Catch Composition And Effects On Fishery Resources Of Stow Net In Qiongzhou Strait

Stow net is a traditional fishing gear which is widely used in South China Sea, East China Sea, Yellow Sea and Bohai Sea with various form. The catch were diverse in various sea area, but too much larva and juvenile fish were trapped that seriously damaged fishery resources. The main reason is that the mesh size of stow net is too small for juvenile fish. Qiongzhou strait is located Hainan island and Leizhou peninsula where fishery resources are abundand, Furthermore, the current is strong which is for stow net fishery, therefore, there are a lot of stow nets seted in the Qiongzhou strait. But up to now the research on catch composition and effects on fish resources of stow net in the Qiongzhou Strait is nearly blank, It is difficult to assess the current status of damage on fish resources by stow net in Qiongzhou Strait, So it is necessary to carry out related research and to provide important theoretical basis for making reasonable fishery management measures and realizing the sustainable utilization of marine fishery resources.The study is working at classification and bological determinationa of fish, it researches the catch composition based on monthly survey and sampling of sow net catch of in Qiongzhou Strait for November 2013 to October 2014. The main purpose is to understand the main dominant species in each quarter, quarterly variation of species diversity and fishery biological characteristics of 18 kinds of major economic fishes, VonBertalanffy growth parameters are estimated by length frequency methods, the total mortality and exploitation rates are estimated by Baranov method. The main results are as follows:1. Fish species composition and characteristics:(1)A total of 126 fish species were identified, which belong to 2 classes, 11 orders, 49 families and 89 gennuses. Chondrichthyes has 1 species and osteichthyes have 125 species, accounting for 0.8% and 92.2% of the total fish species respectively, The osteichthyes have 10 orders, 48 families, 88 gennuses and 125 species, The perciformes species is in a dominant position, They are 29 families, 52 gennuses and 81 species, accounting for 64.3% of the total of fish species. The clupeiformes is the next, it has 2 families and 10 species, followed by scorpaeniformes( 8 species), Pleuronectiformes( 7 species), Anguilliformes(6 species), Aulopiformes(4 species), Tetraodontiformes(3 species), Mugiliformes(3 species), Siluriformes(2 species), Beloniformes(1 species), Myliobatiformes(1 species).(2)There are no cold water species and cold temperature species. Fish species mainly consist of warm water species and warm temperature species. There are 111 kinds of warm water species and 15 kinds of warm temperature species, accounting for 88% and 12% of the total fish species respectively. They mainly consist of near demersal fishes and demersal fishes. Pelagic fishes have 24 species(accounted for 19%), near demersal fishes have 44 species(accounted for 35%), demersal fishes have 58 species(accounted for 46%).2. The main dominant species composition:(1) In spring, the dominant species have 9 kinds: Stolephorus commersonnii、Saurida tumbil、Epinephelus Lanceolatus、Johnius belengerii、Arius sinensis、Plotosus lineatus、Gerres limbatus、Trachurus japonicas、Sardinella jussieu.(2)In summer, the dominant species have 9 kinds: Atule kalla、Muraenesox cinereus、Siganus canaliculatus、Johnius belengerii、Plectorhynchus pictus、Setipinna taty、Arius sinensis、Pelates quadrilineatus、Konosirus punctatus.(3)In autumn, the dominant species have 5 kinds: Siganus canaliculatus 、 Nibea albiflora 、 Stolephorus tri 、 Konosirus punctatus 、 Pampus nozawae.(4)In winter, the dominant species have 5 kinds: Konosirus punctatus、Johnius belengerii、Siganus canaliculatus、 Stolephorus commersonnii、Muraenesox cinereus.3. Quarterly Various of fish species diversity, the relationship between selectivity of stow net and diversity:(1) Number of species(S): The maximum value of number of species presents in autumn, there are 86 species. The minimum value of number of species presents in spring, there are 26 species.(2)The species richness index(D): The maximum value of the species richness index presents in autumn, it is 11.28. The minimum value of the species richness index presents in spring, it is 4.32.(3)Shannon-Winner index(H′): The maximum value of Shannon-Winner index presents in autumn, it is 4.14. The minimum value of Shannon-Winner index presents in spring, it is 3.37.(4)The species evenness index(J′): The maximum value of the species evenness index presents in spring and summer, it is 0.72. The minimum value of the species evenness index presents in autumn, it is 0.64.(5)The species diversity index is between 3.37 and 4.14 in Qiongzhou Strait from November 2013 to October 2014, It is above the scope of the species diversity index(1.5~3.5)presentde by Ma keping. It shows that the species is abundant in the sea area, the distribution of biomass is even between species, community structure is stable.(6)Comparision: Species diversity of poor fishing gear selectivity is higher than that of good fishing gear selectivity.(7)Species diversity in low latitude area is higher than that in high latitude area inferred from catch composition of stow net.4. Stow net is detrimental to the fishery resources:(1)The juvenile fish is above 88% in 18 kinds economic fishes.(2)The discard juvenile fish and larve is accounting for 34.51% among the catch.(3)Among 18 kinds of major economic fish species, there are 11 species belonged to overfishing with the exploitation rates is grater than 0.5, which is accounting for 61.1%. They are Konosirus punctatus、Stolephorus commersonnii、Nibea albiflora、Saurida tumbil、Stolephorus tri、Terapon jarbua、Trichiurus lepturus、Setipinna taty、Sardinella jussieu、Secutor ruconius、Trachurus japonicas. The one species belonged to full exploitation with exploitation rates is 0.5, which is accounting for 5.6%. It is Siganus canaliculatus. Six species are not fully exploited with the exploitation rates is less than 0.5, which is accounting for 33.3%. They are Johnius belengerii、Atule kalla、Trypauchen vagina、Sillago sihama、Pennahia anea、Terapon theraps.(4)The weighted average rate of the exploitation rate for 18 kinds main economic fish species is 0.58 by stow net, it belongs to overfishing.(5)Comparision to trawl fishery in the area, the exploitation rates of major economic fish species captured by stow net are higher than that by bottom trawl, it shows that damage to fishery resources by stow net is more serious than by bottom trawl. The main reseason is that too small codend mesh size of stow net causes poor selectivity and its fishing target is maly composed of juvenile fish with poor swimming abillity.