Coastal zone occupancy by double-crested cormorants on a Laurentian Great Lake before, during, and after a food web regime shift

Abstract

The Lake Huron food web has been undergoing change since the invasion of driessenid mussels (Dreissena spp.) (late 1990s), especially in 2003 featuring the lake-wide loss of alewife (Alosa pseudoharengus) among other elements that year. Collectively the changes in 2003 satisfy a number of criteria for a regime shift. Based on multiflight surveys (2001–2005), we modeled coastal zone occupancy of foraging double-crested cormorants (Phalacrocorax auritus) in the North Channel and Georgian Bay of Lake Huron during the regime shift period. At the start of the regime shift (2003), there were a number of plausible occupancy models based on a set of spatial covariates, the only year in the study when this occurred. Annual shifts in the magnitude and sign of coefficients indicated movement of cormorants between the two coastal regions, especially during and immediately after the regime shift period. Declines in cormorant occupancy of coastal habitat were not confined to 2003 but extended through 2004, with declines in occupancy in the North Channel (2003) preceding that in Georgian Bay (2004). Declines in occupancy in offshore areas in both coastal regions preceded declines in nearshore areas, possibly reflecting the loss of alewife at the time. The spatial response of predators or prey in regime shifts could serve as early indicators of tipping points in ecosystems.