| Understanding of stock is fundamental to fisheries management and endangered species conservation.Various methods have been applied to stock identification.Among them,otolith morphology analysis has its unique advantages including convenience,high efficiency and low cost.Additionally,otolith morphology analysis can be used for stock identification at different tempo-spatial scales.Wavelet transform is commonly used in signal processing.When adopted in otolith morphology,it can not only extract fine otolith information,but also locate the feature region.Compared to other two methods,Fourier transform and shape indices,wavelet transform has not been widely adopted in otolith morphology analysis.Traditional otolith morphology analysis commonly focuses on the whole otolith outlines,whereas some characteristic structures are less concerned.The sulcus is a longitudinal depression on the medial side of the otolith.It is also the major area where otolith interacts with auditory cilia.The morphology of sulcus differs among species or geographic stocks,and therefore has potentials in discriminating stocks.So far,there are few studies concerning sulcus morphology analysis,which generally deal with species identification.Moreover,the adopted parameters for sulcus morphology are also simpler than otolith morphology.Different methods of otolith or sulcus have their own merits or demerits,and thus the efficiency of these methods in stock discrimination also varies.The present study analyzed and evaluated the efficiency of different otolith morphology analyses in stock discrimination of three sciaenids along Chinese coast: the small yellow croaker(Larimichthys polyactis),the yellow drum(Nibea albiflora)and the white croaker(Pennahia argentata).They included shape indices,elliptic Fourier coefficients and discrete wavelet coefficients.In addition,the feasibility of adopting sulcus morphology analysis for stock discrimination was investigated and evaluated.The main goal of this study was to provide new insights into selecting different methods of otolith morpholy analysis and to improve stock discrimination.The main results are as follows:1)Whether using it alone or combining it with shape indices,the stock discrimination rate of elliptic Fourier coefficients or discrete wavelet coefficients was on the same level(differences ?3.0%),which is higher than that obtained by using shape indices alone(?16.1%).This indicated that elliptic Fourier transform and discrete wavelet transform could extract fine information of otolith morphology at comparable levels,whereas shape indices had less ability to describe the fine structure of otolith morphology.When combining different parameters,the stock discrimination rates were commonly promoted.Different types of parameters could extract different otolith information,which might be complementary to each other.Combining them could extract more and finer otolith morphology information than using them alone,and thus promoted the stock discrimination efficiency.2)When adopting sulcus morphology analysis(combing elliptic Fourier coefficients and shape indices)for discriminating stocks of the three sciaenids,the overall discrimination rates reach 70.8%-80.4%.This indicated that sulcus morphology analysis can discriminate stocks as effective as otolith morphology analysis.The efficiency of these two methods in discriminating stocks of yellow drum(73.8% vs 73.8%)and white croaker(80.4% vs 86.5%)were similar,whereas they showed relatively large difference in the small yellow croaker(70.8% vs 88.2%).The accuracy of acquiring sulcus outline was a decisive factor that could affect stock discrimination rate.Since the contrast between sulcus and other parts of otolith was usually low,it was sometimes difficult to identify sulcus outlines automatically by the software.Manually identifying and depicting the outlines could limit the accuracy of sulcus outline.Therefore,it is crucial to develop effective methods for extracting fine sulcus outline,which could improve the stock discrimination efficiency using sulcus morphology analysis.3)When comparing the reconstructed mean otolith outlines and wavelet coefficients variance among stocks,it was found that major differences along otolith outlines corresponded to large variance of wavelet coefficients.There were two levels of wavelet coefficients that were adopted to compare the variance among stocks.The variances that existed in level four wavelet coefficients were consistent with those in otolith outlines.But in level five,several small inconsistences were found between them.These might be determined by the property of wavelet transform: finer scale(level 5)of wavelet coefficients were more susceptible to “noise signals”.In this study,the noise signals could be induced by variances of otolith outlines among different individuals in each stock.Overall,wavelet transform could locate otolith feature regions well,and this could not be achieved by Fourier transform.4)In the otolith morphology analysis by elliptic Fourier coefficients,the number of Fourier harmonics adopted in each of three sciaenids was 10(small yellow croaker),10(yellow drum)and 9(white croaker).These Fourier harmonics explained more than 99.99% of the total Fourier power.By reconstructing the otolith outline based on elliptic Fourier harmonics,the change of otolith outline caused by adding each harmonic was obvious.It could also be found that the adopted Fourier harmonics could describe the otolith outline delicately.In the sulcus morphology analysis,the number of elliptic Fourier harmonincs adopted in each of three sciaenids was 12(small yellow croaker),9(yellow drum)and 14(white croaker).This indicated that the sulcus outlines of three sciaenids were more complicated than the corresponding otolith outlines.Generally,it could be validated whether the number of Fourier harmonics adopted in the study was appropriate by reconstructing the otolith and sulcus outlines.Moreover,the features along otolith and sulcus outlines could also be accurately depicted.5)The differences of otolith and sulcus morphometrics(weight,length,width,perimeter,area and relative area ratio)were significant(P<0.05)among the stocks of three sciaenids.Overall,the morphometrics of Changjiang Estuary stock were usually the largest.The deposition of otolith is regulated by genetics but are also influcenced by environmental factors.The conventional morphometrics of otolith and sulcus could provide biologically significant indicators for describing differences among geographical stocks and reflect habitat differences.This makes otolith and sulcus morphometrics useful parameters for stock discrimination. |
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