Thanks! Giomen. With your help, I found that all sub-maps with a prio including the letter "T" (witch could stand for Topographic info) they are the maps keeping the fishing chart contours. Removing those from the folder, do the trick.
I used GMapTool and selected the folder where I keep the bluechart. All the .img files show up in the folder view. Select all files and hit the info button. Then you will see, for each .img what prio it has. In my case it was "28 T" or just "28". then I deleted all files with "28 T" and then everything was just fine.