Kuo-Wei Lan

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.

Temporal Scale of Sea Surface Temperature Fronts Revealed by Microwave Observations

Sea surface temperature (SST) data for three years from the Advanced Microwave Scanning Radiometer for the Earth Observing System are used to statistically evaluate the temporal scales of the global SST fronts (SSTFs). Using the entropy-based edge detection method which is very resistant to impulsive noises, temporal autocorrelation of the dissimilarities of two SST groups across the SSTF is calculated in 10-km-gridded map. In general, the derived temporal scales, defined as e-folding scales in this study, range from 10 to 40 days. Long temporal scales of up to 100 days are found in areas where the stationary ocean currents maintain the frontal structures.

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.

Erratum to: Effects of climate variability and climate change on the fishing conditions for grey mullet (Mugil cephalus L.) in the Taiwan Strait

Grey mullet (Mugil cephalus L.) is one of the most important commercial species of fish in the coastal fisheries of Taiwan. In this study, we analyzed the long-term (1967–2009) records of grey mullet catch per unit effort (CPUE) in the Taiwan Strait (TS) to investigate the influences of climatic indices on the annual catch of grey mullet at multiple timescales. A wavelet analysis revealed that variations in climatic indices, namely the Pacific Decadal Oscillation (PDO), the Oceanic Nino Index, and sea surface temperatures (SSTs) might have affected the abundance and migration behavior of grey mullet in the TS in winter. The CPUE of grey mullet showed significant high correspondence with the annual PDO index (R 2 =0.82, p >0.01). The results suggested that the PDO affects the migration of grey mullet, but that increases in SSTs are a more important influence on the decreased catches of grey mullet after 1980. Mean SSTs increased 1.01 °C at the Chang-Yuen Rise in the TS from 1984 to 2009. The 20 °C isotherms in the TS in the winter also shifted from 23–24°N in 1958–1978 to north of 25°N after 1998. The fishing grounds of grey mullet also shifted to the north following changes in the 20 °C isotherm in the TS. Copyright The Author(s) 2014

Environmental effects on yellowfin tuna catch by the Taiwan longline fishery in the Arabian Sea

In this study, we collected environmental variables to investigate their effects on the catch per unit effort (CPUE) of yellowfin tuna in the Arabian Sea during the period 1980–2005. We used an advanced time series analysis, including a state-space approach to remove seasonality, and wavelet analysis to investigate transient relationships. For large-scale environmental effects, we used the dipole mode index (DMI) to represent the Indian Ocean dipole; for local environmental factors, we investigated sea surface temperature (SST), thermocline depth and chlorophyll-a (chl-a) concentration. The main factors causing interannual variations in the CPUE might change with time. CPUE showed positive correlations with SST and DMI from the beginning of the 1980s to the middle of the 1990s. It also showed a significant coherence with chl-a, especially a long-term positive correlation for the regular longline fishery in 1998–2005 with a periodicity of 2 years. Both regular and deep longline CPUEs were found to have significant coherence with thermocline depth having a periodicity of 3 years. The relations were of opposite signs such that the shallow thermocline depth produced a high CPUE for the regular longline fishery and deep thermocline depth caused a high CPUE for the deep longline fishery.

INTERANNUAL VARIABILITY OF WINTERTIME SEA SURFACE TEMPERATURES IN THE EASTERN TAIWAN STRAIT

Using Advanced Very High Resolution Radiometer (AVHRR) sea surface temperatures (SSTs) at a 1.1-km spatial resolution, we investigated the long-term SST variability in winter in the eastern Taiwan Strait (TS) in 1995~2008. We performed an empirical orthogonal function (EOF) analysis of spatial and time series variances for 15-day mean images.The first three modes in the EOF analysis respectively accounted for 25.9%, 16.61% and 4.59% of variances. In the first mode, the spatial amplitude showed positive values in the northeastern TS and negative values south of Chang-Yuen Rise (CYR). This suggests that warmer water occurred south of the CYR and a southwesterly intrusion of cold water extended north of study area. In the second mode, the lowest SSTs appeared over the CYR from the middle of December to the end of January. Results of the EOF analysis revealed heat exchange and the formation of an oceanic front boundary at the CYR in winter. We also found that average SSTs at the CYR were highly and positively correlated with latitudinal variations of 20°C isotherm, implying that variations in SSTs at the CYR could be applied to examine variation in the China Coastal Current in the eastern TS.

Using Remote-Sensing Environmental and Fishery Data to Map Potential Yellowfin Tuna Habitats in the Tropical Pacific Ocean

Changes in marine environments affect fishery resources at different spatial and temporal scales in marine ecosystems. Predictions from species distribution models are available to parameterize the environmental characteristics that influence the biology, range, and habitats of the species of interest. This study used generalized additive models (GAMs) fitted to two spatiotemporal fishery data sources, namely 1° spatial grid and observer record longline fishery data from 2006 to 2010, to investigate the relationship between catch rates of yellowfin tuna and oceanographic conditions by using multispectral satellite images and to develop a habitat preference model. The results revealed that the cumulative deviances obtained using the selected GAMs were 33.6% and 16.5% in the 1° spatial grid and observer record data, respectively. The environmental factors in the study were significant in the selected GAMs, and sea surface temperature explained the highest deviance. The results suggest that areas with a higher sea surface temperature, a sea surface height anomaly of approximately −10.0 to 20 cm, and a chlorophyll-a concentration of approximately 0.05–0.25 mg/m3 yield higher catch rates of yellowfin tuna. The 1° spatial grid data had higher cumulative deviances, and the predicted relative catch rates also exhibited a high correlation with observed catch rates. However, the maps of observer record data showed the high-quality spatial resolutions of the predicted relative catch rates in the close-view maps. Thus, these results suggest that models of catch rates of the 1° spatial grid data that incorporate relevant environmental variables can be used to infer possible responses in the distribution of highly migratory species, and the observer record data can be used to detect subtle changes in the target fishing grounds.

Satellite Observation on the Exceptional Intrusion of Cold Water and Its Impact on Coastal Fisheries Around Peng-Hu Islands, Taiwan Strait

We used satellite-derived sea surface temperature (SST) data of the winters of 1996–2008 to examine the exceptional intrusion of China Coastal Current into the Taiwan Strait (TS). The long term observation reveals an exceptional cold water intrusion into the southern TS happened in February 2008. The warm Kuroshio Branch Current, which dominates the water around Chang-Yuen Ridge year round, was restricted to the southern Strait. Comparing the SST and wind speed during El Nino/La Nina events, we found that SST was warmer in the El Nino winters (1998, 2003, 2007) than in the La Nina winters (1996, 2000, 2008), and wind speed was more intensive in the La Nina winters than in the El Nino winters. It is suggested that in the winter of 2008, the strong and continuous northeasterly wind caused by La Nina event probably drove the cold Mainland China Costal Current more southward to penetrate into the southern TS north of the Chang-Yuen Ridge and a portion of this current intruded eastward south of the Peng-Hu Islands. The low SST event also significantly damaged marine life and cage aquaculture, causing the death of more than 73 m of resident and coral reef fishes; and at the same time brought increased abundance of migratory species.