Michigan Department of Natural Resources
Fisheries Research Report No.1832, 1976
Ricker Equilibrium Yield for Unexploited Populations of Crayfish

Walter T. Momot and Howard Gowing

Abstract.-The Ricker model of equilibrium yield, programed for the computer by Paulik-Bayliff (1967), prescribed a sustained yield of crayfish in two small limestone sink lakes (located in the Pigeon River Area, Otsego County, Michigan). A crayfish fishery was created subject to specific regulations. The fishery developed according to theory. After the initial high first-year yield in West Lost Lake, the subsequent harvest decreased, then stabilized. Similar fishing regulations in North Twin Lake during the first 2 years resulted in an underharvest. The computer model prescribed a minimum 2-fold increase in fishing effort. Doubling the effort increased the harvest 1.5-fold and, in the fourth year, produced a yield comparable to those of the first 2 years.

The biotic response, of an unexploited population of crayfish to exploitation, subsequently showed (1) both an increase in the proportion of young animals in the population, and an increase in their growth, and (2) a decrease in annual production of the population. Egg production and recruitment did not respond.

The exploited crayfish population adjusted mainly through change in survivorship rather than a change in fecundity. Lack of a response in egg production was due to the selective nature of the fishery for males. By selectively fishing for males, the relative and absolute proportion of females in the population increased. Unexpectedly, the increase in females eventually resulted in low recruitment to the exploited stock.

Greater numbers of age-0 recruits were consistently produced in West Lost Lake than in North Twin Lake due to the greater annual availability and quality of the microhabitat.