Population Structure Of Oysters In The South Of The Yellow River Estuary Oyster Reefs And Fish Attraction Effects Of Artificial Oyster Reef Model

Oyster reefs are among the most valuable and threatened marine habitats globally in terms of ecosystem services,and growing interest in restoring the coastal habitat of high importance has been developed in many regions.However,basic information on the oyster population distribution and status of natural oyster reefs along the coasts of China is still lacking,which is the fundamental scientific background for developing restoration strategies.From 2021 to 2023,a survey was conducted on a natural live oyster reef recently discovered in the inlet waters of the Xiaodaohe River,southwest of the Yellow River Estuary.And there is also an important breeding and feeding ground for many typical estuaries fishes.The oysters on the reef were analyzed for the species composition and population structure.In addition,in view of the destruction status of habitat and fishery resources,a preliminary study on the restoration technology of oyster reef was carried out.The artificial oyster reefs were designed to determine whether these could be a means for the conservation of this species’resource.Through simulation experiments,the behavior of juveniles seaperch（Lateolabrax maculates）and redlip mullet（Liza haematocheilus）in the presence or absence of its predator with different oyster reef models.These results can provide a scientific basis for evaluating the survival status and ecological service potential of the oyster population in the natural oyster reef in the southwest of the Yellow River Estuary,and developing and formulating technologies and strategies for protecting the natural oyster reef,as well as fishery habitat restoration technologies.The main research results are as follows:（1）Population characteristics of oysters in the natural oyster reefs on the south side of the Yellow River EstuaryIn the samples,two oyster species were identified:the Jinjiang oyster Crassostrea ariakensis and Pacifica oyster Crassostrea gigas.This reef was composed of over 50%of live oysters.Live oysters had a high density with an abundance of 2367.60 ind./m² and biomass of 23.20 kg/m²,respectively.The shell high distribution of two oysters was different（P<0.05）.From the annual growth line on the shell cross-section,ages of the oysters were discriminated with 0+to 6+yr for C.ariakensis and 0+to 2+yr for C.gigas.But both oyster species were composed（>60%）mainly of younger individuals of age<1+yr in terms of density.And in March,the condition index（>8%）of oyster was higher than other months,means that the gonad of oysters are about to enter the spawning stage.C.ariakensis grew upward mainly with higher ratio of shell height/length at low ages,and then extended in volume with lower shell height/length ratio and higher shell height/inflation ratio at old ages（2+to 4+yr）.The logarithmic relationships between shell height and shell mass of both oyster species had slopes<3,indicating that the oyster species on the reef had negative algometric growth.From the fitted von Bertalanffy growth models of the populations,the asymptotic length(SH_max)and inflection ages for C.ariakensis were 286 cm and 5.47 yr with an asymptotic mass(W_max)of about 812 g.For C.gigas,the SH_maxand inflection age were 173 cm and 2.56 yr,and its W_max was about 129 g.The covariance analysis of the von Bertalanffy growth curves showed a significantly different（P<0.001）between two oyster species.Analysis of reef building capacity of oysters indicates that younger oysters（<2+）contribute 50.27%of the reef accretion,but the 2+population of C.ariakensis also contributes 27.59%of the reef accretion.The results above showed that natural oyster populations of the oyster reef are abundant and have the potential reef building capacity.Especially during the period of shell extended,the C.ariakensis of age 2+yr were the main contributors to the oyster reef.For the restoration of oyster reef habitat on the south side of the Yellow River Estuary,C.ariakensis can be preferentially selected as reef building species,combined with scientific monitoring programs on a>3 year cycle.（2）Aggregation response to artificial reef models by wild sea perch（Lateolabrax maculatus）juvenilesA total of six artificial reef models with the shapes of prism-,cubic-and star-and attached oysters or not were applied in the experiment.The behavioral traits and locomotion performance of sea perch juveniles were analyzed and compared among the experiment groups with different reef models.Results showed that in the tank without a reef model,the juveniles scattered and distributed evenly,with a maximum distribution rate of only 6.65%per grid and an average distribution rate of 1.19%in the blank reef grids.When the reef model was placed,the L.maculatus juveniles gathered into the reef after a short test cruise with the aggregation center in the reef areas.All the average distribution rates were greater than 20%,and colony occurrence frequencies were greater than 75%in the reef grids.In the oyster reef models,the juveniles had longer response and aggregation time,and higher average distribution rates（32.2%～49.9%）than those in the oyster-free ones.Of the six reef models,the cubic-oyster reef model had the highest distribution rate in the reef grid and the shortest distance between juvenile individuals and the reef area,implying that this reef type could be the most effective for juvenile aggregation.Although the fish had an obvious gathering reaction to the reef models of different structures,there was no significant difference in the attractive parameters among the six reef models according to the analysis of variance（ANOVA）.The evidence presented above suggests that wild juveniles prefer to gather and have behavioral responses to the artificial reef,indicating that the species could be a target fish species for stocking in reef areas.In addition,it should be taken into consideration that enough space for the fish to swim and appropriate attachment be provided when setting up artificial reefs for the juvenile perch.（3）Attraction effects of artificial reef model on Liza haematocheilus juvenile under predation stressIn this study,a cross-over experiment was designed with oyster reef models of three types（square-,con-and star-）to explore their attraction effects on the target fish species redlip mullet（Liza haematocheilus）in the presence or absence of its predator seaperch（Lateolabrax maculates）.The mullet distribution patterns were analyzed and characterized with MDR（mean distribution rate）and AR（attractive rate）,and its behavioral features were analyzed and consisted of swimming speed,nearest neighbor distance（NND）,and population polarity Pol.Results showed that without reefs and sea perches,the redlip mullet juveniles swam very slowly and were distributed evenly in the experimental tanks.However,in the pools without perches,the mullet mostly gathered in the middle water layer of the reef area,and they had relatively slow swimming speed and small NND and low population polarity.The attractive rates of reefs showed in turn of star-shape>con-shape>square-shape.Furthermore,in the pools with predators and reefs,as the predator perch occupied the reef in the bottom water layer,three reef models had low attractive rates for the mullets.Also,the juvenile L.haematocheilus dispersed away from the reef area and increased in occurring frequency in the surface water,swimming faster and increasing the nearest-neighbor distance between individuals.Furthermore,the mullets had a relatively higher swimming speed and NND than those in tanks without predators.Especially for the pool with a square-shaped reef model,the mullets had the highest swimming speed and the smallest impact area by prediction.Multivariate analyses showed that both predator and shape of reef models had significant effects on the behavior and distribution of juvenile mullets,especially on the Pol,MDR,and AR,vertical distribution of the mullets.From the results above,the artificial reefs have significant effects on gathering juvenile L.haematocheilus in predators absence,and they may be more pronounced in a complex model.But in the presence of predators,although the reefs may affect the behavior and distribution of juvenile L.haematocheilus to some extent,the reefs’attraction effects are significantly weaker.It suggested that when analyzing and evaluating the attraction effects of artificial reefs,it is necessary to examin the interspecific relationship to optimize the resource conservation strategy.