James F. Glahn

Distribution and Relative Impact of Avian Predators at Aquaculture Facilities in the Northeastern United States

Abstract We conducted on-site interviews at 61 randomly selected aquaculture facilities in the northeastern United States and initially censused fish-eating birds at 58 of those facilities in June 1995 to determine the distribution of avian predation problems. To examine the relative impact of these birds, we continued to study bird populations and the fish consumption rates of these birds at 30 facilities during the summer of 1995 and at 7 facilities during the spring of 1996. Approximately 80% of the fish culturists interviewed perceived bird predation to be a problem at their facility; this view was even more widely held by trout producers. Consistent with these results, 81% of the facilities surveyed during the first phase of study had at least some fish-eating birds present. When asked to list bird predators in order of importance, 76% of 49 managers responding named the great blue heron Ardea herodias as the most important predator of concern. Consistent with these findings was the presence of great...

Captive Great Blue Heron Predation on Farmed Channel Catfish Fingerlings

Abstract In a series of experiments, we examined feeding behavior of captive great blue herons Ardea herodius and estimated their ability to affect commercial production of fingerling channel catfish Ictalurus punctatus. Specifically, we determined the following: (1) the biomass of channel catfish fingerlings necessary to maintain the body mass of wild-caught captive great blue herons, (2) capture rates of captive great blue herons foraging on channel catfish fingerlings, (3) evaluation of the losses of catfish fingerlings to heron predation, and (4) the effects of selected catfish pond conditions on heron capture rates and body mass changes. Consistent with previous studies, captive herons required approximately 300 g of live catfish daily to maintain their body mass. Based on evaluation of seine haul indices and inventory data, there was no significant difference (P > 0.05) in numbers of catfish lost over time between control ponds in which herons were excluded and test ponds where herons foraged freely...

Foraging ecology and depredation management of great blue herons at Mississippi catfish farms

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.

Impact of Great Blue Heron Predation at Trout-Rearing Facilities in the Northeastern United States

Abstract Stomach content analysis and raceway exclusion trials were conducted to evaluate the impact of great blue herons Ardea herodias at each of five trout-rearing facilities in the northeastern United States. Forty-two great blue herons, collected from July through September 1995 at five facilities, averaged 1.6 trout/bird or about half of their daily food requirement of 300 g/bird, in a single feeding. With two crepuscular foraging periods per day, great blue herons probably ate about three trout/d. Great blue herons consumed trout averaging 21.6 cm in total length, which is consistent with published visual estimates. However, great blue herons consumed trout ranging from 12 cm to 38 cm, suggesting that most commercially produced trout would be vulnerable to heron predation. By comparing trout inventories between pairs of net-protected and unprotected pools, we measured trout losses due to great blue heron predation at the unprotected pool and extrapolated losses for each facility. Negligible trout l...