Prestocking assessment of the prevalence and intensity of Diphyllobothrium ditremum (Creplin) plerocercoids in freshwater barriered lakes in Alaska

Abstract

Plerocercoids of the pseudophyllidean cestode Diphyllobothrium ditremum (Creplin, 1825) have significantly affected the success of using certain barriered lakes for the rearing, overwintering and smolting of juvenile coho (Oncorhynchus kisutch) and Chinook (Oncorhynchus tshawytscha) salmon by causing mass mortalities of these host fishes. The prevalence of cestode procercoids in copepods, the first intermediate host, was propose as a method for assessing the potential for cestode caused losses of salmon prior to stocking a lake. However, no procercoids were found in a total of 15,276 Diaptomus and 435 Cyclops spp. from three lakes on south Baranof Island examined for procercoids. Diaptomus kenai is suggested as the first intermediate host for Diphyllobothrium ditremum. despite the absence of procercoids in any specimens examined. Diaptomus kenai was the predominant copepod in the three lakes studied, and was the prey item occurring most frequently (percent occurrence, 73.7%) in the stomach contents of 95 resident coho. Coho, Chinook, and rainbow trout (Salmo gairdneri) were obvious second intermediate hosts of D. ditremum. Among three species of piscivorous birds examined from the lake sites, a single common merganser (Merqus merganser) contained seven mature worms resembling D. ditremum. A bioassay study using coho salmon fingerlings in net pens suspended within a "cestode infested" lake proved successful as an assessment method. Plerocercoids of D. ditremum were observed in 91% of the planted coho within twenty days of exposure in Osprey Lake. Coho mortalities of 46.2% and 22.4% were observed in two pens. Mean plerocercoid intensities for apparently normal, moribund, and dead coho were 11, 28, and 32 respectively. Moribund and dead coho each had significantly larger worm loads than apparently normal coho. Primary lesions observed from gross and histopathological examinations of parasitized coho from Elfendahl and Osprey lakes included: ascites with marked distension of the abdomen; hemorrhaging of viscera primarily adipose tissue and liver; and focal necrosis of organs from migrating plerocercoids.