Native pipefish Entelurus aequoreus are promoted by the introduced seaweed Sargassum muticum in the northern Wadden Sea, North Sea

After the major loss of eelgrass habitats in the 1930s the snake pipefish Entelurus aequoreus was considered to be rare in the Wadden Sea (south-eastern North Sea, German Bight). We hypothesize that a sudden increase in pipefish abundance observed in the area since 2004 is related to new habitats provided by the introduced Japanese seaweed Sargassum muticum. SCUBA observations conducted near the Island of Sylt during July 2006 showed that approximately every second S. muticum thallus was inhabited by E. aequoreus, whereas no pipefish could be found on sedimentary areas dominating the Wadden Sea. In a field experiment, we removed thalli from a S. muticum bed and transplanted them to an unvegetated site and to the donor S. muticum bed, respectively. Nine days after the transplantation, abundances of pipefish per thallus were almost equally high within all transplanted S. muticum units at vegetated and originally unvegetated sites. Abundances of E. aequoreus in transplanted thalli did not significantly differ from those found in unaffected control thalli located in the donor S. muticum bed. To explain high pipefish densities within S. muticum beds we sampled the zooplankton community inside and outside the S. muticum bed and compared it with gut contents of pipefish. Zooplankton densities were significantly higher inside the S. muticum bed. Harpacticoid copepods were the dominant group associated with S. muticum and also represented the major prey of E. aequoreus. We conclude that stocks of E. aequoreus are promoted by S. muticum beds since they benefit from higher food supply during the spawning season, probably in addition to increased shelter by habitat complexity. Results of the present study suggest that introductions of habitat-forming species such as S. muticum may cause beneficial effects on abundance and diversity of native species.