Temporal And Spatial Patterns Of Oyster Larval Settlement And Recruitment In Three Typical Bays,China

The temporal and spatial pattern of oyster larval settlement and recruitment,and its influencing factors are of great significance to the formulation of oyster reef restoration strategies.There are often huge differences in the attachment patterns of oyster larvae in different restoration areas.It is necessary to conduct a comprehensive investigation of the temporal and spatial patterns of local oyster breeding and larval attachment in a specific research system and potential influencing factors before formulating a restoration plan.This study explored the temporal and spatial patterns of oyster breeding and larval attachment in three typical bays in central and eastern China,and conducted a preliminary analysis of potential influencing factors.The main research results are as follows:1.Temporal and spatial patterns of oyster recruitment on the coast near a natural intertidal oyster reef in ChinaThe overall understanding of oyster recruitment dynamics provided invaluable information on site-selection and strategies-employed for oyster reef（OR）restorations.The study described the temporal and spatial patterns of oyster and barnacle recruitment on the coast near the Liyashan（LYS）OR in China during 2019-2020,and furthermore explored the mechanism driving low oyster recruitment at the LYS OR by fine-grained analysis of environment condition（food availability,hydrodynamic）coupled with epiboint competition（barnacle-cover vs no-barnacle）and predation（predator-exclusion vs control）.In each of 2019 and 2020,the oyster Crassostrea sikamea recruitment appeared as a continuous process from June through late November or early December,with the peak in August.We documented extreme spatial heterogeneity in oyster recruitment rate on a much smaller scale of just 10 km.Greater densities of oyster spat occurred on the sheltered inshore site than on the open coast（P<0.05）.The duration for barnacle spawning and recruitment extended from spring to early-winter,with two clear settlement peaks（mid-spring and mid-summer）.The spatial patterns of barnacle recruitment were totally converse with those observed for oysters.Higher barnacle recruitment appeared at the open coast than on the sheltered inshore coast.The cumulative recruitment of barnacles in each of 2019 and 2020 negatively correlated with those of oysters（P<0.05）,and the field interspecific interaction experiment showed that settled barnacles in experimental plats significantly inhibited oyster settlement and recruitment（P<0.05）.The predator-exclusion treatments significantly increased the survival（73%±11%）of oyster spat in the LYS OR compared to the control group（18%±3%）（P<0.05）.The study concluded that significantly lower oyster recruitment at the LYS OR could be produced by the stronger hydrodynamic regime,higher predation pressure as well as lower levels of phytoplankton abundance,larval supply and substrate quality relative to the inshore sites.It could be recommended that late-July was the most favorable window for capturing spat through substratum addition at a favorable site.The inshore sites with high oyster recruitment should be protected and considered as the natural spatfall sites for the population enhancement at the LYS OR.2.Temporal and spatial pattern of oyster larval settlement and recruitmentin the upstream of Jiantiao Bay,Zhejiang ProvinceThe study examined the temporal and spatial variations in the spawning and recruitment of C.sikamea,C.angulata and C.ariakensis at the intertidal zone in the upstream of Jiantiao Bay from May 2020 to March 2021.There is no significant difference in the gonad maturity between C.sikamea and C.ariakensis at each developmental stages（P>0.05）.The gonadal cross-cutting indexes of the two oysters were consistently at a high level in the June,July,and August,with the highest value for C.ariakensis in the June and for C.sikamea in the August.Tt is found that the undifferentiated stage of C.sikamea last from January to March.while C.ariakensis is relatively late,with an undifferentiated stage from February to March.;both oysters entered the differentiation stage in the April and May;In the mid and late May,the gonads of the two oysters matured and began to enter the discharging stage.C.sikamea at the mature and discharging stage can still be collected in the November and December,while C.ariakensis only last until mid-to-early October.The two oysters are common in a resting state from November to January of the following year.The recruitment and growth patterns of the three oysters showed significant spatial and temporal differences（P<0.01）.The high recruitment for the C.ariakensis and C.sikamea occurred from the early July to the late August,with the peak recruitments for C,sikamea in the late August and for C.ariakensis in the early July.C.angulata had earlier recruitment and reached its peak in May.The settlement abundance of juvenile C.sikamea was significantly higher at 1.6 m than at other tidal areas,followed by higher settlement at 0.1 m,0.6 m and 1.1 m;the settlement preference of C.ariakensis occurred at lower tidal zone（0.1 m-0.6 m）,and the settlement abundance of C.ariakensis decreased as the tidal zone rose.The best tidal areas for C.angulata settlement were widely distributed,with similar settlement abundance at 0.1 m,0.6 m,1.1 m and 1.6 m elevations.The peak settlement period for barnacles occurred in mid-to-late June and the optimum settlement elevation was 0.6 m,which overlapped with the optimum settlement tidal zone for all three oyster species.In addition,the spatial pattern of settlement of C.sikamea larvae in the early stages（1 d and 3 d）was similar to the cumulative monitoring（15 d）results,with higher settlement in the 1.6m tidal zone,further confirming that the vertical distribution pattern of C.sikamea in the intertidal zone is the result of selective settlement of larvae.The growth rate of C.sikamea larvae was significantly higher in the low tide area than in the high tide zone（P<0.01）,and the prime period for larvae growth was late August.Considering that the abundance of C.sikamea larvae dominates throughout the breeding period,C.sikamea should be selected as the preferred reef-building species for restoration activities,with the best reefing period being late July.3.Spatial pattern in larval distribution,settlement and growth of Crassostrea sikamea in Xiangshan Bay,Zhejiang Province,China.The migration and settlement of oyster larvae significantly affect the distribution,recruitment,establishment and ecological services of oyster populations.We investigated the variations of C.sikamea larvae abundances between the two water layers and between the two experimental sites（seed bed and spawning bed）in Xiangshan Bay within a tidal cycle（9 sampling times）,and explored the spatial patterns in larval settlement and spat growth at three intertidal zones（top,T;middle,M and bottom,B）of both experimental sites by a field settlement experiment.At the seed bed and spawning bed,oyster larvae abundance at the surface water significantly changed with sampling time（P<0.05）.At the seed bed,oyster larvae abundance(20.8±5.6 larvae·L^-1)was greatest at the high flood tide,and was significantly greater than that at the other eight sampling times.Contrasting results were found at the spawning bed where oyster larvae abundance was the lowest(0.1±0.1 larvae·L^-1)at the high flood tide.At each site,oyster larvae abundance at the bottom water did not change within the tidal cycle（P>0.05）.At the seed bed,total abundance,survival rate and shell height of the settled oysters significantly varied across intertidal zones,ranking as B>T=M for total oyster abundance,and T>M>B for survival rate and shell height（P<0.01）.There was no significant difference in live spat abundance among intertidal zones（P>0.05）.At the spawning bed,the total and live abundances,survival rate and shell height of the settled oysters had no difference among intertidal zones（P>0.05）.Our results indicate that oyster larvae commonly settle at the high flood tide,and the abundance of settled spats are similar between seed bed and spawning bed.4.Substrate selection for oyster reef restoration:Differences in oyster settlement and recruitment on fresh and old oyster shellsThe field experiment of 2020 examined the effects of oyster shell and tidal zone on the densities of settled oysters and barnacles.Two-way ANOVA found that shell types significantly affected the densities of settled oyster spats,while tidal zone had little effect on oyster settlement.At each of the two tidal levels（0.6 m and 1.1 m）,there were consistently greater densities of settled oyster spats on the fresh shell than on the old shell on the 15 d,45 d and 70 d（P<0.05）.Results from two-way ANOVA demonstrated that neither shell types or biofilm had significant effect on the shell height of settled oysters.Settled spats measured on 45 d and 70 d had significantly greater size than those on 15 d（P<0.05）.Results from two-way ANOVA found that oyster shell had no influence on barnacle settlement,while tidal zone significantly affected barnacle setlement.Higher densities of settled barnacles were found on the fresh shell than on the old shell in the 0.6m tidal zone on 15 d（P<0.05）,whereas the difference disappeared at each of the two tidal levels on the 45 d and 70d（P>0.05）.The study indicated that there were generally higher oyster settlement and recruitment on the fresh shell than on the old shell,and the evident differences extended to the recruitment season.Therefore,it concluded that the fresh shell were more preferable substrate for oyster reef restoration than the old shell.