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 eva luate 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, aver aged 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, g reat blue herons probably ate about three trout/d. Great blue herons consum ed trout averaging 21.6 cm in total length, which is consistent with publis hed 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 gr eat blue heron predation at the unprotected pool and extrapolated losses fo r each facility. Negligible trout losses at two sites were associated with either no great blue heron use of the unprotected pool or inventory shortag es from the protected pool exceeding 2,800 fish. At the remaining three sit es, trout losses ranged from 9.1% to 39.4%. The economic impact of these lo sses relative to great blue heron use patterns is discussed. Because large fish losses were documented from both bird predation and other causes, furt her controlled studies of this nature are recommended as a procedure for aq uaculture managers to assess their losses to great blue herons and other av ian predators.