Age identification for Larimichthys polyactis based on different 

sections of sagittal otolith and comparison 

with result of age backward inferring from growth equation 

Abstract: Fish age is a basic parameter in the study of fish population dynamics. Overestimation or underestimation of fish age undoubtedly can lead to overexploitation or waste of the resources. The clarity of the annual ring of otolith section affects the identification of fish age. At present, the accuracy of age inferred from the fish growth equation by fish growth parameters has not been verified. In this paper, small yellow croaker (Larimichthys polyactis), a model species for fishery resources research in marine economic fish, was taken as the research object. To explore the accuracy of age identification by sagittal otolith and age backward inferring by the growth equations for L. polyactis, we collected 45 individuals in the southern Yellow Sea to analyze three otolith sections, including sagittal section, frontal section and transverse section of sagittal otolith. 15 individuals were caught in Apr. 2018 with stow net of double stakes in Lvsi fishing ground and 30 individuals caught in Sep. 2019 with stow net in Dasha fishing ground. Three sections of L. polyactis sagittal otolith were analyzed to compare the effect on age identification as the experimental materials. Simultaneously, the Von Bertalanffy growth equations of L. polyactis during recent 30 years in China were summarized for age backward inferring based on body length of 45 L. polyactis. Taking the clear annual ring pattern and intuitive ring as the highest standard, the results showed that the annual ring clarity was the lowest in sagittal section (53.33%), higher in frontal section (66.67%) and the highest in transverse section (73.33%), respectively. The annual ring of three sections became more distinct after acid etching. L. polyactis sagittal section had the advantage in reflecting the whole life history based on the otolith microchemistry over the frontal section and transverse section because of its large surface area. However, the surrounding area of sagittal section core had many interferential rings, which resulted in misjudging the daily age of larva and juvenile of L. polyactis. Without the pretreatment by etching with 5% EDTA, the frontal section had clearer plane to identify the age more easily than the sagittal section. And the microstructure of frontal section could be used to study daily age identification and growth pattern. The transverse section had a smaller surface area than the aforementioned two sections and less affected by the secondary nuclei of L. polyactis, which was helpful in showing the clear annual ring even in thinner plane. The transverse section was more efficient for age identification of L. polyactis.

Comparing the age identification results of three otolith sections with the age backward results of 19 growth equations, the Von Bertalanffy growth equation Lt=366［1-e-0.117 77(t+1.764 29)］ showed the highest similarity levels with 53.33 % for sagittal section, 86.67 % for frontal section and 86.67 % for transverse section, respectively. In sagittal section, the results showed that the two growth equations Lt=366［1-e0.117 77 (t+1.764 29)］ and Lt=230［1-e-0.26 (t+0.367 8)］ had the similarity levels of 53.33% and 66.67%, respectively. In frontal section, the results showed that the growth equations Lt=206.827［1-e-0.454 13 (t+0.353 4)］ and Lt=366［1-e-0.117 77 (t+1.764 29)］ had the higher accuracy of 66.67% and 86.67%, respectively, whereas the other growth equations had less accuracy than 50%. In transverse section, the results showed that the growth equations Lt=206.827［1-e-0.454 13 (t+0.353 4)］ and Lt=366［1-e-0.117 77 (t+1.764 29)］ had 80% and 86.7% accuracy, respectively. To sum up, the growth equation Lt=366［1-e-0.117 77 (t+1.764 29)］ was verified to have the absolute advantage in age identification matched with the results of annual daily of three otolith sections. Three possible reasons accounted for the results of the highest similarity for the growth equation Lt=366［1-e-0.117 77 (t+1.764 29)］. First, the experiment samples of the growth equation were from the Yellow Sea and the East China Sea and the range of research area basically covered the sampling area of this study. Second, the growth equation was fitted by onevariable linear regression method, and the prediction accuracy was higher than that by FordWalford growth transformation method. Third, the growth parameters in the growth equation involved the fish age from the otolith besides the fish body length and weight in experiment, which was one of important reasons for the high accuracy. 

Age identification played a key role in fishery resource assessment, fishery management and the rational exploitation of resources. Age identification materials and methods were particularly important for fish age research. This paper revealed that the transverse section of otolith was the preferred methods followed by the frontal section for age identification of L. polyactis. And Lt=366［1-e-0.117 77(t+1.764 29)］ for age backward inferring was the optimal growth equation for L. polyactis population in the Yellow Sea. As a research method for fish age identification and growth research, age backward inferring from growth equation can quickly and accurately reflect the dynamic changes of biological resources and population structure. It is suggested that we should not choose arbitrarily the growth equation to estimate fish age on the basis of growth parameters with fish body length and body weight, but prefer the grow equation with the high similarity verified by age identification under the consideration of the sampling time and area for the fish. The reliability of age identification is crucial to study the population structure and growth characteristics of fish. Although the current trends of miniaturization and younger age of L. polyactis stock have been relieved in China, the simplification of population structure is still serious. This study provides a scientific option for strengthening the basic research of fish age and population dynamics.

Keywords: Larimichthys polyactis; otolith; sectional features; age identification; age backward inferring