The Impact Of Breeding Densities, Substratum, Predators On The Grown And Survival Of The Bloody Clam (Scapharca Subcrenata)

Scapharca subcrenata is important marine bivalve resources with tremendouseconomic value in China. It is fished for human consumption because it is delicious intaste and rich in nutritional value, thereby win warm praise from customers. However,legal and illegal exploitation of this resource have resulted in the severe decline ofS.subcrenata population quantity. To meet the needs of the market and resourcesprotection, nowadays, breeding programs are being developed on both local andindustrial scales. The study includes three parts. In the first part, in order to determinethe optimal bottom seeding area, season and size, water quality and substrateconditions of bloody clams (S.subcrenata) bottom seeding area are investigated. Thesecond part, the effect of different larvae breeding densities, substratum types andcolors on the settlement and metamorphosis of bloody clams (S.subcrenata) wasevaluated in the course of large-scale hatchery production. The third part, impact ofpredation by crabs and sea stars on the survival of the bloody clams is tested in pondand intertidal zone respectively, by the study of the interaction of predators and preyto determine best seeding area, season, density, size, thereby decrease the losses frompredation effectively. The major results and conclusions are as follows:1. The study of S. subcrenata enhancement and releasing technology.At present, the repairation of shallow sea bottom ecological environment mainlyrely on enhancement and releasing repairation tool species, improve bottomenvironment and even whole ecological environment. In this study, bloody clam isselected as the important repairation tool species. Between September and Novemberis the best seeding period, which evade the season of crabs reproduction in spring, aswell as high temperature, rainy season. Large size juvenile S. subcrenata are selectedadvisably, when shell length is higher than20mm, survival of overwintering can be improved, also the ability of resistance predation is enhanced. The selection of properbottom seeding area not only need to consider appropriate substrate conditions but thechange of seawater salinity, DO, COD, thereby improving the survival of seedingjuvenile S. subcrenata.2. The study of bloody clam larvae breeding density.Five different breeding densities are designed,2,5,8,11,14larvae/mL at differentdensities. Through recording the change of shell length and shell height continuously,draw the grown curve graphs of larvae. Comparing the larvae daily average growncurve at different densities, the change of densities before settlement andmetamorphosis, the size of settlement and metamorphosis, the rusults confirm that thesignificant increase of shell length and shell height are at three, five and nine daysafter hatching. Significant difference of size are found when larvae settlement andmetamorphosis at different densities. Larvae spend shorter settlement time in5and8breeding densities, which density is stable before settlement, juvenile S. subcrenatasize is uniform, be full of energy and a little deformity, so5and8larvae are moresuitable breeding densities.3. The impact different substratum types on the settlement and metamorphosis oflarvae.In the present study, the effect of various substrates on the settlement andmetamorphosis of bloody clams, Scapharca subcrenata, was evaluated. The resultsshowed that PVC meshes, red coir rope, plastic plates were preferable substrates. Bycontrast the settlement densities difference between horizontally and verticallypositioned substrates, we found that vertically positioned substrateattract more larvaesettlement. The difference of settlement densities on upper layer, middle layer andlower layer of vertically positioned substrate was significant, the lower layer ofvertically positioned substrate attracted more larvae. In addition, the settlement oflarvae in the experiments with three different color substrates was compared andshowed that deep color PVC meshes (black) attracted significantly more larvae thanlight color (white and green).4. Impact of predation by crabs and sea stars on the survival of the bloody clam. Crabs (Sesarma haematocheir) and sea stars (Asterias amurensis) are commonpredators of bottom seeding bivalve resources. Experiments are carried out toinvestigate the effect of predator density, size, sex, prey density, size, and temperature,substrate condition on predation. The second types functional response is shown bycrabs, feeding rate increase significantly with the increased prey densities in lowdensities treatments, but sea stars feeding rate are not increased significantly, displaythe first type functional response. Feeding rate of crabs and sea stars decrease withincreased prey size, bloody clam achieve “size refuge”. Feeding rate of predatorincrease with increased body size, prey survival decrease significantly. Sex have asignificant effect on consumption rate, the feeding rate of male crabs is higher thanfemale crabs, which result in more mortality. Temperature has a significant effect onprey survival, feeding rate increase with rised temperature. Substrate of complexityand heterogeneity provid more spatial refuges for clam survival, thereby decrease thelosses from predation. Crabs who are crushed, bloody clam, and clams (Ruditapesphilippinarum) are filled in cages as an inducing material, can induce and capturenumerous crabs, may become an effective method to capture and remove predatorsbefore seeding.