Report No FR/D0011

Aug 1993

Ammonia is highly toxic to fish and is released to fresh waters by agricultural and industrial activities. Water quality standards for ammonia e.g. 25 µg l^-1 NH₃ (95 percentile) for salmonids (EIFAC, 1970), have in the past been derived from mortality studies. However unfavourable chronic physiological and sublethal effects may be caused by lower concentrations of ammonia.

The objective was to assess sublethal effects of ammonia on sensitive fresh water species such as wild populations of salmonids, especially at early stages of the life cycle which are regarded as most vulnerable, with the aim of recommending water quality standards for ammonia.

Laboratory experiments were based on relevant ammonia and pH values obtained from field data. Techniques were developed to detect sublethal responses of fish to ammonia including measurement of sodium fluxes (unidirectional influx and efflux). The rates of these fluxes control body sodium balance. The method is non invasive and measures perturbations in sodium balance of fish provoked by the presence of pollutants in the water.

Results

1. The most sensitive stage of the early life cycle of salmonids is the late alevin/first feed fry, especially when it has yet to feed.
2. The juvenile stages of wild Atlantic salmon (Salmo salar L.) and brown trout (Salmo trutta) are more sensitive to ammonia than cultured rainbow trout (Onchorhynchus mykiss). Salmon and brown trout alevins and start feed fry responded physiologically to ammonia levels as low as 12.5 µg l^-1 NH₃ (pH 7, 10ºC). Ammonia calculations are based on EIFAC (1970).
3. Chronic exposure to 50 µg l^-1 NH₃ (pH 7) resulted in delayed hatching and up to 100% mortality of salmon and brown trout (Salmo trutta) juvenile stages. In some cases these effects were observed at 25 µg l^-1 NH₃ (pH 7).
4. Salmon alevins showed significant post exposure mortalities at and above 50 µg l^-1 NH₃ (pH 7, 10ºC).
5. The sublethal effects of ionised ammonia (NH₄⁺) are recognised e.g. NH₄⁺ is a competitive inhibitor of Na⁺ uptake.
6. Nutritional state is an important factor in ammonia tolerance of start feed fry.
7. For late rainbow trout alevins a 24 hour episode of ammonia (500µg l^-1 NH₃ pH 7, 10ºC) resulted in increased body levels of ammonia together with decreased Na⁺, K⁺ and Cl^-. Episodic ammonia loading and post-exposure unloading were out of phase with Na⁺, K⁺ and Cl^- loss and recovery. In particular recovery of K⁺ was incomplete even 1 week post-exposure. Subsequent ammonia episodes during the period of ionic recovery would have severe implications.

Conclusions

Future standards for ammonia should be based not solely on NH₃ but on total ammonia. Previous recommendations suggested that unionised ammonia levels should not exceed 25 µg l^-1 and total ammonia 1 mg l^-1. However our study suggests that even these levels may be too high to prevent chronic sublethal effects on salmonids, particularly wild populations.

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