The effect of various metals on Gyrodactylus salaris (Platyhelminthes, Monogenea) infections in Atlantic salmon (Salmo salar).

Atlantic salmon (Salmo salar) parr (age 0+), infected by the ectoparasite Gyrodactylus salaris, were exposed to aqueous aluminium (Al), copper (Cu), zinc (Zn), iron (Fe) and manganese (Mn), at 4 different concentrations. There was a negative correlation between G. salaris infections and metal concentrations in both Zn- and Al-exposed salmon. In the Zn-experiment, all 4 concentrations tested caused a decrease in the G. salaris infections, while in the Al-experiment the G. salaris infection did not decline at the lowest concentration. The number of G. salaris increased continuously during the experiments in all control groups, and in all groups exposed to Cu, Fe and Mn. At the highest concentration, however, copper seemed to impair the growth of G. salaris infection. The results show that aqueous Al and Zn are environmental factors of importance controlling the distribution and abundance of the pathogen G. salaris. Other pollutants might also have an influence on the occurrence of G. salaris. Finally, the results demonstrate that aqueous Al and Zn have a stronger effect on the parasite than on the salmonid host, suggesting that both metals may be used as a pesticide to control ectoparasites such as G. salaris.

Size and charge fractionation of aqueous aluminium in dilute acidic waters: effects of changes in pH and temperature.

In natural waters, aluminium occurs in different physico-chemical forms, depending on pH, temperature and the presence of inorganic and organic ligands. Conventional methods for fractionation of AI species do not fully succeed in separating monomeric, i.e., low relative molecular mass (Mr) AI species, from polymeric colloidal AI species. In the present work, hollow-fibre ultrafiltration and acid digestion steps are introduced prior to the ordinary Barnes-Driscoll procedure. By this method, a colloidal AI fraction is separated from the particulate fraction, i.e., the fraction that is able to precipitate by the force of gravity. Both the monomeric and colloidal AI fractions are characterized according to Mr and the chemical properties such as hydroxyquinoline-isobutyl methyl ketone-extractability and Amberlite IR-120 cation exchangeability. Changes in pH and/or temperature were found to be critical for the analytical results of AI. By combining size and charge fractionation techniques, increased information concerning AI species in dilute freshwaters was obtained. One important investigation is the presence of high Mr AI species in the operationally defined monomeric AI fractions.

The mixing zone between limed and acidic river waters: complex aluminium chemistry and extreme toxicity for salmonids.

When liming running waters, dosers must compensate for different flow and water qualities and for the downstream inflow from acid tributaries which creates mixing zones. At a certain point in the mixing zone, a constant or fluctuating chemical disequilibrium will appear due to transformation processes. In laboratory assays, over-saturated solutions of aluminium with ongoing active precipitation of aluminium have been found to be especially toxic to fish. Recent experiments in a mixing zone in the limed River Audna, Norway, have confirmed this phenomenon. Atlantic salmon (Salmo salar L.) and sea trout (Salmo trutta L.) smolts were exposed to acid and limed waters and mixtures of the two waters downstream from the point of connection. In the acid tributary (mean values: pH=4.8, Ca=1.3 mg litre (-1)), Ali 236 microg litre(-1)=), LT5) was 22 and 40 h for Atlantic salmon and sea trout, respectively. In the mixing zone (pH=4.8-6.5, Ca=1.2-3.2 mg litre(-1), Ali=50-240 microg litre(-1)), LT50 was 7 h for both species, masking the normal species difference in tolerance. Osmoregulatory failure and rapid gill lesions occurred in the mixing zone as an effect of the transformation of Al into high molecular weight precipitating species. This is the first documentation of the existence of such highly toxic mixing zones in nature, and the results clearly show that the mixing zone is even more toxic to fish than acid aluminium-rich waters.