Proceedings of the Third World Fisheries Congress: Feeding the World with Fish in the Next Millenium—The Balance between Production and Environment
Selectivity, Survival and Stress of Japanese Whiting Sillago japonica after Simulated Capture by a Sweeping Trammel Net
Ari Purbayanto, Daniel R. Monintja, Atsuhiro Tsunoda, Seiji Akiyama, Takafumi Arimoto
The sweeping trammel net is a type of entangling gear by which the sweeping procedure increases the possibility of an encounter between the gear and fish. It is operated in the coastal areas of Japan for capturing demersal fish such as red horse head Branchiostegus japonicus and Japanese whiting Sillago japonica as target species. Historically, this fishing gear was introduced to Ishikawa Prefecture around 1955 for targeting red horse head. Years later, it was further developed in Wakasa Bay, then widely spread along the coast of San-in districts and some other areas, such as Niigata and Ehime Prefecture (Matuda 1972). In Tateyama Bay, Chiba Prefecture, capturing Japanese whiting using trammel net began around 1965 on a smaller scale (Purbayanto et al. 2000a).
Since the trammel net became popular as an effective gear for catching red horse head and subsequent overfishing, some studies were carried out for the efficient management of this fishery resource (Kitahara and Matuda 1967; Matuda and Kitahara 1967; Kitahara 1968a, 1968b). Despite these reports, however, little attention has been paid to the conservation of Japanese whiting by improving mesh selectivity performance of the trammel net. The fish captured by this gear in Tateyama Bay varied in species and size, and the Japanese whiting as target species was a small, immature fish of less than 160 mm in total length (Purbayanto et al. 2000a; Sulistiono et al. 1999).
Mesh size regulations are commonly used as a management tool to reduce nontarget catch. Ham-ley (1975) stated that knowledge of gear selectivity is important in the choice of an optimum mesh size for use in a fishery to provide maximum yield and minimum losses from nonlanded fishing mortality. However, improving the gear selectivity without reducing damage and stress incurred during capture and escape may not be the most appropriate way of protecting immature fish. In most cases, mechanical selection through fishing gear modification has been carried out without research into the survival of fish after escape (Chopin et al. 1995, 1996b).
Nowadays, survival research becomes an important factor that should be conducted for the successful improvement of fishing gear selectivity without waste or damage to the resources. The survival studies done previously, however, were limited to a few fishing methods (Chopin et al. 1995), and most of them concentrated on the fish escaping from the trawl cod end (Sangster 1994). Very little work on the survival and stress of fish after escape has been done to validate the effectiveness of improving gear selectivity, especially for gill nets and trammel nets. Broadhurst et al. (1997) suggested a simpler method of simulated capture to measure the survival of fish after escape from the trawl-meshed cod end in the laboratory because of the difficulty of conducting survival experiments in field. Chopin et al. (1996a) investigated the stress response and mortality of red sea bream Pagrus major captured by hook and line and by trammel net in a laboratory. Furthermore, Tsunoda et al. (1999) examined the stress response of Japanese whiting captured with a sweeping trammel net in the field.
The previous study on the mesh selectivity of a sweeping trammel net suggested that the master curve of selectivity for Japanese whiting had a wide selection range of 50% relative efficiency compared with the mesh selectivity curve of the gill net. This wider selectivity curve was considered to be caused by large fish, which were mostly entangled or entrapped in the pocket formed by the loose inner net as a result of slackness (Purbayanto et al. 2000b).