Cases of mass fish death, mostly flat head mullet (Mugil cephalus), were observed multiple times in Israeli micro-estuaries. A fish kill event that took place in a short span of time in multiple micro-estuaries during the spring of 2021 is used in this research as a test case to identify the cause of these events. In the past, lack of oxygen, morbidity induced by pathogens, and toxicity of organic and inorganic pollutants were tested by experts as causes for mass fish kills, with no conclusive results. Some experts suggested ammonia toxicity as a possible cause. Ammonia is released during organic matter decomposition and is abundant in Israeli coastal streams, which are frequently subjected to effluents. Dissolved ammonia exists at an acidity-controlled equilibrium between two forms: the ammonium ion (NH4+) and the non-charged ammonia (NH3). High concentrations of the non-ionized form damage nerve cell membranes and can lead to damage to the central nervous system and even death.
In the present study, the concentration of the non-ionized (toxic) form was calculated based on measurements of total ammonia concentration, temperature, and salinity in the Alexander micro-estuary during 2014-2021. The calculation was conducted based on typical acidity values in Israeli micro-estuaries. The calculated concentration did not reach toxic values at pH 7.5, but was higher than the acute toxicity threshold (LC50 48h) during 25% of the time at pH 8.5. Total ammonia and non-ionized ammonia concentrations in the estuary water were higher during spring than in other seasons, coinciding with observations of significant mass kill events.
The findings indicate that ammonia, directly and in combination with other stress factors, may be a significant contributor to mass fish kill events. This study joins the conclusions of other research describing the consequences and damage to the aquatic environment from prolonged effluent discharge. Note: the full article was published in Hebrew.
Citation: Suari, Y., Milstein, D., Gilboa, M., Sade, T., Zedaka, H., & Topaz, T. (2022). Ammonia toxicity as a possible explanation for mass fish kills in the Israeli micro-estuaries [In Hebrew]. Ecology and Environment, 13(2). https://magazine.isees.org.il/?p=52203