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and Tatarskiy Strait link the Sea of Japan to theOkhotsk Sea in the north. The major oceanographicfeatures in the Sea of Japan are a polar front at latitude40°N and the inflow of the Tsushima Warm Current(TWC) from the East China Sea (Isobe et al. 1994). TheTWC forms 3 branches (Kawabe 1982), and Isobe(1999) showed that 66% of the volume transported bythe TWC comes directly from the Kuroshio region inautumn. The oceanographic features of the Sea ofJapan probably influence the spatial and temporal distributionof Todarodes pacificus. The locations of anumber of squid fishing areas have been shown to berelated to environmental phenomena such as SSTdetected by satellite remote-sensing (Kiyofuji et al.2001, Waluda et al. 2001a,b). Satellite imagery isextensively used in fisheries studies to identify relationshipsbetween spatial distribution and environmentalvariables such as SST and ocean color. Satelliteremote-sensing of SST is also a useful tool for describinglarge-scale oceanic phenomena and related fishdistributions. Despite such studies, the large-scale distributionof T. pacificus is still difficult to determine, assurveys by research vessels are asynchronous in termsof spatial and temporal observations. New methodologiescapable of deriving data on the spatial and temporalvariability of T. pacificus distributions are required,especially given the increased need for large-scaleaccurate stock management.
Squid fishery is of 3 types: distant, offshore and
coastal. The Japanese squid fishing vessels operate at
night, using powerful lights to attract the squid. These
lights can be observed on nighttime OLS (Operational
Linescan System) images of the DMSP (Defense Meteorological
Satellite Program). Although the number of
squid fishing vessels has been decreasing from 1994 to
1999, total numbers were still above 20 000 in 1999
(Fig. 3). In the images, bright-light areas around Japan
are believed to be fishing vessels, especially the lights
of the squid vessels targeting Todarodes pacificus.
DMSP/OLS images have previously been used to identify
urban areas (Imhoff et al. 1997, Owen et al. 1998).
In terms of fishery oceanography, Cho et al. (1999),
Kiyofuji et al. (2001), Rodhouse et al. (2001) and
Waluda et al. (2002) examined nighttime visible
images to determine the spatial distribution of
fishing vessels. Cho et al. (1999) and Kiyofuji et al.
(2001) determined that the bright areas in the OLS
images, created by 2-level slicing, were caused by
light produced by the fishing vessels. Rodhouse et
al. (2001) reported the frequency of light occurrences
in cloud-free imagery, and associated these
lights with fishing vessels. Waluda et al. (2002)
analyzed a relationship between the number of lit
pixels in DMSP/OLS nighttime visible images and
the number of fishing vessels around the Falkland
Islands’ Illex argentinus fishery. Kiyofuji et al. (in
press) examined the relationship between the numbers
of pixels in the DMSP/OLS imagery and the numbers
of fishing vessels, and demonstrated that fishing vessel
numbers can be estimated from DMSP/OLS nighttime
visible images in the Sea of Japan. However, there
remains the problem of transforming a wide range of
digital numbers in images for lighted pixels into
classes differentiating the actual fishing vessels from
light reflected by the sea surface.
For this study, we assumed that squid were caught in
areas where fishing vessels were located. Thus, based
on fishing vessel locations, we believe it is possible to
estimate the spatial and temporal distribution of
Japanese common squid. This study also aimed at
developing a methodology for identifying the fishing
fleet in DMSP/OLS visible images of the Sea of Japan,
and examining seasonal variability in fishing area. We
sought to provide a new perspective on the specific
distribution of fishing areas, and an innovative analysis
that not only examines fishing area formation, but also
traces migrations throughout the Sea of Japan.
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