Essential Trace Elements in Three Species of Dolphins Stranded in the Croatian Part of the Adriatic Sea from 1995 to 2013

Trace elements are widely distributed in the environment and are considered essential when their deficiency leads to impaired biological function. This study aimed to quantify concentrations of two essential trace elements-copper (Cu) and zinc (Zn)-in the tissues of three toothed whale (<i>Odontoceti</i>) species: bottlenose (<i>Tursiops truncatus</i>), striped (<i>Stenella coeruleoalba</i>), and Risso's dolphins (<i>Grampus griseus</i>) found deceased along the Croatian coast of the Adriatic Sea between 1995 and 2013. A total of 190 individuals were analyzed, comprising 159 bottlenose, 25 striped, and 6 Risso's dolphins. Concentrations of Cu and Zn were determined in liver, muscle, kidney, skin, lung, spleen, and fat tissues using inductively coupled plasma optical emission spectrometry (ICP-OES). The highest Cu concentrations were observed in the liver and kidneys of bottlenose dolphins, followed by striped and Risso's dolphins. Zn concentrations were the highest in the skin of bottlenose and striped dolphins, whereas the liver exhibited the highest levels in Risso's dolphins. In 14 bottlenose and 2 striped dolphins, Cu and Zn concentrations in liver tissue exceeded critical thresholds typically regulated by homeostatic mechanisms. Regression analysis indicated significant relationships between element concentrations, and both body length and body mass. In addition, trace element concentrations were positively correlated across individuals within the same tissue type, as well as among different tissues within the same individual. Overall, Cu and Zn concentrations exhibited a declining trend over the studied period across all tissue types. These findings provide important baseline data for future ecotoxicological investigations and contribute to conservation strategies for cetacean populations inhabiting the Adriatic Sea.