Satellite observation on the exceptional impact of cold water intrusion on coastal fisheries around Peng-Hu Islands, Taiwan Strait
Ming-An Lee
1, 2*, Yi Chang
1, Kuo-Tien Lee
1, Kuo-wei Lan
1, and Wei-Juan Hsieh
21Department of Environmental Biology and Fisheries Science, National Taiwan Ocean University. Pei-Ning Rd., Keelung 20224, Taiwan R.O.C.
2Remote Sensing Lab. of Taiwan Ocean Research Institute, Keelung 20224, Taiwan R.O.C.
*
Present email address:: [email protected]
Abstract
An exceptional intrusion of cold water was happened around Peng-Hu islands in February 2008. This cold water event induced a fishery disaster with about more than 70 tons of died fishes. This study was tried to observe this ocean phenomena by using remote sensing data and its exceptional impact on coastal fisheries around Peng-Hu islands. The monthly maps in the winter of 2008 were contrary to the typical features of long-term composites. The continues strong wind drove the cold China Coastal Current intruded more southward into southern Peng-Hu Islands resulted in the SST (sea surface temperature) decreased to 13°C in February of 2008, which blocked the warm current near the north end of the Peng-Hu Channel.
Four major fisheries (Pole Line fishing, Gill net, Long Line, and Set net) were investigated. The Pole Line fishing, Long Line and Gill net, mainly caught the benthic species but the Set net totally caught the pelagic species. Their catches of the former three fisheries in January to March 2008 fell more 83%, 50% and 74%, respectively, while the catches of set net fishery was increased twice than that of average in 1998 to 2007. It suggested that the exceptional cold water intrusion impact on the benthic fishes, but not for pelagic fishes of coastal fishery in this fishery disaster.
Introduction
Trends in large-scale climate change can affect local oceanic environment and marine ecosystems on different scales. A continues strong wind drove the cold China Coastal Current intruded more southward into southern Peng-Hu Islands resulted in the SST (sea surface temperature) decreased to 13°C in February of 2008, which blocked the warm current near the north end of the Peng-Hu Channel (Jan et al., 2002; Chang et al., 2009). Otherwise, it is clear to see that the SST varied in the Taiwan Strait is similar trend with the variation of ONI (Chang et al., 2009). In other words, SST was colder while the cold event (La Nina) occurred (negative ONI), especially sharp decreased in February of 2008. The rapid change in current circulation caused a large SST drop and ecological disaster around the waters of Peng-Hu Islands, that there were more than 70 tons of fish died in a short period of cold SST event (Figure 1). This study attempted to understand circumstances of fishery resource and impact in the waters of Peng-Hu after fishery disaster due to this exceptional intrusion of cold water in February 2008.
Data and Method
Wintertime (January and February) SST images derived from AVHRR (Advanced Very High
Resolution Radiometer) sensors (Figure 2, Chang et al., 2009). For the Peng-Hu fisheries-based data, the monthly catch data of different fishing gears (Pole Line fishing, Gill net, Long Line, and Set net) were collected from the Peng-Hu Fishery association during the period of January to March (wintertime), 1998~2008. All fishes composition caught by these four fisheries were collected and sorted into pelagic and benthic species, for evaluating the influence impacted by this exceptional cold water intrusion event.
Result and conclusion
12-year (1996-2007) monthly mean composite SST maps in winter are shown in Figures 2a and 2b. In January (Figure 2a), the cold China Coastal Current (<18°C) flowed southward along the Chinese coast, while the warm Kuroshio Water and South China Sea Water (>20°C) were surrounding the Peng-Hu Islands in the eastern Taiwan Strait. By February (Figure 2b), the water of lower than 15°C was still occupying waters along the Chinese coast, however, the cold water tongue north of CYR faded gradually.
The monthly maps in the winter of 2008 were contrary to the typical features of long-term composites. SST in January (Figure 2c) illustrated that the warm water guided by Peng-Hu Channel extended more northward and intruded into the northern end of Chang-Yuen Ridge. On the other hand, clearly a cold water of lower than 10°C flowed into the western Taiwan Strait along the Chinese Coast in February of 2008 (Figure 2d). Unlike the feature of long-term average, the cold water tongue intruded into the south of Chang Yuen Ridge (Figure 2d) with the SST lower than 13°C in February of 2008, which blocked the warm current near the north end of the Peng-Hu Channel. In consequences, the water of lower than 15°C dominated the most waters around Peng-Hu Islands.
Comparing with long-term average production of 1998 to 2007, those of catch by using Pole lines, Long line and Gill net fell more 83%, 50% and 74%, respectively, than that in 2008 (Table 1). However, the production in 2008 by using Set Net fishery was increase twice than that of the 11-year average production. In figure 3, it appeared the time series of total production (ton) in January to March (square bar) from 1998 to 2008 with its predicated production (solid triangle) in 2008 for these four fisheries. It showed a decline trend by year for Pole lines, Long line and Gill net fisheries, and sharply decreased in 2008 after the cold SST event occurred. Figure 4 showed the daily catch weight of Hairtail fish (Trichiurus lepturus) caught by Set net with SST from January to February, 2006~2008. In generally, the catch of Trichiurus lepturus was higher in January with slight lower SST (< 22°C) and then dropped off in February due to increase SST in 2006~2007.
Shih (2004) indicated that the Trichiurus lepturus is characterized by long-distance migration from
East China Sea to south Taiwan Strait for over wintering and spawning. It seems that in general the
Trichiurus lepturus will disappear from the waters around Peng-Hu islands in February. But in
February 2008, it is interesting that the Trichiurus lepturus was immediately increased as the cold
water intruded into the waters around Peng-Hu islands. It is thus suggested that the more southward
intrusion of cold current from north may bring the school of Trichiurus lepturus more southward
into Peng-Hu waters. Currently however, it is difficult to give an answer that how much of the catch fluctuation is actually due to climate change induced cold SST event. The long-term environmental information, ecological interactions, and the fishing effort data are necessary for further analyzing in the future.
References
Chang, Y., K. T. Lee, M. A. Lee and K. W. Lan, 2009: Satellite observation on the exceptional intrusion of cold water in the Taiwan Strait. Terr. Atmos. Oceanic Sci., 20(4), 661-669. doi:10.3319/TAO.2008.08.07.01(Oc).
Jan, S., J. Wang, C. S. Chen and S. Y. Chao, 2002: Seasonal variation of the circulation in the Taiwan Strait. Journal of Marine System, 35, 249-268.
Shih, N. T., 2004: Age, growth and reproduction of cutlassfishes, Trichiurus spp. in the waters off Taiwan. Master thesis, Department of Environmental Biology and Fisheries Science, National Taiwan Ocean University, Keelung, Taiwan, 101 pp.
Table 1 The pelagic and benthic species composition and the average production of 1998-2007 and 2008 for the four fisheries
Fishing methods Pole and Lines Boote
Misc. Fish Long
Line Gill Net Set Net
Pelagic species 4 (%) 1 (%) 60 (%) 99 (%)
Benthic species 96 (%) 99 (%) 40 (%) 1 (%)
Average production
(1998-2007) 597.62 (t) 351.3 (t) 1073.5 (t) 191.67 (t) Production of 2008 101.9 (t) 175.1 (t) 278 (t) 562.3 (t)
