Conditions which determine the efficiency of ammonium sulphate in the control of Prymnesium parvum in fish breeding ponds.

The phytoflagellate Prymnesium parvum has been reported in Europe (Otterstroem et al., 1939, Liebert et al., 1920), where it occurs sporadically and causes mass mortality among fishes. In 1947 Prymnesium parvum first appeared in Israel fish ponds, rapidly spreading through regions where water was brackish. Since then it has caused extensive damage to the fish breeding industry and is now the most serious natural obstacle to fish breeding in Israel. Prymnesium kills fish by means of an extracellular protein-like toxin. The control of Prymnesium parvum can be based upon methods which destroy the organism, or upon methods which destroy only the toxin (Shilo and Aschner, 1953; Shilo et al., 1953). After the first appearance of Prymnesium parvum in Israel it was found by Reich and Aschner (1947) that ammonium sulphate exerted a lytic effect on Prymnesium parvum. Low concentrations of the substance lysed Prymnesium parvum. Its effect on other forms of life present in the fish ponds was negligible; in addition, the relatively low cost, high solubility, ease of dispersion and concomitant fertilizer effect of ammonium sulphate made it a most desirable means of controlling Prymnesium. Although the killing effect of ammonium sulphate was found to be of short duration, constant vigilance entailing immediate application of ammonium sulphate upon appearance of Prymnesium in ponds has enabled fish breeding to be continued in the infected regions comprising over onefourth of the country's total fish-breeding area. Of late, it has been observed repeatedly that ammonium sulphate may in some cases prove ineffective as a control measure, even when used in 8-fold, the usual concentration. Failure of control by ammonium sulphate has occurred largely in a particular region (Beisan Valley) and has been found to be especially marked in winter. In order to determine the factors governing the lytic action and the most effective use of ammonium sulphate, a program of investigation, both in the laboratory and in the field, was instituted. The first step was to determine whether the ineffectiveness of ammonium sulphate was connected with the random appearance of sensitive strains of the phytoflagellate, or with a natural selection of a resistant mutant in the course of the years of continued use of ammonium sulphate. Strains of Prymnesium were collected from scores of ponds where resistance to ammonium sulphate was observed, and these cultures were tested in the laboratory for sensitivity to ammonium sulphate. All of the strains, when tested under controlled conditions of temperature, pH, and illumination, were found to be equally and highly sensitive to ammonium sulphate. From the foregoing, the explanation of reduced effectiveness of ammonium sulphate by development of resistant strains appeared unlikely and it became evident that external conditions must play a major role in the lytic action of the ammonium salt on the microorganisms. Hence, it was decided to determine the relative importance of these various conditions for ammonium sulphate activity.