The importance of parasite mortality in regulating the population size of the AcanthocephalanPomphorhynchus laevisin goldfish

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Abstract
Experiments to study the mortality rate of the acanthocephalan Pomphorhynchus laevis in goldfish are described. Infections with different population densities of parasite were given to fish on one occasion only and on several occasions with different time intervals between infection.The parasites established and grew in the experimental hosts at rates comparable to those in natural hosts on all occasions. The proportion establishing did not decrease with increasing population size and bore no relationship to the presence of an existing infection. The greatest loss of parasites occurred during the period of establishment. Thereafter, mortality was gradual, and the proportion of the population lost was not related to either the density of infection or to the intervals between infections. No evidence of reduced growth rate was found at the higher population densities.It is concluded that parasite mortality does not function as a feedback control and so is relatively unimportant in regulating the population size of fish acanthocephalans. Both field and laboratory evidence indicate that this is controlled by factors influencing recruitment. The equation of mortality and recruitment rates observed in many field populations must therefore be a matter of chance, and the implications of this with respect to the stability of fish-acanthocephalan systems is discussed.

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