Michigan Department of Natural Resources
Fisheries Research Report No.1812, 1974
Disappearance of Dead Fish and Assessment of Mortality
Clarence M. Taube
Abstract.-Assessing the loss of fish that results from a sudden, extensive kill in a lake or stream usually is a difficult task. The estimate probably seldom approximates the actual number, and it is apt to be low. Disappearance of many of the victims is often responsible for much of the difficulty. Concealment by water depth is one of the main causes of disappearance. Adult coho salmon that were introduced into a 1.4-mile stretch of the Platte River in Benzie County afforded a test of how completely the introduced salmon could be recovered after they had spawned and died. Twelve hundred coho were released here each fall in three successive years. After the fish began to die, the stream was patrolled to search for carcasses, at least once every 7 days and until spawning activity had ceased. Despite good conditions for retrieval (clear water, mostly shallow depths, large size of the fish), less than 50% of the salmon were found each year. Evidently, the bulk of those not found were hidden in restricted areas of deep water and beneath cover. Better methods are needed for evaluating fish mortalities. The potentiality for developing such techniques appears to be good. Several approaches are suggested.