Diseases of fishes caused by Aeromonas spp. are common, have broad host ranges and may cause high mortality. Treatments of captive-reared populations using antimicrobials are limited with concerns for bacterial resistance development and environmental dissemination. This study was done to determine whether selected plant-derived essential oils were bactericidal to Aeromonas spp. Initially, twelve essential oils were evaluated using a disk diffusion assay to an isolate of A. salmonicida subsp. salmonicida, cause of fish furunculosis. The greatest zones of inhibition were obtained with oils of cinnamon Cinnamomum cassia, oregano Origanum vulgare, lemongrass Cymbopogon citratus and thyme Thymus vulgaris. Minimum bactericidal concentrations (MBC’s) were determined for these four oils, Allimed® (garlic extract, Allium sativum) and colloidal silver to sixty-nine isolates representing nine Aeromonas spp. The lowest mean MBCs (0.02–0.04%) were obtained with three different sources of cinnamon oil. MBCs for three sources of oregano and lemongrass oils ranged from 0.14% to 0.30% and 0.10% to 0.65%, respectively, and for two thyme oils were 2.11% and 2.22%. The highest concentration (5%) of Allimed® tested resulted in MBCs to twelve isolates. A concentration of silver greater than 15 mg/L would be required to determine MBCs for all but one isolate.

We investigated the effect of temperature on gastric evacuation rate and growth at ration of ruffe Gymnocephalus cernuus because of its importance for estimating food consumption by this invasive nuisance species. Gastric evacuation rate was estimated at 8, 14, 19, and 23ЊC using groups of 16-48 fish that were fed chironomid larvae, an important prey species for many ruffe populations. Growth at ration was estimated at 14, 19, and 23ЊC for daily rations of 2, 4, and 8% of body mass using groups of 7-8 fish. Gastric evacuation rate (R ) was related to temperature (T ) by the exponential function R ϭ 0.024 ϫ e 0.075 ϫ T ; r 2 ϭ 0.90. Between 8ЊC and 23ЊC, the magnitude of the temperature response of R was less for ruffe than for yellow perch Perca flavescens and European perch P. fluviatilis, but the European perch and ruffe R versus temperature functions intersected at approximately 4ЊC. Growth of ruffe ranged from Ϫ0.24 to 0.55% of wet body mass per day, increased with ration, and declined with increasing temperature. Our results suggest that, compared with perch, ruffe are better adapted to cooler temperatures and that their metabolism is less sensitive to temperature.

No abstract

Ruffe (Gymnocephalus cernua) were introduced to North America from Europe in the mid-1980s and based on similar diets and habit use may compete with yellow perch (Perca flavescens). To examine competitive interactions between invasive ruffe and native yellow perch, individually marked perch and ruffe were placed in mesocosms in a small lake. Mesocosms allowed fish to interact and feed on the natural prey populations enclosed. In the first experiment, four treatments were assessed: 28 perch, 14 perch + 14 ruffe, 14 perch, and 7 perch + 7 ruffe. Yellow perch growth was significantly lower in the presence of ruffe (ANOVA, p = 0.005) than in treatments containing only perch. In a second experiment, an increasing density of one species was superimposed upon a constant density of the other in parallel treatment series. Growth rates of both ruffe and perch declined when ruffe density was increased (t test, p = 0.006). However, neither ruffe nor perch growth was affected by increasing perch density. Total stomach content mass of perch was significantly decreased by ruffe in both years (p < 0.02), but no effects of ruffe on the composition of perch diets were observed. Ruffe growth and food consumption was greater than that of perch for both experiments. Ruffe can outcompete yellow perch when both species depend on a limited benthic food resource. Thus there is reason for concern for the ecological effects of ruffe if they expand their range into Lake Erie or North American inland lakes that contain yellow perch.