Fig. 1

Sampling sites in the Odder River (freshwater) and Norsminde Fjord (marine). The three major groups of samples are as follows: (1) OddUa and OddUb are the two upstream localities in Odder River, pooled into the OddU group for some analyses. OddM denotes the group of samples that were collected continuously in the lower part of the river (as shown in the inserted box). Nor represents marine sticklebacks, sampled at three neighbouring localities and pooled into a single sample