Anthropogenic factors have contributed to the precipitous decline of wild Pacific salmon stocks, although the mechanisms and processes at work are largely unknown. Pollution may be one of these factors. Sediments in estuaries are known to act as repositories for contaminants , and estuaries are important habitats for ocean-and river-migrating salmon. We have shown that juvenile salmon Oncorhynchus spp. and their prey bioaccumulate chlorinated hydrocarbons and aromatic hydrocarbons-important classes of toxic xenobiotics. Furthermore, we have shown that exposure to these pollutants can lead to immunosuppression and increased disease susceptibility in juvenile salmon. Whether pollution influences natural disease outbreaks in host populations, including salmon, is currently unknown. It is postulated that the occurrence of disease depends on the interaction of the host, the environment, and the pathogen. Absence of pathogens would reduce the potential for adverse environments to influence disease outbreaks. However, a recent reconnaissance survey of juvenile chinook salmon Oncorhynchus tshawytscha from Oregon coastal rivers revealed that pathogens were an integral component in all systems studied, although the prevalence of the pathogens varied. Furthermore, recent studies of natural fish populations have demonstrated that infectious-disease-induced mortality can significantly reduce the size of the host population. By creating adverse environments (e.g., polluted estuaries) which alter the susceptibility of the host to pathogens that are integral and ubiquitous components of the habitat, pollution increases the probability of disease-related impacts on fish populations.
Dermocystidjum salmonis is a gill pathogen of salmonid fishes in the U.S. Pacific Northwest where it has been associated with mortality of adult and juvenile chinook salmon Oncorhynchus tshawytscha. The previously unknown mode of D. salmonis transmission was determined and demonstrated in the laboratory. Uniflagellated zoospores developed within spores obtained from gill cysts and produced infections in pink salmon 0. gorbuscha fry. These infections were lethal, and histological examination of infected gill tissue revealed large numbers of D. salmonis cysts in gill epithelia. Electron microscopic examination of immature spores from experimental infections showed that they were identical to immature spores in naturally infected juvenile chinook salmon.
The microsporidan Glugea stephani is a common parasite of juvenile English sole (Parophrys vetulus) in Yaquina Bay, Oregon. Field observations indicated that fish became infected only in the upper estuary where summer temperatures were above 15C and the incidence of infection reached 79.8% in the late fall. Laboratory infections developed and parasite growth occurred only at or above 15C. The parasite was successfully transmitted to juvenile English sole by brine shrimp (Artemia salina) and amphipod (Corophium spinocorne) vectors as well as by direct ingestion of spores by the host. Infections that resulted from ingestion of spore-carrying vectors were much heavier than those resulting from the direct ingestion of spores. The speckled sanddab (Citharichthys stigmaeus), a nonpleuronectid flatfish, and chum salmon (Oncorhynchus keta) were refractory to G. stephani infection in the laboratory.
The biota associated with Monochamus species collected in Maine include species of Acarina, Diptera, Hymenoptera and Nematoda, as well as fungal and bacterial pathogens.Mites belonging to seven families and thirteen genera were found in association with the sawyers. Two species were previously unknown and have since been described as the new genus and species Longoseius cuniculus Chant of the family Digamasellidae and the new genus and species Mucroseius monochami Lindquist of the family Blattisocidae. Parasitic diptera belonged to three genera in three different families, and of these one contained a new species, Forcipomyia innom., family Ceratopogonidae, which was found as an intermittent parasite of adult Monochamus notatus and Monochamus scutellatus. This constitutes a new host record for the biting midge family. Only one hymenopterous parasite, a species of Dolichomitus was reared from a sawyer larva. The five genera of nematodes isolated from larvae, pupae, and adults of Monochamus species include specimens of the genus Hexamermis. Parasitism by this group is thought to be unusual because of the relative inaccessibility of the host. Isolates from eight fungal genera were obtained, and strains of Beauveria bassiana, and Isaria farinosa, were shown to be pathogenic. The bacteria isolated from dead sawyers included Aerobacter aerogenes, Bacillus cereus var. mycoides, and one unidentified species. Only the tachinid E. monohammi and the mite Uropoda have been previously reported of the large number of organisms found associated with sawyer beetles in this study. The fungus Beauveria bassiana proved to be the most important biological control agent.
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