On 30 July 1994, lake trout (Salvelinus namaycush) were discovered in Yellowstone Lake in Yellowstone National Park, Wyoming, the core of the remaining undisturbed natural habitat for native Yellowstone cutthroat trout (Oncorhynchus clarki bouvieri). Data from this and other lake trout subsequently caught by anglers and the U.S. Fish and Wildlife Service suggest lake trout have reproduced in Yellowstone Lake since at least 1989 and now number in the thousands, perhaps tens of thousands. A highly piscivorous, nonnative species, lake trout will probably thrive in Yellowstone Lake and reduce the lake's cutthroat trout stocks substantially unless preventive management actions are taken. A team of scientists that convened in February 1995 to discuss the lake trout problem concluded that there is little chance lake trout can be eliminated from Yellowstone Lake. The team projected a decline of 90% or more in cutthroat trout numbers in 20 years–100 years if the lake trout population is not controlled. The team considered mechanical removal methods, either gillnetting or some combination of gillnetting and trapping, to be the management actions most likely to control lake trout.

No abstract

We conducted a field trial to evaluate the effectiveness of Aquaflor (50% florfenicol) for controlling mortality associated with Streptococcus iniae in freshwater-reared subadult sunshine bass (female white bass Morone chrysops X male striped bass M. saxatilis). Bacterial samples collected from moribund fish representing a reference population were presumptively identified microbiologically and were later confirmed to be S. iniae by biochemical characterization and polymerase chain reaction. The trial comprised a 1-d acclimation period, 10-d treatment period, and 14-d posttreatment period. During the treatment period, Aquaflor-medicated feed was administered to treated tanks (N = 3) at a target dose of 10 mg of florfenicol x kg of fish(-1) x d(-1), and nonmedicated feed was administered to control tanks (N = 3). At the end of the posttreatment period, mean (+/- SD) cumulative mortality in treated tanks (9 +/- 11%) was significantly (P = 0.040) less than that in control tanks (52 +/- 13%). Analysis of medicated feed samples revealed that treated tanks had received an actual dose of 8.3 mg florfenicol x kg fish(-1) x d(-1) (83% of target). No florfenicol was detected in control feed samples. Although the actual florfenicol dose administered to treated tanks was less than the target dose, the trial was accepted by the U.S. Food and Drug Administration Center for Veterinary Medicine as demonstrating the efficacy of Aquaflor to control mortality associated with S. iniae in cultured sunshine bass populations.

Salmincola spp. infestations can adversely affect freshwaterreared salmonids. Control methods tested to date have had limited success; consequently, we conducted a pilot field trial to evaluate SLICE (0.2% emamectin benzoate [EB])-medicated feed to reduce a natural infestation of S. californiensis in freshwater-reared rainbow trout Oncorhynchus mykiss. Before the trial started, 96 of 1,500 rainbow trout broodstock held in a flow-through raceway were impartially captured, sedated, uniquely tagged, and returned to the raceway. Pretreatment S. californiensis infestation prevalence and intensity were 97% and 10.4 ± 7.6 (mean ± SD) adult female parasites per fish, respectively. Treatment was administered at 50 µg EB·kg fish −1 ·d −1 for 7 d. By the end of the trial (43 d posttreatment), infestation prevalence and intensity had decreased to 32% and 1.6 ± 1.1 adult female parasites per fish, respectively. These results suggest that SLICE-medicated feed can be used to reduce natural infestations of S. californiensis in freshwater-reared rainbow trout.

Bacterial gill disease (BGD), caused by Flavobacterium branchiophilum and other species of yellow‐pigmented, filamentous bacteria, is a common and potentially catastrophic disease of hatchery (freshwater)‐reared fish. Chloramine‐T (Chl‐T) is a biocide proven effective for controlling mortality in freshwater‐reared fish diagnosed with BGD. However, Chl‐T is not approved by the U.S. Food and Drug Administration for such use. To generate data in support of a U.S. approval, we evaluated the effectiveness of Chl‐T (administered at 12 mg/L of static bath water for 60 min/d on three alternate days) to control mortality caused by BGD in freshwater‐reared chum salmon Oncorhynchus keta, Apache trout O. gilae apache, and rainbow trout O. mykiss. For each species, the mean percent total mortality in Chl‐T‐treated tanks (N = 3) was significantly less than that in control tanks (N = 3): Chum salmon = 8.9% versus 99.7%, Apache trout = 39.2% versus 97.9%, and rainbow trout = 5.7% versus 25.8%. Because the Chl‐T treatment regimen was efficacious for each species, we conclude that our findings support the approval of Chl‐T for use in the USA to control mortality in freshwater‐reared salmonids diagnosed with BGD.