Improving size selectivity of round pot for Charybdis japonica by configuring escape vents in the Yellow Sea, China

Sustainable development of the important economic species, Asian paddle crab (Charybdis japonica), has attracted attention in the coastal waters of the Yellow Sea, China. The commonly used round pots are almost nonselective, resulting in severe bycatch of juveniles. In this study, we explored a method to improve the size selectivity for C. japonica by mounting escape vents on the side panels of each pot. The selectivity of pots with escape vent sizes of 70 mm × 20 mm, 70 mm × 25 mm, 70 mm × 30 mm, and 70 mm × 35 mm was tested using a catch comparison method. The estimated minimum landing size (MLS) of carapace height (27 mm), according to the regulated MLS of carapace length (50 mm), was used as a reference point to explain the results. Significant increases in the size of crabs caught by pots were found with the enlargement of escape vent size (Kruskal–Wallis test, P < 0.01). The pots with 70 mm × 20 mm, 70 mm × 25 mm and 70 mm × 30 mm escape vents released nearly 50%, 75% and 95% of undersized individuals, respectively, and these three types of pots retained approximately 90% of legal-sized individuals compared with the control pots without escape vents. The pots with 70 mm × 35 mm escape vents released nearly all undersized individuals, but they also released most legal-sized individuals. Pots with an escape vent size of 70 mm × 30 mm were recommended for the sustainable development of C. japonica in the Yellow Sea of China. The results of this study reiterate the importance of carapace height for determining the size selectivity, which can serve as a reference to formulate management regulations in the coastal waters of the Yellow Sea, China.

UJI LABORATORIUM KESESUAIAN UKURAN CELAH PELOLOSAN PADA BUBU LIPAT TERHADAP TINGKAT PELOLOSAN RAJUNGAN (Portunus pelagicus)

Rajungan (Portunus pelagicus) merupakan salah satu komoditas perikanan yang memiliki nilai ekonomis penting yang ditangkap dengan alat tangkap pasif, seperti bubu lipat. Penggunaan celah pelolosan pada bubu diharapkan mampu memenuhi ukuran rajungan yang diperbolehkan untuk ditangkap sebagaimana yang tertulis pada Peraturan Menteri Kelautan dan Perikanan 12/2020 (lebar karapas >10 cm). Tujuan dari penelitian ini adalah untuk mengetahui ukuran celah pelolosan yang sesuai untuk meloloskan rajungan dengan lebar karapas 10 cm serta mampu menahan rajungan dengan lebar karapas >10 cm. Sebanyak 30 ekor rajungan dengan lebar karapas 9-10,8 cm diuji cobakan terhadap tiga ukuran celah pelolosan yang berbeda (4,6 x 2,6 cm; 5 x 3 cm; dan 7 x 2,5 cm), dengan tiga kali ulangan. Uji coba dilaksanakan di laboratorium pada bulan Maret 2020. Hasil penelitian menunjukkan bahwa tidak ada perbedaan tingkat pelolosan rajungan pada ketiga ukuran celah pelolosan. Meski demikian, ukuran 4,6 x 2,6 cm diyakini merupakan ukuran yang paling efektif karena mampu meloloskan rajungan dengan lebar karapas 10 cm (46,9%), serta paling bagus dalam menahan rajungan dengan lebar karapas >10 cm tetap berada di dalam bubu. Percobaan lapang terhadap ukuran celah pelolosan ini pada beberapa perairan diperlukan untuk mengkonfirmasi penggunaannya pada perikanan rajungan.The blue swimming crab (Portunus pelagicus, BSC) is one of fishery commodities caught by passive gears, such as collapsible traps. The use of escape-vent on traps was supposed to comply with the legal size of BSC’s catch declared in the Ministerial Decree 12/2020 (i.e. >10 cm of carapace width). To determine an appropriate escape-vent size which is able to release BSC of 10 cm Carapace Width (CW) and to retain BSC of > 10 cm CW, 30 samples of BSC were tested on three different sizes of escape-vent (4.6 x 2.6 cm; 5 x 3 cm; and 7 x 2.5 cm) with three replications of each size. The results show that there was no significant difference of BSC’s escape rate among the escape-vent sizes. However, the size of 4.6 x 2.6 cm was assumed as the most effective escape-vent as it could release BSC of 10 cm CW (46.9%), and best to retain BSC of >10 cm CW inside the trap compared to other sizes. Field investigation might be needed to confirm the effect of the appropriate escape-vent size of collapsible trap on BSC fishery.

III.—The Fissure Theory of Volcanoes

Dr. Hans Beck has adduced an example of a volcano which, according to him, has been formed independently of a fissure. The volcano pierces the centre of a faulted block, the Herdubreid, in Iceland, and on the vertical fault-faces there is no sign of any fissure. The example is probably unique in the world, and seems at first sight to negative the hypothesis that the escape of gases which tear through the earth's crust and form the chimneys of volcanoes is in the first place initiated by a fracture; on closer examination, however, the fact that the volcano stands in close relation to the faults which bound the horst, and the many cases which are known to occur where a fracture in the earth's crust may be healed at the surface so that the rocks about the fracture are subsequently more resistant than before, seem to point to the Herdubreid volcano being a normal fissure-formed volcano, only that it stands in the same relation to the fracture as a parasitic cone stands to the central crater. In other words, the chimney is an escape vent leading below the surface to one of the bounding faults of the horst.