Management effects on breeding and foraging numbers and movements of double-crested cormorants in the Les Cheneaux Islands, Lake Huron, Michigan

The yellow perch fishery of the Les Cheneaux Islands (LCI) region of Lake Huron, MI suffered a collapse in 2000, attributed in part to the increase of double-crested cormorants (Phalacrocorax auritus) in the region. A management program involving egg-oiling and lethal culling was initiated in 2004 to reduce cormorant foraging on yellow perch in the LCI. Counts of cormorant nests, nests oiled, cormorants culled, and aerial counts and telemetry surveys were used to evaluate management. Management contributed to a 74% reduction of cormorants on breeding colonies from 2004 to 2007. Cormorants used the LCI area significantly more (P<0.05) than surrounding areas. Aerial counts of foraging cormorants declined significantly (P<0.05) over the entire survey area but not within the LCI proper. However, aerial counts of cormorants in the LCI were five-fold less than cormorant counts in the same area in 1995. Reduced cormorant numbers were attributed in part to the elimination of cormorant nesting on a large colony due to the introduction of raccoons. Although the numbers of cormorants using the LCI did not decline, flocks were significantly smaller and more dispersed after management began. The reduced number of cormorants from 1995 levels and more dispersed foraging likely reduced predation on fish stocks including yellow perch in the LCI. Our findings indicate that the relationship between reduction in cormorant breeding numbers and reduced cormorant foraging in a given area is complex and may be influenced by density dependent factors such as intraspecific competition and quality of the forage base. Published by Elsevier B.V.