The uptake, accumulation and retention of 137-caesium by salmonid fish in fresh water

Abstract

The Chernobyl disaster on 26ih April 1986, caused contamination over much of the U.K. with radiocaesium, principally via rainfall in upland areas such as N. Wales and S.W. Scotland. 137-Caesium was of particular concern in the freshwater environment as it has a long physical half-life (30yr) and previous studies had shown that 137-caesium accumulated in freshwater animals and that the levels of accumulation increased with trophic level up the food chain. This thesis presents the results of studies on the uptake, accumulation and retention of 137-caesium in salmonid fish in fresh water. A number of previous studies both m vitro and in vivo have observed biochemical similarities of caesium to potassium. The uptake of 137-caesium and 86- rubidium (as a tracer for potassium) by erythrocytes of the rainbow trout (Oncorhynchus mykiss Walbaum) was studied. The total caesium influx was much smaller than that of potassium ( 14.4 and 796.0nmoles min ' ml ' packed cells respectively), at an external concentration of .3mM. Potassium influx was significantly inhibited by caesium and vice versa, at concentrations >0.1 mM. The results indicated that caesium behaved as potassium in a qualitative but not quantative manner. This conclusion, together with evidence from the literature, was used to justify biochemical compansons of the two elements later in the thesis. The accumulation of 137-caesium was studied in the early stages of development of Atlantic salmon {Saimo salar L.) and brown trout {.Salmo trutta L.). The accumulation of the isotope in the eggs of brown trout was relatively small until a few days prior to hatching, when the 137-caesium concentration factor (C F.) increased rapidly. The accumulation of 137-caesium in juveniles of Atlantic salmon and brown trout followed a first order rate equation, ie. the rate of increase of accumulation decreased until a constant, equilibrium C.F. was reached. 137-Caesium accumulated several times above the concentration in water, reaching equilibrium after 4-6 months at C.F.s of approximately 10-12 at ''normal" pH (•7.4), The uptake was greatest in gills, muscle, liver and kidney. The majority of the radiocaesium was however, deposited in muscle tissue and this had consistently the longest biological half-life (t,,,)- Accumulation was significantly reduced at low" pH (-5.0) in both species. This was attributed primarily to an inhibition of caesium uptake by protons in a manner similar to that recorded in the literature for other group 1 metals. The elimination of 137- caesium from juvenile Atlantic salmon was best described by a single exponential equation and was little affected by increased acidity. The accumulation of 137-caesium in alevins of the two species also followed first-order kinetics. The accumulation was much more rapid however, and reached much greater C.F.s (>50) at equilibnum. This difference between the stages of development was attributed to the greater metabolic rate (MR) of the alevins and/or their extra dependence on water as a soua'e of ions. No consistently significant differences were recorded between the two species. The branchial and intestinal influx of 137-caesium was measured in a perfused, whole-body preparation of the rainbow trout {O mykiss). The in vitro caesium fluxes recorded in these experiments were in line with values in the literature, Branchial influx displayed saturation kinetics, with and K* values of 1.17^molcs Cs kg ' fish h ' and 3,93mM respectively, and was therefore concluded to be a mediated process dependent upon a limited number of carriers or active sites, In contrast, intestinal influx was not saturable but was directly proportional to the mucosal concentration of caesium and was thought to occur via paracellular. "leak" pathways. Intestinal influx was consistently greater than branchial influx; this difference increased with caesium concentration as the branchial influx approached saturation. Reduction of ambient pH had no significant effect on short-term branchial influx. Caesium binding to intracellular muscle protein in rainbow trout (O mykiss) was relatively weak and the majority of the caesium remained "free" at in vivo caesium concentrations. It was therefore concluded that intracellular binding was not a significant factor in the long-term retention of radiocaesium seen in nature. It was suggested that caesium transport occurred via ion-specific potassium channels and that the relatively slow uptake and long retention of radiocaesium was due to its large size with respect to such channels. The results of this thesis were used to speculate on the consequences of a single input of 137-caesium into a previously uncontaminated lake. Initially, direct branchial uptake would be the greater source of radiocaesium in fish, to which larval forms would be the most susceptible. As the dissolved concentration of the isotope decreased however, due to flushing and loss to sediments, the radiocaesium concentrations in food organisms would gradually increase. Food would then become the primary source of radiocaesium in fish, although the uptake would obviously vary significantly depending on the diet.