Ammonia toxicity in rainbow trout (Salmo gairdneri)

• Main Author: Hillaby, Betty Ann
• Language: English
• Published: 2010
• Subjects: Ammonia; Rainbow trout; Oncorhynchus mykiss
• Online Access: http://hdl.handle.net/2429/21077

Acute ammonia toxicity in rainbow trout was studied. This was carried out by injecting fish with various concentrations of ammonia dissolved in isotonic saline. In order to approximate conditions of natural toxicity, where ammonia would enter the fish at the gills, without the additional problems of environmental factors, the ammonium solutions were injected via a cannula implanted in the dorsal aorta. To determine if a differential toxicity existed in fish in relation to high pH and low pH ammonium solutions, ammonium bicarbonate and ammonium chloride were chosen for injection. Hydrogen ion, and total ammonia, concentrations were measured in blood sampled from the dorsal aorta, both before and after injection. In order to determine if, during the course of the injection, normal excretion rates would remove all of the injected ammonia, total ammonia and hydrogen ion concentrations were measured in blood sampled from both the dorsal and ventral aortae, and rates of extraction of ammonia from blood at the gills were calculated. Ammonia is toxic to fish. There was no significant difference between the dose of NH₄Cl and NH₄HCO₃ which killed fish. Therefore, unlike mammals, fish exhibited no differential toxicity to the ammonium compounds tested. Injection of NH₄Cl decreased pHa and injection of NH₄HCO₃ increased pHa. Both compounds increased the total ammonia concentration in the blood. Although in water the fraction of ammonia which exerts the toxic effects is unionized ammonia, within the fish it is the ionized fraction which exerts the toxic effects. The same dose of ammonium killed fish, but NH₄HCO₃-injected fish which survived had a much higher concentration of unionized ammonia in the blood than NH₄Cl-injected fish which died. Ammonia extracted from the blood in control fish was about one-fifth the amount which killed fish. This, together with the measured increases in blood ammonia following injection, demonstrate that, although ammonia is a normal excretory product of rainbow trout, the trout cannot increase excretion rates sufficiently to rid themselves of an ammonia load. Symptoms observed in fish following injection of ammonium solutions led to the conclusion that ammonia acts at the neural level. === Science, Faculty of === Zoology, Department of === Graduate