Fischreiser (fish brush-wood)

"Fischreiser," a word for which I have found no translation on the Internet. My free translation is therefore "fish brush-wood". And that's what it's all about:
 
In order to provide spawning grounds for certain fish species, so-called “Fischreiser” were established in Lake Constance decades, if not centuries ago, especially in shallow water zones of 5 to 15 m depth. An area of several square metres was bordered with piles and the place was filled with brush-wood. With stones the Fischreiser were weighted and fastened. These Fischreiser formed real protective castles for young fish against enemies and also against the weather. There are more than 100 such Fischreiser in our area and some can become tricky, especially for rowers, as not all are well visible (as the one here in the blip). Some piles only look a small piece out of the water surface. If the lake level rises, they can even disappear completely and then become a risk for those inexperienced in the area.