Otolith Morphology Analysis And Its Application In Stock Discrimination Of Sciaenidae Fishes

Otoliths are calcium carbonate crystals functioning hearing and balance of teleosts,which are located in their inner ears.Since the deposition of otolith is affected by the genetics and the living environment of the fish,the morphological characteristics of the otoliths are species-and stock-specific.Therefore,the shape of fish otolith is widely used in fish species identification and stock discrimination.So far,a variety of studies have proved the validity and efficiency of otolith shape analysis for fish stock identification.However,the techniques have their own suitable applying conditions,advantages and disadvantages.And the otolith morphology analysis is no exception.In this paper,a series of factors influencing the accuracy of the stock discrimination in the process of otolith morphological analysis are investigated and discussed.To achieve this goal,length groups from two known geographical stocks of yellow croaker,Larimichthys polyactis,along the Chinese coast were subjected to otolith shape analysis.And 5 Sciaenidae fishes(Collichthys lucidus,Collichthys niveatus,Miichthys miiuy,Argyrosomus argentatus,Nibea albiflora)were used for empirical study.The main conclusions are as follows:1)In the analysis of otolith shape,fish size is a potential source of variability in morphometric measures because size is associated with individual ontogeny.Thus,removal of the length effect in otolith shape analysis for stock identification using length scaling is an important issue and some statistical procedures have been used to eliminate the size effect of fish individuals.In the present study,two known stocks of L.polyactis were subjected to otolith shape analysis to evaluate whether commonly used size scaling methods could effectively remove the size effect of fish in stock discrimination.The results indicated that the variation of otolith shape caused by intra-stock fish length might exceed that due to inter-stock geographical separation,even when otolith shape variables are standardized with length scaling methods.This variation could easily result in misleading stock discrimination through otolith shape analysis.2)In order to remove the size-related effects,shape indices(e.g.roundness,circularity,rectangularity et al.)were used in numerous studies.The shape indices were obtained from traditional morphological variables(e.g.length,width,area,perimeter et al.)through specified mathematical equations.However,our present study showed that shape indices cannot achieve the original intention of eliminating the influence of body size only if the relationship between numerator and denominator is a straight line and the y-intercept equals zero and the indices changed raw dada and data error distribution thus misrepresent the true relationship between numerator and denominator.Shape indices differentially transform the metric relationship among samples inter-or intra-size classes make it difficult to evaluate the statistical and biological meaning of quantitative differences.Therefore shape indices should not be included as a multivariate variable in the discriminant analysis.Fourier analysis can be an effective method for describing outline shapes,but does not encourage intuitive understanding of the reason for subtle shape differences.Therefore,shape indices could be analysed with one-way ANOVA to provide an intuitive understanding of the morphological differences among sample sites.3)Normalized elliptical Fourier harmonics(NEFDs)can efficiently describe the contour information of the shape,which can correct the morphological variation caused by linear growth,but cannot correct the allometry growth which is common in nature.Therefore,it is necessary to minimize the difference of individual growth among sample groups when the normalized elliptical Fourier harmonic is used as a multivariate variable for discriminant analysis.4)As case studies,morphology analysis was applied to the interspecific discrimination between the C.lucidus and C.niveatus,and a classification success rate of 97.8% was obtained and modest classification success rates(67.7% and 65.2% for C.lucidus and C.niveatus,respectively)for discriminating among the Liaodong Bay,the Yellow River Estuary and the Jiaozhou Bay of the two species were obtained.Stock identification conducted on another three Sciaenidae species among sample sites of the Yellow River Estuary,the Jiaozhou Bay and the Changjiang River Estuary gained modest overall classification success rates of 71.5%,72.2% and 71.1% for M.miiuy,A.argentatus and N.albiflora.The finding that significant variations of otolith shapes occurred among samples with high degrees of genetic homogeneity(C.lucidus in the Bohai Sea and the Yellow Sea;M.miiuy in the Yellow Sea and the East China Sea)indicated that an otolith shape analysis could yield information complementary to that derived from genetic studies.Therefore,otolith morphology analysis could be used as an important supplementary tool to the study of genetic structure,which was to provide information for efficient management of fishery stocks.5)The stock discrimination based on otolith morphology analysis is easily susceptible to growth variation.At present,researchers have different criterions on sample selection and data scaling,which in turn affects the accuracy and consistency of the stock discrimination results.Therefore,conclusions about fish stock structure should be carefully drawn from otolith shape analysis because the observed discrimination may primarily be due to length effects,rather than differences among stocks.A holistic approach involving a broad spectrum of complementary techniques(e.g.,otolith shape analysis,otolith microchemistry and genetics)should be applied for accurate stock discrimination and structure analysis,which is essential to effective fishery management.