Hsueh-Jung Lu

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.

Species identification of fish shoals from echograms by an echo-signal image processing system

Abstract Echograms of 229 fish shoals, including 35 skipjack, 43 round scad, 49 horse mackerel, 60 anchovy and 42 larval fish, were collected in the coastal waters of northeast Taiwan from 1991 to 1993. Using these data, the echo-signal image processing system (EIPS) was developed for species identification of echograms of fish shoals. The system consisted of programs for digital image transformation, digital image processing, measurement and computation of shoal descriptors, and discriminant functions for species identification. Also, principal component analysis, variable clustering analysis and stepwise discriminant analysis were conducted to determine the relationships among the descriptors. The important shoal descriptors for species identification were those related to external structure of the shoals (area, perimeter, height, width, axis length, circularity, rectangularity and number of pixels) and to internal structure of the shoals (mean value, standard deviation, skewness and curtosis of signal amplitude). The former explained more variances than the latter. The accuracy of species identification by the system was 98% for round scad, 97% for anchovy, 94% for skipjack, 91% for larval fish, and 67% for horse mackerel.

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).

On the relationship between El Niño/Southern oscillation and South Pacific albacore

The influence of El Nino/Southern oscillation (ENSO) episodes on South Pacific albacore (Thunnus alalunga) is analyzed using historical catch per unit effort (CPUE) data collected from the Taiwanese distant-water longline fishing fleet, Southern oscillation indices and large scale measurements of seawater temperature. Two effects were identified from a time series analysis: (1) relatively low CPUE followed the onset of ENSO episodes with 8 and 4 yr time lag in fishing grounds between 10°S and 30°S and south of 30°S, respectively; and (2) towards the beginning of ENSO episodes CPUE was slightly higher in fishing grounds between the equator and 10°S. The time lag of the first effect corresponds to the time intervals that would be expected before the recruitment for the fish spawned during ENSO episodes. The second effect is interpreted as a result of the anomalous temperature structure of the seawater. The rise of the mixed layer depth (MLD) in the equatorial region as well as the narrowing volume of water comprising the albacores temperature preference in the subtropical region would both be expected to lead to a (slight) increase in the hook rate of albacore in the two regions.

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.

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.

Relationships between CPUE fluctuation of southern bluefin tuna and ocean temperature variability in the Central Indian Ocean

In this study, catch and effort data of southern bluefin tuna (SBT) from Taiwan longliners operating in the Central Indian Ocean (CIO) during 1982 to 2003 were compiled and their catch per unit effort (CPUE) was standardized using the generalized linear model (GLM). The GLM includes factors such as year, season, by-catch, latitude, sea surface temperature (SST) and the interactive effects among factors. The standardized CPUE and its relationship with SST fluctuation were then analyzed to understand the effects of fishing ground SST variations on CPUE of SBT, as well as their connection to El Niño-Southern Oscillation (ENSO) events. The standardized CPUE in the CIO seemed to oscillate with the sea surface temperature anomalies (SSTA) between 30 and 50°S where SSTA fluctuations were prolonged and slower than the ENSO cycle. It is then very likely that fishing conditions at the CIO fishing ground were influenced by the expansion of the cold water mass from the Southern Ocean, and the colder SST is beneficial to increasing SBT catch rate.

A Study on the Effect of Moon Phase to the Hatching of Swordtip Squid (Uroteuthis Edulis)

The catch of swordtip squid (Uroteuthis edulis), one of the major target species of torch-light fisheries in the northern Taiwan, showed declining trend in recent decades and became a highly concerned issue. Swordtip squid artificial breeding is still impossible at present stage. Pool understanding of the control factor for spawning behavior is an important reason. In order to understand the growth rate, timing of hatch among different spawning seasons as well as its relationship with environment, we sampled 1,369 swordtip squids from January, 2010 to October, 2010. We back calculated the hatching day of swordtip squid through growth increment reading of the statoliths. The results showed that spawning and hatching occurred all year-round in the waters off northeastern Taiwan, but two massive hatching occurred in late fall and early spring. The growth rate for winter cohort (0.015/yr) was higher than the other groups hatched in the other seasons. No matter the squids hatched in what seasons, the hatch frequency during full moon period (lunar days 11-19) were higher than those during new moon (lunar days 25-5) and crescent moon (lunar days 6-10 and 20-25) periods. Under natural environment, ambient luminance and tidal condition of spawning ground are important control factors for spawning behavior of swordtip squid.

The impact on food security and future adaptation under climate variation: a case study of Taiwan’s agriculture and fisheries

According to Food and Agriculture Organization and Intergovernmental Panel on Climate Change reports, climate change will lead to a severe food-supply problem. In the future, food production will continually decrease because of aggravated effects of climate change, causing food production to continually decrease. Food production will be unable to satisfy the demand of the global population, leading to a food-security crisis. As the world population continues to increase, the shortage of food will become increasingly severe, particularly for those located in “climate impact hotspots” of tropical, subtropical, small-island countries, and countries that are dependent on imports to meet domestic demand such as Taiwan. Numerous Taiwanese studies have suggested that agricultural and fishery productivity has declined because of climate variation, which may cause changes and instability in food quantity and quality, and increase deficiency and uncertainty in the food supply. Therefore, to discuss the risks posed by climate change to the stability of food supply and demand, this paper, taking Taiwan as a case, explored the impact of climate variation on food security and future adaptation strategies. TaiCCAT’s supportive system for decision-making (TSSDA) was adopted here to assess and analyze the current situations of agricultural and fisheries production and supply, as well as future food supply risks, in addition to evaluating the deficiencies in the existing climate adaptation strategies in order to plan and revise feasible future adaptation alternatives. Based on the rule of risk management, the adaptation strategies recommended in this study were differentiated into two categories: proactive adaptation and planned adaptation. Proactive adaptation is emphasized to counter the uncertainty of food production, which increases the difficulty of production and necessity to import food. Conversely, planned adaptation can be used to manage the uncertainty of food supply to implement adjustments in production and marketing, as well as to mitigate the impact of climate variation.