Otolith Sr:Ca Ratios as a Natural Mark to Discriminate Restocked and Naturally Recruited European Eels in Latvia
Yu-Jia Lin, Jen-Chieh Shiao, Maris Plikshs, Atis Minde, Yoshiyuki Iizuka, Isaak Rashal, Wann-Nian Tzeng
Abstract.—Restocking programs of the European eel Anguilla anguilla have been conducted for nearly one century in Latvia. To evaluate the efficiency of the eel restocking program and reveal the migratory life histories of European eels in Latvian waters, a total of 75 individuals was collected from the mouth of River Daugava (Daugavgrīva, brackish), a nearby lake (Lake Ķīšezers, freshwater), and a coastal site (Mērsrags, brackish). The naturally-recruited eels consisted of two saltwater types: eels that lived in saltwater and did not enter freshwater (SW, 0–7%) and eels that experienced both freshwater and saltwater, referred to as inter-habitat-shifter (IHS, 60–85%). Restocked eels consisted of purely freshwater types (FW, 7–36.7%)without any exposure to saltwater. Average Sr:Ca ratios at the edge of otoliths were different among sites and origins, and corresponded to the salinities of sampling sites. The mean age at first freshwater entry of IHS was 4.8 ± 2.3 years and was similar to previous studies in Lithuania. The growth rates of FW eels in the river mouth and coast were significantly slower than that of IHS eels (p < 0.001 and 0.012). The use of otolith Sr:Ca ratios as a natural mark to distinguish restocked eels here enabled the assessment of restocking efficiency.