Effect of Roost Harassment on Cormorant Movements and Roosting in the Delta Region of Mississippi

Double-crested Cormorant (Phalacrocorax auritus) predation on Channel Catfish (Ictalurus punctatus) at commercial farms has been estimated to cost the Mississippi aquaculture industry approximately $5 million annually to replace consumed fingerlings. In 1997, catfish producers assumed responsibility for the dispersal of cormorants in night roosts in the eastern (interior) delta region of Mississippi, where catfish farms are concentrated. We documented movements of 50 cormorants marked with radio transmitters in the delta region from January through March 1997. We obtained 161 post-capture day locations and 176 post-capture night roost locations. Cormorants that were harassed at their night roost flew farther to their next day’s location than birds that were not harassed the previous night. Of the cormorants for which we had more than one night roost location, only 11% of cormorants that were harassed returned to the same roost within 48 hours, compared with an 81% return of cormorants to a previously un-harassed night roost. Moreover, cormorants in the eastern portion of the delta (where all harassment was conducted) changed night roosts more frequently than cormorants in the western (non-harassed) delta. Since cormorants in our study foraged relatively close to their night roosts and only 11% of the birds that we observed roosting in the western delta traveled to the eastern delta to forage the following day, coordinated and intensive dispersal of cormorants from the interior delta may, temporarily, limit cormorant impacts to Mississippi aquaculture