Recruitment failure of mottled sculpin Cottus bairdi in Calumet Harbor, southern Lake Michigan, induced by the newly introduced round goby Neogobius melanostomus

This study documents a local extinction of mottled sculpins, apparently due to round gobies, and presents data pertinent to the mechanism of extinction. Mottled sculpins, Cottus bairdi, were assessed using SCUBA standardized diving transects during the invasion of the round goby, Neogobius melanostomus, into Calumet Harbor, southern Lake Michigan. Laboratory stream studies were conducted in which gravid male and female mottled sculpins were allowed to nest, then were exposed to one male round goby. Diet studies were conducted to assess the potential for competition for food at small sizes of both species. The SCUBA surveys showed that mottled sculpin populations rapidly, declined, after the first round gobies were found in the area in 1994, despite the presence of a well established population prior to the round goby arrival. Mottled sculpins have been almost totally extirpated from the area in 1998 due to three proposed mechanisms: competition for food resources at small sizes, for space at intermediate sizes, and for spawning space at large sizes. The laboratory stream study confirmed that round gobies interfered with nest-guarding male mottled sculpins, seized their spawning shelters, changed to spawning coloration in preparation for spawning, and caused near loss of all the mottled sculpin eggs. It is concluded that recruitment failure and subsequent demise of mottled sculpins was most likely caused by spawning interference by, round gobies.