The study area is situated 400 m north of a small rocky island (Helgoland, 54°11'N, 07°52'E) in the southern North Sea. Fish and crustacean abundances were sampled using SCUBA at 3 introduced tetrapod fields (54°11.622' N, 07°52.652' E; 54°11.578' N, 07°52.707' E; 54°11.552' N, 07°52.740' E) in the experimental area MarGate. Sampling was done along 20 m long line transects starting at the specific tetrapod field. The transects were fixed in four directions away of the fields. Every 5 m the diver observed the demersal fish and decapod crustacean abundances in a 2 m**2 field. Further sampling took place from June - October 2010 and 2011 (doi:10.1594/PANGAEA.821915). A pre-study took place at the same positions before the tetrapods were introduced in 2009 (doi:10.1594/PANGAEA.821909).