黄河口西南侧小岛河河口天然牡蛎礁的牡蛎种群结构

Population structure of oysters in the natural oyster reef near the mouth of the Xiaodaohe River,southwest of the Yellow River Estuary

牡蛎礁是生态系统服务价值高、但退化最严重及受关注度最高的海洋生境之一,牡蛎礁修复亦成为国际海洋生态修复的热点。掌握牡蛎自然种群状况及动态变化是评估牡蛎礁修复效果的基础。目前,我国天然牡蛎礁的牡蛎种群状况相关的背景资料较为缺乏。在黄河口西南侧的小岛河河口新发现天然活体牡蛎礁,但该牡蛎礁曾被大规模的商业采捕,亟需推进针对性的保护和修复研究工作。基于2021年11月对该牡蛎礁开展的牡蛎种群生态调查,分析其种类组成、年龄结构及生长特征。结果显示:该牡蛎礁分布有近江牡蛎(Crassostrea ariakensis)和长牡蛎(Crassostrea gigas)。牡蛎礁上以活体牡蛎为主,死亡牡蛎壳体数仅占6.1%—6.7%。活体牡蛎的密度和生物量分别为(2811±778)个/m^(2)和(21.97±30.43)kg/m^(2),近江牡蛎较多,其密度和生物量分别占比55.7%和76.4%。近江牡蛎和长牡蛎的年龄分别介于0+—4+龄和0+—2+龄,它们都以壳高介于30—40 mm及壳质量<5 g的0+龄个体数量居多(>80%)。近江牡蛎的壳体形态参数均值都高于同龄组长牡蛎的相应值。两种牡蛎壳体均呈负异速增长,不同龄级的壳体延展方向不同。拟合von Bertalanffy生长方程得到,近江牡蛎和长牡蛎的渐近壳高分别为286 mm和173 mm,估算的拐点年龄分别为5.47龄和2.56龄,两种牡蛎的生长曲线分布存在极显著差异(P<0.001)。以上结果表明,小岛河河口的天然牡蛎礁的牡蛎自然种群资源较丰富,具有高密度、低龄和低死亡率等特点,有较好的活力和扩张潜力,有利于被采捕后的礁体的恢复。两种牡蛎中,近江牡蛎因其具有较高的生物量和较长的生长年龄,对礁体形成和扩繁可能更为重要。建议对该天然牡蛎礁及牡蛎种群开展周期>3年的原位保护、修复和连续监测计划。

Oyster reefs are among the most valuable and vulnerable marine habitats worldwide in terms of ecosystem services.More interests in restoring coastal habitats of high importance have developed substantially in many regions.However,basic information on the structure and distribution characteristics of oyster populations in natural oyster reefs along the Chinese mainland coasts is still limited,even though it is crucial for establishing ecological reference baselines for evaluating oyster reef restoration.In recent years,a natural live oyster reef has been discovered in the intertidal zone near the mouth of the Xiaodaohe River,southwest of the Yellow River Estuary.As the oyster reef has been commercially harvested on a large scale,it is necessary and urgent to carry out effective conservation and restoration efforts.By surveying the exposed oyster reef in November 2021,this study investigated the species composition,age structure,and growth characteristics of the oyster populations on the reef.Based on morphological and molecular analyses,two oyster species were identified,including the Jinjiang oyster Crassostrea ariakensis and Pacifica oyster Crassostrea gigas.Only 6.1%—6.7%of the oysters sampled were dead oysters.The density and biomass of live oysters were(2811±778)ind./m^(2) and(21.97±30.43)kg/m^(2),respectively.Of the two species,C.ariakensis had higher density and biomass,accounting for 55.7%and 76.4%of the total live oyster density and biomass,respectively.In addition,the annual growth line on the shell cross⁃section indicated that the ages of oyster C.ariakensis ranged from 0+to 4+years,and those of C.gigas ranged from 0+to 2+years.Mean values of shell height,length,and inflation of C.ariakensis were relatively higher than those of C.gigas at the same age.Both oyster species were dominated(>80%in number)by young individuals of 0+years old,with shell heights between 30—40 mm and shell masses less than 5 g.They also exhibited similar negative allometric growth patterns due to their individual linearly fitted model coefficients for shell mass and height<3.The ratios of individuals′shell height/length and shell height/inflation reflected differences in shell expansion between the two oyster species at different ages.According to the fitted von Bertalanffy(VB)population growth models of the two species,the estimated asymptotic shell height(SH_(max))and inflection growth age(ttp)for C.ariakensis were 286 cm and 5.47 years,respectively,with an asymptotic shell mass(W_(max))of 812 g,the SH_(max) and ttp for C.gigas were 173 cm and 2.56 years,respectively,with a Wmax of 129 g.The test of covariance revealed that the distributions of fitted VB growth curves in the two species were significantly different(P<0.001).Therefore,it can be inferred that the oyster populations in the natural oyster reef were characterized by high density,young age,and low mortality,implying that the natural oyster reef was relatively rich in oyster resources.The present oyster populations could provide potential capacity for reef expansion and restoration near the mouth of the Xiaodaohe River,southwest of the Yellow River Estuary.Owing to its higher shell mass and growth,the oyster species C.ariakensis may hold greater importance than C.gigas in shaping and nurturing the natural oyster reef.An in⁃situ conservation,restoration,and long⁃term(the cycle>3 years)monitoring program is recommended for this oyster reef and oyster population.