The Chronic Effects of Dietary Lead on Freshwater Juvenile Rainbow Trout (Oncorhynchus mykiss): Physiological and Toxicological Approaches

Abstract

<p> The aim of this research was to determine how freshwater fish respond to dietary lead (Pb) in their environment with a respect to both physiological and toxicological endpoints. Previous acute exposure studies have shown that waterborne Pb accumulates in the internal tissues, resulting in hematological alterations and disruptions in Na+, cr and Ca2+ ion-regulation, with waterborne Ca2+ being protective against Pb tissue burden. In comparison with other metals such as silver, zinc, cadmium and copper, very little is known about Pb toxicity via the trophic food chain. </p> <p> Using rainbow trout as the model freshwater fish species, it was discovered that dietary Pb accumulated mainly in the gastrointestinal tract, with the intestine and the stomach being suggested as the primary tissues involved in Pb toxicity. Mild physiological disruptions in terms of plasma Ca2+, Mg2+ and Na+ levels were evident. On a per fish weight basis, the bone accumulated the greatest Pb burden (38%) followed by the anterior intestine (19%) and white muscle (12%). The order of Pb accumulation reflected the exposure pathway of dietary Pb in the internal tissues after 42 days; anterior intestine> stomach> mid intestine> posterior intestine> bone> posterior kidney> anterior kidney> liver> spleen> gill> carcass> brain> white muscle (JJ.g Pb/g tissue wet weight). Despite this accumulation of Pb into the internal tissues, there were no effects on growth and survival rates, indicating that growth is not a sensitive indicator of dietary Pb toxicity in fish. Elevated dietary Ca2+ levels had an overall effect in reducing dietary Pb burdens in the whole body and most of the internal tissues analysed. The red blood cells (RBC) accumulated 99% of the Pb in the blood, with less than 1% in the plasma. The enzyme 8-aminolevulinic acid dehydratase (ALAD), important in hemoglobin synthesis was significantly inhibited by the end of the experiment in fish exposed to high concentrations of dietary Pb. In contrast to waterborne Pb studies, ALAD was not a sensitive biomarker of dietary Pb toxicity. </p> <p> Overall, this study has shown the mild physiological and toxicological disturbances of dietary Ph exposure to juvenile rainbow trout. It encourages studies on transport mechanisms at the intestine and in the blood of fish exposed to dietary Pb. It is the hope that the work from this thesis will be used in risk assessments and environmental policies that mitigate the toxic effects of dietary Pb in the aquatic environment. </p>