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.

Gyrodactylus salmonis is a monogenean ectoparasite that can infest a variety of captive‐reared salmonid fishes. The physical damage inflicted during severe infestations can cause osmoregulatory disturbances and potentially render individuals more vulnerable to secondary pathogens. If not treated, G. salmonis infestations can reduce growth and survival in affected fish populations. Many chemical compounds have been used to treat Gyrodactylus infestations; however, little information has been published about the use of hydrogen peroxide (H2O2) for Gyrodactylus control. Consequently, we conducted a trial to evaluate the efficacy of H2O2 in reducing a natural infestation of G. salmonis in freshwater‐reared, adult rainbow trout Oncorhynchus mykiss. Triplicate tanks of adult rainbow trout (20 fish per tank; length = 45.6 ± 5.8 cm and weight = 1.3 ± 0.4 kg, mean ± SD) were exposed to a static bath of H2O2 at a target dosage of 50 mg/L or hatchery water (sham treatment) for 30 min/d on two alternate days. Treatment efficacy was assessed at 2 and 7 d posttreatment via light microscopy examination of skin scrapes (one per fish) taken from 10 fish per tank on each day. At 2 d posttreatment, the mean abundance of G. salmonis in the H2O2‐treated group (0.1 ± 0.3 G. salmonis individuals per skin scrape) was significantly different from that observed in the sham‐treated group (34.4 ± 43.2 individuals per skin scrape). Also, at 7 d posttreatment, the mean abundance of G. salmonis in the H2O2‐treated group (0.1 ± 0.3 individuals per skin scrape) was significantly different from that observed in the sham‐treated group (38.5 ± 77.4 individuals per skin scrape). The percent reduction in mean abundance (treated group compared with control group) was greater than 99% at both 2 and 7 d posttreatment. In conclusion, the H2O2 treatment regimen that we used significantly reduced a natural infestation of G. salmonis in freshwater‐reared, adult rainbow trout.

Columnaris (causative agent, Flavobacterium columnare) is a widespread fish disease of concern among fish culturists in the USA. If left untreated, an entire population of fish may become infected, and morbidity and mortality may reach high levels. In virtually all instances, columnaris outbreaks require intervention to prevent significant losses. A number of sanitizing agents, most notably chloramine‐T (CLT) and hydrogen peroxide (HP), have been used to control mortality associated with a variety of bacterial pathogens causing external infections. However, the majority of trials conducted to demonstrate the effectiveness of these chemicals, thereby gaining U.S. Food and Drug Administration approval for their use in treating fish infected with columnaris, have been conducted on salmonids. Accordingly, we conducted seven experiments to evaluate the effectiveness of CLT or HP to control mortality associated with external columnaris in Florida Largemouth Bass Micropterus salmoides floridanus and Bluegill Lepomis macrochirus. Treatment with CLT or HP significantly reduced cumulative mortality in five of the seven experiments. Cumulative mortality was strongly correlated to pretreatment mortality in treated and control tanks in the five Largemouth Bass experiments, suggesting that intervention at later stages of columnaris progression may result in less favorable outcomes. Odds ratios calculated for individual experiments indicated varying degrees of success in controlling mortality; however, meta‐analysis of all experiments indicated treatment with either CLT or HP significantly increased probability of survival, regardless of fish species or test article. These results demonstrate that both chemicals can be effective in controlling mortality associated with external columnaris in Largemouth Bass and Bluegills and that timely treatment of fish will likely result in lower overall mortality.

Salmincola spp. are ectoparasites of major concern in wild and cultured salmonids. These parasites can cause respiratory distress and facilitate the entry of secondary pathogens. Of particular concern in the United States is S. californiensis, which can infest all Oncorhynchus spp. and is restricted largely to freshwater. Bath treatments with formalin and hydrogen peroxide have traditionally been used to control infestations of some parasitic copepods in cultured salmonids; however, these treatments can be difficult to apply, expensive, and stressful to fish and have not been shown to be effective against S. californiensis. A more effective and efficient treatment method needs to be developed. SLICE (0.2% emamectin benzoate [EB]) is a commercial in-feed treatment that has been shown to be effective for the control of sea lice infestations in seawater-reared farmed salmon and trout. We postulated that EB might also be efficacious for the control of parasitic copepods such as S. californiensis on freshwater-reared salmonids. Four trials were conducted to evaluate the effectiveness of SLICE-medicated feed administered at a target dosage of 50 µg EB·kg fish −1 ·d −1 for seven consecutive days to control infestations of S. californiensis on freshwater-reared rainbow trout. At the end of each trial (after either a 30-or 42-d posttreatment period), copepod prevalence in treated tanks was substantially reduced compared with the pretrial prevalence recorded in the reference populations. Additionally, a significant difference was detected in mean abundance between treated and control groups, with a 79-96% reduction in mean abundance among fish offered the EB-medicated feed. Based on the results of these trials, it was concluded that SLICE was efficacious in reducing infestations of S. californiensis on freshwater-reared rainbow trout.Copepods are the most prevalent of the parasitic crustaceans affecting a wide range of marine and freshwater finfish worldwide (Lester and Hayward 2006). Salmincola spp. are parasitic copepods of major concern in wild and cultured salmonids. It has been reported that the incidence and spread of these parasites can be exacerbated by degraded environmental conditions such as increased water temperature, high organic loading, low dissolved oxygen, and overcrowding (Sutherland and Wittrock 428 CONTROL OF Salmincola ON RAINBOW TROUT