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.
