Abstract.-The exotic parasite Myxobolus cerebralis was first detected in native adult Yellowstone cutthroat trout Oncorhynchus clarkii bouvierii from Yellowstone Lake in 1998, seriously threatening the ecological integrity of this pristine, naturally functioning ecosystem. We immediately began to assess the prevalence and spatial extent of M. cerebralis infection in Yellowstone cutthroat trout within Yellowstone Lake and to determine the infection risk of age-0 Yellowstone cutthroat trout, the relative abundance and actinospore production of tubificid worms, and the basic environmental characteristics of tributaries. During 1999-2001, juvenile and adult Yellowstone cutthroat trout were infected throughout Yellowstone Lake; the highest prevalence (15.3-16.4%) occurred in the northern and central regions. Exposure studies in 13 streams indicated that Pelican and Clear creeks and the Yellowstone River were positive for M. cerebralis; the highest prevalence (100%) and severity was found in Pelican Creek during mid-July. Sexually mature individuals of the oligochaete Tubifex tubifex were most abundant in early summer, were genetically homogenous, and were members of a lineage known to produce moderate to high levels of M. cerebralis triactinomyxons. Only 20 of the 3,037 sampled tubificids produced actinospores after 7 d in culture, and none of the actinospores were M. cerebralis. However, one non-actinospore-producing T. tubifex from Pelican Creek tested positive for M. cerebralis by polymerase chain reaction. Stream temperatures at Pelican Creek, a fourth-order, low-gradient stream, were over 208C during the first exposure period, suggesting that T. tubifex were capable of producing triactinomyxons at elevated temperatures in the wild. Although the infection of otherwise healthy adult Yellowstone cutthroat trout within Yellowstone Lake suggests some resistance, our sentinel cage exposures indicated that this subspecies may be more susceptible to whirling disease than previous laboratory challenges have indicated, and M. cerebralis may be contributing to a significant recent decline in this population.

Histopathology was most severe in cartilage of the cranium and the lower jaw, whereas cartilage of the nares and gill arches was seldom damaged. This study suggests that Yellowstone cutthroat trout are highly vulnerable to M. cerebralis and that current population declines in the Yellowstone Lake basin may, in part, result from whirling disease. Our results answer important questions in fish health and will aid in the development of diagnostic tools and management efforts against this pathogen in native cutthroat trout and other vulnerable salmonids.

A combination of stomach content and nitrogen (δ 15 N) and carbon (δ 13 C) stable-isotope analysis was used to assess the trophic interactions and feeding habits of three notothenioid coastal fish (Champsocephalus esox, Patagonotothen tessellata and Patagonotothen cornucola) and one exotic salmon species (Oncorhynchus tschawytscha) with diverse life habits (benthic and benthopelagic) in the Francisco Coloane Coastal Marine Protected Area, southern Chile. The stomach contents of C. esox were mainly fish; those of P. cornucola and O. tschawytscha were crustacean decapods, Munida gregaria. A cluster analysis on isotope data and stable-isotope Bayesian ellipses detected two different predator groups, one with benthopelagic habits (C. esox and O. tshawytscha) and one with benthic habits (P. cornucola and P. tessellata). These results were supported with similar isotopic trophic level of each group. We suggest that the exotic salmon O. tschawytscha is a generalist predator with a broad trophic niche that may compete with the native notothenioid C. esox, as both have equivalent trophic levels with substantial overlap. This preliminary study is the first on trophic relationships of a subtidal fish assemblage within a remote ecosystem of fjords and channels in Chile's southern Patagonia.

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Infection by the invasive parasite Myxobolus cerebralis (causing whirling disease in salmonids) is strongly influenced by a stream's physico-chemical characteristics, which might affect host pathology. We examined whether environmental variables of a M. cerebralis-positive tributary to Yellowstone Lake, Yellowstone National Park, USA, correlated with the histopathology of naturally infected native cutthroat trout Oncorhynchus clarkii bouvieri. Host inflammatory response and cranial cartilage lesions were the main correlates with whirling behavior. Canonical correlation analyses showed that the prevalence of trout with severe lesions in the cranial and jaw cartilages was highest in stream sites with a combination of high temperature and low specific conductivity. Our results reveal that environmental components can affect when and where a pathogen resides within the host, and manifestation of disease. Recognition of the synergism among environmental and histopathology factors most conducive to whirling disease will increase our prediction and detection abilities for M. cerebralis in salmonid hosts.KEY WORDS: Whirling disease · Pathogen · Invasion-environment synergism · Canonical correlation · Yellowstone ecosystem Resale or republication not permitted without written consent of the publisherDis Aquat Org 93: [225][226][227][228][229][230][231][232][233][234] 2011 Hydrothermal and high elevation streams are examples of systems where the physical and chemical environments are variable (e.g. severe temperature or pH fluctuations) and can inflict additional stresses on the native biota beyond seasonal changes. In Lake Wabamun in Alberta, Canada, thermal effluents facilitated parasite transmission between hosts throughout the year and increased prevalence of certain parasites (Sankurathri & Holmes 1976). Synergisms between infection by metacercaria and exposure to pesticides and herbicides were reported as the cause of vertebral deformities in frogs (Kiesecker 2002). These are examples of environments where natural (e.g. hydrothermal) and anthropogenic (e.g. pollutant) stressors often have a strong influence on parasite-host interactions and development of diseases.However, synergisms between the effects of environmental stressors and development of disease from parasitic infection in wild fish are seldom examined through a histopathology approach (but see Roubal 1994). Histopathology is key to identifying target organs of parasitic infection and pathogens' mode of action at the biochemical level of organization (severity of disease), which may affect ecosystem function at the population level of organization (Schwaiger 2001). Histopathological anomalies in fish, for example, are frequently used as indicators of chemical pollution in marine and fresh water environments (Schwaiger 2001, Wester et al. 2002, Kent et al. 2004, identifying risks of population declines. But we found no examples in the scientific literature of studies on the potential relationships between environmental stress and histopatholog...