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

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 losses at two sites were associated with either no great blue heron use of the unprotected pool or inventory shortages from the protected pool exceeding 2,800 fish. At the remaining three sites, trout losses ranged from 9.1% to 39.4%. The economic impact of these losses relative to great blue heron use patterns is discussed. Because large fish losses were documented from both bird predation and other causes, further controlled studies of this nature are recommended as a procedure for aquaculture managers to assess their losses to great blue herons and other avian predators.