Fatty acid compositions of Ethmalosa fimbriata oocytes sampled inside the Sine Saloum estuary (Senegal)

Inversion of estuaries in the dry tropics (i.e. salinity increases upstream) in front of the backdrop of a globally changing climate forces migratory clupeid fish species to adapt quickly to modifications in their spawning habitats in order to ensure reproductive success. Hatching success and survival probabilities of marine fish eggs and early life stages are prone to impairment under high temperature and elevated salinity conditions. Essential fatty acids affect osmoregulation in marine fish early life stages and deficiencies can therefore be expected to cause elevated offspring mortality under hyperosmotic conditions, ultimately modifying a stock's reproductive potential. Monthly sampling of Ethmalosa fimbriata females storing hydrated oocytes was conducted at the Senegalese coast and inside the Sine Saloum estuary from February to October 2014, during Ethmalosa fimbriata's main spawning period. Three different sites were sampled; Joal (coast, 14°9.1' N; 16°51.7' W), Djifer (River mouth, 13°57.8' N; 16°44.8' W), and Foundiougne (upstream Saloum River, 14°8.1' N; 16°28.1' W). A supplementary sampling campaign was conducted in November 2016. Here, microphytoplankton was collected by casting a handheld net (20 µm mesh size) three times per station. Additionally, the stomach content of 10 female fishes per sampling site, and the counted oocytes of 12 females were sampled. During this sampling trip, phytoplankton could also be sampled at Ndjiafate (upstream Saloum River, 14°3.7' N; 16°11.4' W). The coastal waters off Joal and in the Saloum River are well-mixed and do not show significant stratification. Thus, only surface water temperatures and salinities were measured for each sampling week. Water temperatures at Foundiougne and Ndiafate were recorded with a digital thermometer (ama-digit ad 15th; precision 0.4%; accuracy 0.4%). For Joal, where catch positions could only be estimated within a radius of ~20 km, and Djifer, average night-time sea surface temperatures were assessed by means of MODIS satellite-data (Aqua, level 2, 0.1 degrees)(https://oceancolor.gsfc.nasa.gov/). For Joal, monthly means in MODIS satellite-derived (Aquarius, level 3, 0.5 degrees) sea surface salinities (https://oceancolor.gsfc.nasa.gov/) were used. At Djifer, Foundiougne, and Ndjiafate salinity was determined with a handheld refractometer (Aqua Medic; precision 0.7%, accuracy 0.2%) using in situ water samples. Fatty acid methyl esters were analysed using GC-FID. To ensure comparability with past studies, results for fatty acids (FA) are given as a percentage of the combined weights of all detected FA.全球气候变化背景下，干旱热带地区河口出现盐度沿上游递增的逆盐度现象，迫使洄游性鲱科（Clupeidae）鱼类快速适应其产卵栖息地的变化，以保障繁殖成功。海水鱼类卵及早期生活史阶段的孵化成功率与存活概率，在高温及高盐环境下易受到损害。必需脂肪酸（essential fatty acids）会影响海水鱼类早期生活史阶段的渗透压调节（osmoregulation），因此在高渗环境（hyperosmotic conditions）下，必需脂肪酸缺乏可能会导致后代死亡率升高，最终改变鱼类种群的繁殖潜力。 2014年2月至10月，即埃氏鲱（Ethmalosa fimbriata）的主要产卵期，研究人员在塞内加尔海岸及锡内萨卢姆河口（Sine Saloum estuary）内部，对携带水化卵的埃氏鲱雌性个体开展月度采样。本次共设置3个采样位点：若阿尔（Joal，海岸位点，北纬14°9.1'，西经16°51.7'）、吉杰尔（Djifer，河口位点，北纬13°57.8'，西经16°44.8'）以及丰杜涅（Foundiougne，萨卢姆河上游位点，北纬14°8.1'，西经16°28.1'）。2016年11月补充开展了一次采样工作：本次采样中，通过在每个站位手持网具（网目尺寸20 µm）拖网三次，采集微型浮游植物（microphytoplankton）；此外，每个采样位点采集10尾雌性鱼类的胃含物，并对12尾雌性个体的卵母细胞进行计数采样。本次补采期间，还可在恩贾法特（Ndjiafate，萨卢姆河上游位点，北纬14°3.7'，西经16°11.4'）采集浮游植物。 若阿尔周边近岸海域及萨卢姆河河水均充分混合，未出现显著的层化现象，因此每轮采样仅测定表层水温与盐度。丰杜涅与恩贾法特的水温通过数字温度计（ama-digit ad 15th；精度0.4%，准确度0.4%）记录。针对仅能在约20公里半径范围内估算采样位点的若阿尔，以及吉杰尔位点，夜间海表平均水温通过MODIS卫星数据（Aqua卫星，二级产品，精度0.1°）获取（数据来源：https://oceancolor.gsfc.nasa.gov/）。若阿尔的海表盐度采用MODIS卫星反演的月平均数据（Aquarius卫星，三级产品，精度0.5°，数据来源：https://oceancolor.gsfc.nasa.gov/）。吉杰尔、丰杜涅及恩贾法特位点的盐度则通过现场采集水样，使用手持折射仪（Aqua Medic；精度0.7%，准确度0.2%）测定。 采用气相色谱-火焰离子化检测器（GC-FID）对脂肪酸甲酯进行分析。为确保与既往研究的可比性，脂肪酸（FA）的检测结果以所有检出脂肪酸总重量的百分比形式呈现。