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Impact of coastal defence structures (tetrapods) on demersal fish and decapod crustaceans

Tetrapods are four-footed concrete

structures used worldwide as

breakwaters. Around the island

Helgoland and its corresponding small

island Dne, about 10 000 tetrapods

protect the coast from erosion and

destruction by storm events. (Foto Uwe

Nettelmann 2011)

This photo represents a storm event

during wintertime on Helgoland. Without

coastal defence structures such as moles

and breakwaters, significant amounts of

coastal habitats would be eroded in a

longer time scale. (Foto Uwe Nettelmann

2011)

Climate change leads to various changes in the temporal and spatial dynamics in

coastal environments. The erosion of natural barriers, the destruction of coastal

structures by extreme storm events but also the installation of coastal protection

measures along coastlines is assumed to significantly affect the structural integrity of

coastal habitats. However, the ecological impacts of such structures on the coastal

ecosystem are hardly understood. In our workgroup, we investigated the impact of

experimentally introduced Tetrapods (Fig. 1; see also MarGate) on the demersal fish

and decapod crustacean community in a hard-bottom area in the southern North Sea.

In our working group, we perform SCUBA supported in situstudies on the temporal and

spatial dynamics of the fish and macro-crustacean community. We focus on the effectsof artificial defence structures on sublittoral communities in near-shore marine habitats.

Our studies show that fish abundances are significantly higher in close vicinity to

artificial structures. After the introduction of 6 tetrapod fields off Helgoland (Fig. 2; see

also MarGate) a significant migration behaviour of fish was observed towards the

artificial structures while a decrease in abundance was observed in the surroundings..

Furthermore, in the second year after the introduction, a distinct increase in

young-of-the-year fish (YOY) was observed in the immediate vicinity of the artificial

structures. This indicates a distinct and longer lasting influence of such artificial

structures on the local fish community.

No significant impacts of artificial structures were observed on the decapod crustaceans

community in general. Observed changes in abundance and species composition in the

area were rather highly species-specific and did not follow a general trend. For the great

spider crab (Hyas araneus) and the common lobster (Homarus gammarus) we

observed an increase in abundance at the tetrapods, indicating that the tetrapod fields

represent an ideal habitat and shelter refuge for these both species. On the other side, a

significant decrease in abundance was observed for small and less vagile crab species

such as the long-clawed porcelain crab (Pisidia longicornis) and the hairy crab (

Pilumnus hirtellus). However, it is not yet clear if these changes in the macro-crustacean

community are caused by natural fluctuations in the population dynamics of these species or if the introduction of the tetrapods

is causal for the observed changes.