Characteristics of trace element in the otolith of 

Sardina pilchardus in waters near Mauritania

SONG Liming1,2, ZHOU Wang1, WANG Di1, Ebango Ngando Narcisse1

(1. College of Marine Sciences, Shanghai Ocean University, Shanghai201306, China; 

2. National Engineering Research Center for Oceanic Fisheries, Shanghai Ocean University,Shanghai201306, China)

Abstract: Sardine (Sardina pilchardus) resources play an important role in the West African fisheries and have high economic value. Mauritania waters is located in the intersection area of Guinea warm current and Canary cold current, with strong upwelling and high primary productivity. It is one of the important fishing grounds in the Northwest African sea area.At present, most of the studies about sardines otolith in the Mauritanian waters are focused on the otolith morphology and age identification, and studies of sardines otolith microchemical in Mauritania waters are mainly used for species identification.In the Mauritania coastal area,the migration pattern of sardine is related to the seasonal changes of upwelling systems and the SenegalMauritania warm front. Sardine spawns all the year round in the Mauritania coastal area, the dominant spawning season is from October to December, and from April to May of the following year.To study the characteristics of trace element of sardine in waters near Mauritania will further understand the habitat of sardine, so as to further develop, maintain and manage small pelagic fishery resources such as sardine. The content of fish otolith elements is affected by the habitat. The habitat can be studied by analyzing the content ratio of fish otolith elements. To better understand the characteristics of trace element in the otolith of sardine in waters near Mauritania, 219 sardines were collected on board the purse seiner from the waters near Mauritania (20°10′N21°10′N,17°10′W17°40′W) from Oct. to Dec. , 2018.The fork length of the samples was (15.24±6.76) cm and the body weight was (103.51±23.48) g.The gender was identified. Fifty eight (25 male, and 33 female)otoliths were selected and polished until the daily ring could be identified. The numbers of daily ring were measured under the microscope. The numbers of daily ring were counted independently by three qualified individuals and identified according to the numbers of broad bands or narrow bands. The otoliths transversely sectioned slices were sonicated in deionized water. The otolith element content in the core region, oneyear region and twoyear region was measured by laser ablation inductively coupled plasma mass spectrometry (LAICPMS) to analyze the ratio differences of otolith element content among different ages and discuss the habitat environment differences among different ages of sardine in waters near Mauritania. The experiment was conducted at Wuhan Shangpu Analysis Technology Co., LTD. During analysis a spot size of 44 μm was used for cell removal. Helium (0.7 L·min-1) was used as a gas sample and argon (0.8 L·min-1) was thus added to the gas flow for sensitivity adjustment. For the calibration of elemental concentration various external methodologies were considered and quantified with certified reference materials (NIST 610, BCR, BIR, BHVO, MACS3and ZS). The instrument parameters were calibrated again after measured 35 samples.The range of the otolith daily ring was from 344 to 789 for 58 samples.The numbers of fish in 01 year old, 12 years old, and more than 2 years old are 2, 40, and 16 respectively, the dominating age was 12 years old.Wilcoxon test was used to analyze the otolith trace elements content of different genders and ratios of dominant elements (Na, Sr, Fe, Mg, Ba) content to Ca content in different measuring points of sardine otolith. Results showed that: 1) A total of 44 elements were detected in the otolith of sardine, mainly including Ca, Na, Sr, Fe, Mg, Ba, etc; 2) There was no significant difference in the ratios of otolith dominant elements content to Ca content between male and female in waters near Mauritania (P>0.05); 3)There was no significant difference in the ratios of Na, Sr, Mg, Ba content to Ca content between the otolith core region and oneyear region of sardine in waters near Mauritania (P>0.05). There were significant differences in the ratios of Na, Sr, Mg, Ba content to Ca content between the oneyear region and twoyear region and between the twoyear region and fringe region (P<0.05); 4) There was no significant difference in the ratio of Fe content to Ca content between the oneyear region and twoyear region or between the twoyear region and fringe region (P>0.05). This study suggests that: 1) The female and male sardine in waters near Mauritania have the same habitat environment and migratory route. When the migratory life history is studied, they don’t need to study separately; 2) There is no habitat environment difference between 0yearold and 1yearold sardine in waters near Mauritania, but there are habitat environment differences between 1yearold and 2yearold, and between 2yearold and capture time for sardine in waters near Mauritania. Sardines might stay in the shallow waters off Cabo Blanco and there is no migration from age 0 to age 1. There are seasonal migratory movement for sardines from age 1 to age 2 and from age 2 to the capture time.In the summer and fall, sardines might migrate along the coastal area of Western Sahara to the north, and to the south in the winter and spring; 3) Sr and Ba element contents in the otolith of marine fish associate with fish habitats. Sr∶Ca and Ba∶Ca are environment fingerprint elements for researching the life cycle of the fish; 4) Na∶Ca and Mg∶Ca ratios can also be used for the study of sardine habitat change at different ages, but Fe∶Ca ratio is not applicable to study the sardine habitat change in the waters near Mauritania.Na content of the fish otolith relates to the water temperature and Mg content of the fish otolith relates to water temperature and salinity. The element composition in the waters near Northwest Africa of the Atlantic Ocean is influenced by the mine dust from the Sahara Desert. Iron ore resource is rich in Mauritania, which may lead to the distribution of Fe content even in this sea area, and Fe content in sardine otolith fluctuates little in the process of sardine growth.