Historic and contemporary use of catfish aquaculture by piscivorous birds in the Mississippi Delta

ABSTRACT Piscivorous birds are the primary source of catfish (Ictalurus spp.) depredation at aquaculture facilities in northwestern Mississippi. Of particular concern is the Double-crested Cormorant (Phalacrocorax auritus), which can cost aquaculture producers millions of dollars annually through the depredation of cultured fish. Historical research conducted in the early 2000s estimated cormorant use of aquaculture ponds in the region, but aquaculture area has decreased by more than 70% since those estimates were made. With less aquaculture available, we predicted cormorant densities on aquaculture would be greater today than historically. Applying a similar methodology as in historical studies, we used aerial surveys to collect data on cormorants at night roosts and using catfish aquaculture ponds during 3 consecutive winter seasons, beginning in 2015. Although the mean annual number of cormorants at roosts in the Delta during our study was 64% less than historically, we found no significant change in densities on aquaculture, suggesting that aquaculture area is likely the factor influencing cormorant occurrence in northwestern Mississippi. During contemporary surveys we also measured the abundance of Great Blue Herons (Ardea herodias) and Great Egrets (A. alba) on the aquaculture clusters, and built predictive models of abundance relative to variables associated with forage at and surrounding the clusters. We found abundance of all 3 species was strongly related to the amount of aquaculture area both within and surrounding a cluster, although patterns varied by species. Cormorant abundance was also greater on clusters with proportionately more food fish (≥20 cm in length) than fingerlings (<20 cm) and was positively related to the proximity and size of night roosts. The relationships described here can be used by producers and wildlife managers to predict the abundance of these piscivorous birds at aquaculture facilities and to design efficient management plans to mitigate potential impacts of depredation and disease. LAY SUMMARY Mississippi contains ∼60% of all catfish production in the U.S., and bird depredation costs producers millions of dollars annually. We studied how Double-crested Cormorants, Great Blue Herons, and Great Egrets use commercial catfish aquaculture in Mississippi. However, catfish production in Mississippi has declined by over 70% since its peak in the early 2000s, raising questions regarding bird use, particularly by cormorants. Does less aquaculture mean more cormorants per pond? Has the aquaculture decline affected cormorant abundance in the region, and is catfish aquaculture a driver of local bird abundance? To address these questions, we compared surveys of cormorants from the early 2000s with contemporary surveys. We found fewer cormorants in Mississippi today than in the past but cormorant density on ponds has not changed, suggesting a link to the amount of catfish ponds. Local abundance of all fish-eating birds surveyed was also related to the amount of catfish aquaculture.