Using stable isotopes to examine movement and prey usage of cormorants breeding in the southeastern United States

Double-crested Cormorant (Nannopterum auritum) numbers have significantly increased in North America since the late 1970s. Analyses of stable isotopes of carbon, nitrogen, and sulfur were conducted on the liver and pectoral muscle sampled from cormorants collected on Guntersville Reservoir, Alabama to infer whether individuals consumed cultured catfish and whether this population was subject to immigration during the spring sampling period. Potential food sources were also sampled from this system and aquaculture farms in west Alabama. Relationships among isotopic values measured in cormorants and prey illustrate that birds sampled from Guntersville Reservoir included both those that assimilated prey from this system long term and individuals that obtained resources elsewhere. Collectively, these results indicate that Guntersville Reservoir could be a migratory stopover site for cormorants and that these birds likely contribute little damage to aquaculture facilities. Management enacted to remediate effects associated with nuisance populations should be tailored throughout this species range focusing on areas where damage is occurring, and evidence supports culpability.