The approximately 99-hectare nature reserve is registered under number 81 in the list of nature reserves of the Ministry of Agriculture, Environment and Rural Areas. It was designated in 1974 (date of the regulation: April 3, 1974). The responsible lower nature conservation authority is the Segeberg district. The nature reserve is looked after by the Schleswig-Holstein State Hunting Association. The nature reserve is located in the Holstein Switzerland Nature Park on the southern shore of Lake Stocksee. It protects a 40-hectare section of Lake Stocksee and the islands of Großrethberg, Kleine Insel and Lange Insel as well as the Kleinrethberg peninsula and the area to the south with Lake Lütten. Lake Lütte is connected to Lake Stocksee by a canal. The islands and the peninsula as well as the area around Lake Lütten are predominantly wooded, the remaining areas are partly used for agriculture. There are also herbaceous meadows, succession areas and reedbeds.
The clear water in the Stocksee means that numerous mollusks can live here, including pond and river mussels, mud snails and marsh snails, and more rarely, boat snails. They are the food source for various diving ducks, rails and fish.
The forest areas have a partly primeval forest character. They have important deadwood stocks, which are the livelihood of many animal, plant and fungal species. Woodpeckers, owls and various songbirds can be found here.