Detecting spiny dogfish in grey seal diets

Grey seal ( Halichoerus grypus ) populations declined dramatically in U.S. waters during the nineteenth and twentieth centuries, but have rebounded in recent decades as a result of legal protection. In the Gulf of Maine, increasing grey seal populations have the potential to impact commercially important groundfish species such as cod and haddock, but the potential also exists for more complex ecological interactions to occur given the recent increase of spiny dogfish ( Squalus acanthias ), another potential groundfish predator. Specifically, seal predation on dogfish could free groundfish species from additional predation pressure. Little quantitative data exists on intraguild predation rates by grey seals on spiny dogfish, in part because of the limitations of current methods of food habits analysis for grey seals. We report the development, validation, and field testing of a non-invasive genetic method for the detection of spiny dogfish in grey seal diets using DNA collected from seal feces. Our method amplified spiny dogfish DNA from 10 of 10 positive control scats from a captive seal with a known diet, and amplified spiny dogfish DNA from 0 of 10 negative control scats. We field tested the method on 20 scats from wild seals and detected spiny dogfish in two of the samples. All positive amplifications were sequenced to verify the species identify, and all were spiny dogfish. Our method has the potential to provide valuable information about changing predator prey dynamics in the Gulf of Maine and elsewhere.