Studies on sustainable development and comparative ecological evaluation of set-net fishery in Taiwan

The set-net fishery of Taiwan was examined in this study in the context of ecological sustainable development of a unique fishery. Set-net fishery, unlike many other active forms of fishing, involves static gear set in near shore coastal environments and targets migratory pelagic fish. This study is the first comprehensive examination of a set-net fishery and draws on a comprehensive literature (published mainly in Taiwanese). Data are presented on annual production in the context of regional fishing cooperators which have been established to manage the set-net fishery. Information on fishing technology and the application of set-net gears in coastal waters is presented including net design and operation through extending questionnaire census responses. Detailed analyses of the catch from representative set-net fisheries present a comparative evaluation of species composition including interannual and seasonal variation. Sources of variation in the catch rates of set-nets are evaluated, particularly meteorological and environmental data (e.g. sea surface temperature). The results of the present study reveal that catches of giant trevally, yellowfin tuna, Hawaiian tenpounder, Japanese barracuda and sword fish are correlated with mean monthly water temperature. Only Spanish mackerel, which were associated with peak catches in winter, were negatively correlated with monthly mean temperature. Sea surface temperature and atmospheric pressure are consistent with the hypothesis that ocean currents and their variation influence the catch rates of set-nets. Thus, pelagic fish including tuna, mackerel and trevally follow near-shore coastal currents making those fish vulnerable to capture by set-nets. Studies on the bathymetry and underwater topography near set-nets revealed an association with depth contours and catch rates of fish. Fish are more likely to be caught in set-nets if the set-nets are located near underwater troughs which in effect influence migration paths. Knowledge of underwater topography and associated ocean currents is important in locating set-nets and maximising catch rates of target species. By means of self-developed fishery detection data processing system (FDPS), an appropriate fishing site for set-net fishing can be selected. Acoustic surveys of fish near set-nets were used to examine fish behaviour near set-nets. The surveys revealed that most fish were active at night and were more likely to be caught over night than during the day. This is consistent with the daily variation in catches as set-nets are usually harvested twice a day (i.e., morning catch and afternoon catch). Whale sharks are occasionally caught in set-nets. Because of their conservation status, mitigation methods to avoid whale shark capture were evaluated. However, such mitigation is in tension with the economic benefits of catching and selling whale sharks (in high demand by premium seafood markets). Studies showed that most fish caught in set-nets were relatively small (30-40cm total length). Mitigation methods (including wide-spaced rope grids to exclude large animals such as sharks) may negatively affect catch rates of target species (e.g. tuna). Thus, there are both economic and practical disincentives to apply whale shark- reducing devices. In all other respects, when compared with other fish methods (particularly demersal trawling), set-net fisheries have a demonstrably benign impact on coastal environments. Little by-catch is taken with almost all fish caught sold. What little by-catch is taken is used as food in co-located fish cage farming operations. As set-nets employ static gears, the quality of the fish caught is very high (compared with active trawl methods). This, and the regular harvest (twice daily) presents set-net fishery as a source of high value species with concomitant economic return to regional coastal communities. Thus, in the context of ecological sustainable development, the results of this study show demonstrable ecological, economic and social benefits from set-net fisheries in Taiwan