Variability in the concentrations of Ca, Mg, Sr, Na, and K in the opercula of perch (Perca fluviatilis L.) in relation to the salinity of waters of the Oder Estuary (Poland)

Macro-ionic composition of fish opercula may be an indicator of the fish stock identity. This is especially interesting in river estuaries, where individual sections of the estuary may differ in terms of their hydrochemical regimes. Diversified macro-ionic tissue composition is likely to provide clues not only about the origin of an individual fish, but also about its possible migration routes. The current study researched this aspect of the chemical composition of perch collected from different sections of the Oder River estuary. A total of 60 European perch, Perca fluviatilis L., were obtained from 3 sampling sites located in the river at Widuchowa, Szczecin Lagoon, and Pomeranian Bay, where waters are subject to dynamic macro-ionic transformations. Fish length ranged from 20 to 25 cm. Opercula of the fish were examined for their content of calcium, magnesium, strontium, sodium, and potassium, with the aid of the flame method using a Perkin-Elmer 3100 atomic absorption spectrophotometer. Perch inhabiting the waters of the Oder River between Widuchowa and Szczecin Lagoon had their opercula composed predominantly of calcium. Calcium was also the predominant ion dissolved in the water in this area; Ca represented 83.6% of all cations in the water and 41.6% of those present in the bones of the perch operculum. The increased levels of salinity near the sea were associated with the replacement of calcium in bones by other, more readily available elements (magnesium or strontium). Differences in the observed macro-ion composition of the opercula were sufficient to assign the perch studied to two distinct groups inhabiting fresh and brackish waters.