Community Structure Of Crab And Trophic Niche Of Portunus Trituberculatus In Laizhou Bay

laizhou bay is located in the South of bohai sea, binzhou is in the west, weifang isin the south,yantai is in the east,it is an important part of the bohai sea,the Yellow River,Xiao Qing river,Sha river, such as many rivers in the sea, Laizhou Bay is an important fishing and aquaculture area, parented the residents around it, and promotes the development of the local fishery economy.In addition,the bait in Laizhou Bay is very rich, the climate is suitable and provided a good condition for fish shrimp and crab‘s reproduction and growth.But in recent years, effected by overfishing, water pollution and aquaculture wastewater emissions,the resources are recession is. since 2005,the relevant departments of shandong province to implement fishing moratorium and the restocking policy, and it is very helpful to the resources of the economic species.Crabs is important to the fishery production and fishery ecological system, large size crab like Charybdis japonica and Portunus trituberculatus is the main economy species,they are the main object and important to the local fishermen,the small size crabs can be used as baits,and discharge of the main varieties.Portunus trituberculatus is a main species for stock enhancement,it is important for resource complement and ecological balance in Laizhou Bay.Based on the data which were collected during 2010~2014 by using bottom trawl on 14 different voyages and 4 different voyages by using beam trawl in 2011. Diversity index such as Margalef richness index(D),Shannon-Wiener index(H‘) and Pielou‘s evenness index(J‘),index of relative importance(IRI),non-metric multi-dimensional scaling(MDS),hierarchical cluster analysis(CLUSTER),SIMPER were used in this paper.Meanwhile according to the survey data of two different trawl,we discussed their effect to the crab community.Using isotope analysis to figure out the niche and trophic level of Charybdis japonica and Portunus trituberculatus. Our work could provide valuable information on maintaining the sustainable development of Laizhou Bay,and offer reference for reasonable stock enhancement.1.This article based on the 8 bottom trawl surveys of crab resources during 2010-2012 in the Laizhou Bay(36°50′~38°40′N,118°40′~120°03′E),where we set 18 survey stations,we researched the crab species, distribution, biomass, dominant species and diversity. The results show that 20 species of crabs were identified in the survey,belonging to 12 families and 18 generas, Portunus trituberculatus and Charybdis japonica are the main economic species.the others are low-value feeded species.Only Charybdis japonica is the dominant species in 7 times,while some others have seasonal variations and changes.The species seasonal composition was obvious. The tendency of the biomass and abundance are similar,they all getting to the top in the summer of 2011 and in the bottom during the winter of 2011.According to the data, the average of Shannon-Wiener index was between 0.63~1.11, Pielo’s evenness index(J) of the community structure ranged from 0.32-0.73,and the Margalef’ species richness index was between 0.42-0.78.According to the biomass of 20 species.The results of the cluster shows that the crab species in Laizhou Bay can be devided into 2 groups(2Dstress=0.14).The results shows that:1) Affected by the fishing moratorium and the restocking policy in Shandong province,the resources of the economic species such as Portunus trituberculatus was rised,but it still has lots of spaces to go.2)The diversity of the crabs in Laizhou Bay is still in a very low status,most of the them are small,low-value,baite-type species.3) Multivariate analysis showed that the community structure of crabs in the Laizhou Bay are obviously,the main reasons are the rebounded of the Portunus trituberculatus and hydrological environment in this area,the baite from Yellow river may also effect the crab community.2.To compare the effect to the crab community of two different trawls,we collected the data from May,August,October and December in 2011 using bottom trawl and beam trawl.The result showed that the main object of beam trawl is benthic shellfish and little size crab which have weaker moving ability,the main object of bottom trawl is the crab which live in bottom water and have strong moving ability,such as Charybdis japonica and Portunus trituberculatus. From the point of species composition,the bottom trawl lack of Carcinoplax vestita,the beam trawl lack of Pilumnus spinulus and Eriocheir sinensis,the rest of others are common species. This shows that there was little different between beam trawl and bottom trawl in species composition.From the perspective of the dominant species, Philyra pisum,Eucrate crenata and Charybdis japonica are the common dominant species,which further prove that Charybdis japonica is absolutely the dominant species in the area of Laizhou Bay in chapter 3.Philyra pisum and Eucrate crenata have higher IRI, on ane hand,their wide distribution in Laizhou Bay,on the other hand,their moving ability is weak and easy to be catch.In addition, Portunus trituberculatus was already become the dominant species in some voyage that using bottom trawl,and that proves bottom trawl is more appropriate for catching the Portunus trituberculatus.Based on biomass, summer and autumn are more similar,winter and spring are similar,the reasons are:1) The time of stock enhancement is on June,and the main species is Portunus trituberculatus,beacause of this,the biomass of summer and autumn is in a high level and have a higher similarity index.2)Summer and autumn are the feeding times for the crab,lots of crabs get back to Laizhou Bay,but in winter and spring they are in Huanghai Sea to pass the winter.3.To achieve a better effect,we figure out the overlap degree of Charybdis japonica and Portunus trituberculatus‘ niche,the data was collected in May,August,October and December in 2011 from 20 position.The samples of isotope are from August and October, the experimental results were determinated by Chinese Academy of Forestry.The results showed that trophic level of Portunus trituberculatus is 3.59, Charybdis japonica is 3.58,and they all belong to the third level,,they are the secondary consumer of the community.The overlap of niche is serious.The Portunus trituberculatus has more widely nutrient niches,and the Charybdis japonica has more widely spatial niches.