Effects of parasites on fish populations: Practical considerations

Abstract

Fish populations are subject to numerous natural and anthropogenic factors which reduce abundance. In wild populations it is difficult to isolate and quantify the effects of any single factor, such as prédation, or starvation, or disease, on fish stock size. Evidence is accumulating, however, which supports the notion that some of the animal parasites, and particularly the Protozoa, can act as severe pathogens, causing direct mortality or rendering the hosts more vulnerable to other environmental or biotic Stressors. Lethal effects of parasites can be determined statistically or by experiment, but actual observation of mortalities is rare—especially in the sea—principally because of prédation or scavenging. Sublethal effects (a partial misnomer since many such effects lead indirectly to mortality) include muscle degeneration, liver dysfunction, interference with nutrition, cardiac disruption, nervous system involvement, castration or mechanical interference with spawning, weight loss, and gross distortion of the body. Study of fish in captivity or under culture conditions has provided much information about the effects of animal parasites on survival. Epizootics occur, mortalities ensue, immunity may or may not develop, and parasites considered inocuous may prove to be not so under added Stressors such as crowding and poor water quality found in the culture environment.