These "sticks" are navigation aids which mark the exact outline of a navigable tidal creek. Contrary to buoys which sway at anchor these don't move and are more precise. They are young birch trees which are driven into the sea bed with a water jet. Normally they need to replaced every other year, since they hardly survive winter storms and ice floes.Tidal creeks shift more or less, they need to be sounded a and marked anew anyway