Stone Reefs

A stone reef is a collection of larger stones embedded in the sea floor. The stones are so deeply imbedded that they do not move with waves and currents. 

The reef can either consist of scattered stones on the seabed – a dispersed stone reef – or it can be composed of stones piled on top of each other, creating a cavity-forming reef with holes and crevices that can attract fish, crabs, lobsters, and similar marine life.

There are also reefs that are not composed of stones. Biogenic reefs are formed by animals, such as the hard tubes of calcareous tubeworms or banks of blue mussels and horse mussels.

Some reefs feature larger, continuous areas of large stones surrounded by sand or gravel, while other reefs may consist of several smaller stone clusters spread over a larger area. There can also be a gradual transition between stones and sandy or gravelly seabeds, with progressively smaller stones and reduced stone coverage.

Reefs are areas where the seabed is elevated and has a rocky or other hard substrate. The reef may occasionally be exposed at low tide. From the seabed upward, reefs often contain a layering of different animal and plant communities. This creates a high biodiversity on individual reefs, often distinct from that of nearby reefs. The decline in salinity from the Kattegat through the Danish straits to the Baltic Sea around Bornholm is a primary reason for the differences in animal and plant life among reefs.

A reef is a protected habitat type
Stone reefs within Natura 2000 areas are designated as a protected habitat type in order to protect a range of threatened, rare, or characteristic animal and plant species, as well as the habitat type itself.

Stone reefs are vital for attached perennial macroalgae (seaweed) and the associated animal life, and they are characterized by a very high species richness. Rocky seabeds play a significant role in oxygen conditions, as seaweed grows and produces oxygen in these habitats if there is sufficient light at the bottom. If stones are removed, seaweed loses its substrate, and the bottom water will lack the additional oxygen production provided by the seaweed. Besides harming animal life, low oxygen levels in bottom water can also lead to the release of nutrients from the seabed, triggering plankton algae blooms.

Among the seaweed on the stone reef, numerous habitats are available for many different animal species. Bottom-dwelling animals live on the stones, while a variety of fish, crustaceans, worms, and other invertebrates use the vegetation and gaps between the stones as hiding places and feeding grounds. Life on stone reefs is in constant motion, with crawling, creeping, pumping, waving, and swimming activity. This vibrancy is due to the extraordinarily rich animal life, which thrives in a diversity of forms and life strategies.

Few studies have been conducted on the animal life of Danish stone reefs, and these primarily focus on the Kattegat and the Belt Sea. The following overview is therefore a preliminary impression of how animals are distributed on reefs in these waters.

Sessile animals

Sessile animals in Danish waters include sea sponges, hydroids, jellyfish, leather corals, sea anemones, bryozoans, calcareous tubeworms, mussels, barnacles, goose barnacles, and sea squirts among others. These organisms attach themselves to solid substrates and are mostly immobile. Some species, like blue mussels and sea anemones, can release their grip and move short distances before reattaching, but they are not designed for swimming.

Sessile animals can often cover large areas. At shallower depths, they compete with seaweed for space on stones, typically with algae occupying the tops of stones and animals inhabiting the sides. Animals also inhabit the algae. Below 20 meters, sessile animals dominate, with the most diverse communities found in the lower part of the algal belt, where a rich mixture of seaweed and sessile animals thrives.

Feeding strategies

Sessile animals rely on prey coming close to them, employing various capture methods. Most are filter feeders, filtering phytoplankton from the water. Sponges, mussels, and sea squirts use cilia to create active inward water currents, filtering food particles from the water inside the animal. Bryozoans and tube-dwelling polychaete worms use cilia on their stationary tentacles to generate external currents that help capture food.

Many sessile animals are predators, particularly cnidarians such as hydroids, leather corals, and sea anemones. They passively wait for prey to come into contact with their tentacles, which are equipped with cnidocytes (stinging cells) that can sting, inject venom, grasp, and adhere to prey.

Free-living animals

Free-living animals on stone reefs and the soft sediment between stones include a wide variety of groups, from primitive worms to fish. While fish are discussed separately, the other groups include nematodes, polychaete worms, mollusks (chitons, snails, and mussels), crustaceans, and echinoderms (sea urchins, starfish, brittle stars, and sea cucumbers). Less common groups include sea spiders and ribbon worms.

Free-living animals on hard substrates exhibit varied behaviors. Some are active feeders, while others remain mostly stationary, though all are capable of movement. Active feeders include shrimp, often forming shoals among seaweed. Some sedentary animals construct homes from sand grains or algal material. Animals in the soft sediment between stones may burrow as they search for food.

Feeding strategies

Feeding strategies vary. Some animals are highly specialized, but many switch to alternative food sources if their preferred food is unavailable. In practice, the feeding habits of a species often change frequently. Many predators often scavenge and may graze on algae, also consuming any animals encrusting the algae. A few species, like certain brittle stars and a single starfish, can both feed on the seabed and capture plankton from the water column.

Fish 

Stone reefs are rich feeding grounds for fish and their juveniles. The sessile animals and especially the algae have a high productivity, providing abundant food for fish. Fishers have long recognized that certain species are associated with stone reefs, wrecks, offshore installations, and protective stone structures around pipelines and cables. Studies show up to ten times more fish, such as cod, on stone reefs compared to surrounding sandy seabeds, and four to five times greater biomasses of flatfish species near artificial reefs than in adjacent areas without stones.

Research also indicates that fish on reefs are generally larger and that populations contain more older individuals. Older, experienced fish have a better chance of ensuring that their offspring reach maturity. Reef areas are thus crucial for fish populations that grow slowly and mature late, such as cod. For example, juvenile cod prefer complex habitats like stone reefs, where they grow faster and have higher survival rates.

The role of stone reefs for fish life

While no comprehensive study has been conducted on the importance of stone reefs for fish life, it is evident that they serve as gathering places for many fish species. Reefs are popular destinations for anglers, and divers observe significantly more fish on reefs than on surrounding flat seabeds. The degradation of stone reefs over the years has undoubtedly negatively impacted not only other reef life but also fish populations.

It is estimated that there are approximately 1,200 square kilometers of stone reefs in Danish waters, but there used to be many more.

During the 20th century, at least 8 million cubic meters of large stones were removed from the seabed through stone fishing. The harvested stones were used for harbor piers and other construction projects. Stone fishing was finally banned with an amendment to the Raw Materials Act on January 1, 2010.

There has never been a systematic survey of stone reefs over time, so the extent of natural stone reefs is unknown. A cautious estimate by the Danish Nature Agency suggests that, over the past 50 years, 40 km² of exposed stone surfaces have been removed from stone reefs in Danish coastal waters (Dahl et al., 2003). Some reefs have lost virtually all their larger stones, while others have primarily seen the removal of larger stones from shallower depths. The removal of larger stones also contributes to the instability of remaining reefs, causing erosion that further disperses the remaining smaller stones.