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Based on bathymetric data collected in recent years, the topographic features and their structural implications on the northern offshore area of Taiwan are described. The Chilung Shelf is representative of the subma rine physiographic units in this region. It occupies the shallow water area eastward to the City of Chilung between the Chilung Valley and the Mien Hua Canyon. To its north and west, the East China Sea Shelf reveals a gentle ocean bottom such that water depths on its edge (the shelf break) are found to be at least 150 meters shallower than those on the Chilung Shelf.Taiwan is a product of orogeny which resulted in compressive imprints throughout the island. On the Chilung Shelf, a series of topographical lin eaments extending from onland Taiwan and oriented in the SW-NE direc tion are present; however, seismic profiles indicate that these lineaments are a factor of normal faulting mechanics. It is suggested that the offshore area of northern Taiwan has been under a different tectonic stress from that represented on the island.

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The relationship between production and reef pile size was analysed using standing stocks of fishes surveyed at three established artificial reef (AR) habitats in the subtropical waters off the northern coast of Taiwan during April-August 1999. For a closer look, the fish assemblage was further divided into visitors and residents, and biomass was used as a parameter to represent production. Visitors were characterised by sporadic occurrences and highly variable school sizes. However, large schools (containing more than 200 kg of biomass) occurred only around small AR piles (consisting of less than 15 units of 2-m 3 concrete reef). Considering visitors alone, the correlation between biomass and pile size was not significant for the combined data from three sites. Changes in the standing stock of residents followed a clearer trend, and the relationship could be fitted to a sigmoidal equation. This trend shows that collections of the residents tended to increase until the pile reached a size of 20-30 reef units. Thereafter, the increase in biomass slowed, and maximum biomass would eventually be reached only with a further increase in the pile size. Predictions of the efficiency of unit reefs for different pile sizes indicate that a pile consisting of 4-10 units of this type of reef would be most effective in terms of biomass of residents. Nevertheless, a size of 15 units is recommended when the behaviour of visitors is taken into account.

Operators tow side-scan sonar transducers at the optimum height from the bottom to produce better quality side-scan sonographs. This can be done only if the bottom is moderately flat. Sonographs with maximum tonal intensity are easily produced on steep slopes e.g. valley banks. Strong signatures are recorded over all ranges due to the poor geometry of sonographs produced without acoustic shadows. In order to create clear scanned images of a steep slope using a side-scan sonar, rotation of the towed fish or tilting the transducer face to the dipping slope surface is suggested. Cases conducted on the shore of the Wushieh Reservoir, Taiwan, in which the bank slopes varied from 10u to 45u are discussed in this work. In general, the angle of the scanned slope determines the degree of towed fish rotation angle. The rotation angle is set at a smaller angle than the slope, giving stronger returns from the lower slope. In contrast, if the rotation angle is set at a larger angle a null return outside the insonified area in the lower slope is produced. A sub-aqueous landslide was observed in the side-scan sonographs using this strategy. This study also gives results that affirm the value of a side-scan sonar as a powerful tool for underwater inspection.