in the families Exocoetidae and Scombridae. Stable isotope analysis indicated that the δ15N values ranged between 6.2‰ and 12.6‰, and the estimated trophic position varied from 3.18 ± 0.24 for tuna with FL < 30 cm, while it reached 4.59 ± 0.50 for those with FL > 50 cm and 4.75 ± 0.06 for those with FL > 90 cm. Based on the distinct diet shift of the juvenile Yellowfin Tuna, demonstrated by both stomach contents and stable isotope analyses, this study concluded that the tuna shift their diet at approximately 50 cm FL.

Ichthyoplankton Community Associated with Oceanic Fronts in Early Winter on the Continental Shelf of the Southern East China Sea

The study of ichthyoplankton community associated with oceanic features on the continental shelf of southern East China Sea (ECS) was conducted in early winter from 2006 to 2009. The species composition of the ichthyoplankton and its spatial distribution was classified and compared with oceanic front. A total number of 1,809 fish larvae individuals were sampled from 5 field surveys, and represented 76 families and 137 species. Using the Bray-Curtis dissimilarity, the abundance of species found to exceed 0.5% of the total sample from 36 stations were classified into four groups: China Coastal Group, Mixed Shelf Group, Taiwan Strait Group, and Kuroshio Group. The shallow water species Sebastiscus marmoratus dominated the China Coastal Group, while the Mixed Shelf Group was dominated by Saurida spp., Trichiurus lepturus, and Bregmaceros spp. The Taiwan Strait Group was dominated by Engraulis japonicus and Benthosema pterotum. The dominant species in the Kuroshio Group was found to include Sigmops gracile and unidentified Myctophid larvae. The spatial distribution of the ichthyoplankton community groups was superimposed on a thermal front map of the study area. This clearly revealed that the China Coastal Front could be the boundary which separated the China Coastal Group on the west and the Mixed Shelf Group on the east. In addition, ichthyoplankton were found to increase in abundance and diversity in the region close to the Kuroshio Front.