Northern pike Esox lucius growth rates in sub populations of freshwater and sea water origins in the Baltic Sea : Otolith method approach

Sammanfattning: Northern pike or pike Esox lucius is total piscivorous fish species found in the brackish and freshwater ecosystems of the Northern hemisphere. It is a top predator and keystone species that causes cascading effects in these ecosystems. Otoliths have been used to estimate age and growth rates in most fishes. Cleithra have generally have been used in Pike due to opaque nature of the otoliths. The aim of this study was to develop a method for age and growth estimation of pike using sagittal otoliths and carry out age and growth rates estimation of four sub populations; Dunöbäcken, Kårehamn, Oknebäcken and Väderön that represent freshwater and brackish water spawning sites. Determination of growth rates differences between sub populations was done for the first 2 years of their life. The methods included burning of the otoliths, fixing the otoliths in the epoxy resin, polishing of otoliths and staining with Alizarin red, Silver nitrate solution, Tetracycline and alkaline Lugol's solution with observation under light, dissecting and epifluorescence microscopes. Four methods tested based on staining with Alizarin red, Silver nitrate, alkaline Lugol's solution and burning otoliths. Provided useful results and not the method with Tetracycline. Alkaline Lugol's solution staining method was chosen and further developed. There was a significant difference in the relative growth rates between the sub population from brackish water spawning site of Väderön and the sub population from freshwater spawning site of Kårehamn in the first year (One-way ANOVA:F value 3.18, p value 0.0356) and after Tukey post hoc test (p value 0.0362). No difference among sub populations from the freshwater spawning sites. There was no difference in growth rates between any other sub populations in the second year. The importance of the study is successful application of Lugol's solution staining method for age and relative growth estimation studies with otoliths in pike.