Foraging Ecology and Depredation Management of Great Blue Herons at Mississippi Catfish Farms

Abstract

Great blue herons (Ardea herodias) occur at high densities at catfish farms in the southern United States. They are perceived by farmers to prey heavily on fish stocks. After a field study at selected catfish farms in Mississippi, we describe (1) the pond conditions where great blue herons intensively foraged, (2) the prevalence of disease in catfish that were captured by herons, and (3) predation rates and economic effects of herons among selected pond situations. Heron abundance was significantly associated with season and was greatest during the fall (Sep-Oct). We characterized catfish ponds as having high (≥6 birds) or low (≤3 birds) foraging activity by herons and characterized pond type, disease prevalence, and water quality. Categorical models showed a significant association of heron activity with disease prevalence in ponds (diseased) and fingerling ponds. Based on model parameters and associated odds ratios, high heron activity was 6.6 times greater at fingerling ponds than at food-fish ponds, and 40.1 times greater at diseased ponds than at those without diseased fish. This was presumably because fingerlings are a more desirable prey size, and disease makes catfish more vulnerable to heron predation. Based on pathology reports, 85% (n = 55) of the live catfish captured by herons from high-activity ponds were diseased, of which 76% were considered to have a terminal condition. In contrast, 75% (n = 63) of the catfish captured by herons congregated at ponds where catfish were being fed were diagnosed as healthy and only 3 (5%) were considered to have a terminal condition. Although both disease and fish feeding bring catfish to the surface and increase their vulnerability to heron predation, we suggest that heron harassment efforts by farmers be focused during fish feeding when heron capture rates are the highest and the greatest predation on healthy catfish occurs. Based on heron foraging rates, average numbers of herons seen, and the duration of foraging activity, we estimated low expected heron predation losses at catfish ponds over time. Assuming that predation losses observed in this study are representative, we conclude that catfish predation losses from great blue herons are either insignificant or readily preventable.