Assessment of factors affecting metal burden in the stone loach (Noemacheilus barbatulus L.)

This thesis is concerned with exposure of stone loach ( Noemacheilus barbatulus L.), a fairly common fish in British rivers, to cadmium and lead. The work described in this thesis was performed to assess the relative importance of different routes of entry (water, food and sediment) on the body burden of cadmium and lead in stone loach under field conditions. These studies were based on the premise that metabolic rate of stone loach is an important factor affecting metal burden. Metabolic rate in fish is dependant on factors which include body weight, temperature, and physiological condition. Therefore, a wide weight range of fish was deliberately used in this study, to obtain information on the effect of body weight, in contrast to common practice in laboratory studies where uniformity of body weight is used to reduce the variation in effect on uptake of pollutants.This thesis is divided into the following parts:- introduction (part I);- laboratory studies (part II);- field studies (part III);- mathematical modelling (part IV);Part I of this thesis describes where lead and cadmium are present in the environment and the effect of human activities on distribution of the metals. This distribution was studied in areas in Derbyshire (UK), where one site (the River Ecclesbourne with mineral veins at the head of the valley), had higher metal levels in sediment and water than two other sites (both with 'background' levels of metals). In general, uptake of cadmium and lead by fish may occur via three pathways, but there is no consensus over their relative importance, not even within one species used in different studies. The literature is reviewed on environmental fate and effects on organisms of cadmium and lead. (chapter 1).Part II (laboratory experiments) describes the laboratory experiments which were aimed at obtaining quantitative information on rates of uptake and loss of cadmium from three possible sources. Chapter 2 reports exposure to water-borne cadmium by the stone loach at different temperatures and over a range of cadmium concentrations. Rates of uptake and loss of cadmium were found to be affected by size of the fish and to increase with temperature. There was some evidence that cadmium burden of the loach reached a maximum after exposure around 16 °C. Fed fish appeared to have a higher rate of cadmium uptake from the water than had starved fish. Published rates of loss after exposure to cadmium cannot be readily compared because duration of exposure may affect rates at which a fish gets rid of cadmium.Metabolic rate was estimated by the oxygen consumption rate. Two levels were distinguished: a low 'routine' level, when the fish were at rest during light, and a higher 'active' level, when fish were swimming, which was observed for a few hours after the onset of darkness. The difference between these two levels narrowed towards the upper end of the temperature range used (6 - 18 °C), suggesting that loach became stressed at high temperatures. A similar effect of temperature on rate of cadmium uptake from Tubifex , used as food items, into fish was found. Absorption efficiency of cadmium from food by the loach decreased with increasing cadmium concentration in the food; also, in spite of increased food consumption at higher temperatures, cadmium burden of stone loach did not rise in proportion because of reduced absorption efficiency of the metal. (chapter 3).Exposure of stone loach to sediment with levels of cadmium and lead higher than 'background' increased body burdens of both metals (chapter 4). High rates of loss resulted in the rapid development of a steady state. The biological half-life for cadmium was longer than for lead in these experiments.The field studies (part III) were carried out to provide information on the growth rate of stone loach, fluctuations in the fishes' body burden and in concentrations of cadmium and lead in the environment (water, food and sediment) in relation to fluctuations in temperature with season. These field studies included a one-year sampling programme of stone loach in Derbyshire (UK) (chapter 5). The selected streams were the River Ecclesbourne, Brailsford Brook South and Suttonbrook. Growth rate of fish, which depended on temperature and age, was greatest during the spring and summer and was highest in young loach. Fish from all streams had reached their maximum length (about 110 mm) after two years and also their maximum cadmium burden. Differences in body size accounted for most of the variation in cadmium levels between loach of different age groups, but was less important for lead levels; cadmium burden fluctuated more than lead burden. Loach caught in the River Ecclesbourne had higher body burdens of both cadmium and lead; no significant differences were measured between the Brailsford Brook South and Suttonbrook.Water samples were taken from all three streams at four periods and on several occasions during each period (chapter 6). Most of the cadmium, in contrast to lead, was in solution. Flow rate of the water appeared to have a greater effect on the cadmium concentration than on the lead concentration.There was some evidence for seasonal fluctuations: metal concentrations were higher in the early spring than in the autumn.Kick-samples from the streams were examined for the range of invertebrate species. Remains of most of these species were found in the gut contents of loach, which suggests that loach are not very selective feeders on invertebrates. Cadmium levels in invertebrates showed seasonal variation: lowest levels were measured in samples taken during the spring and highest levels in those sampled during the autumn. (chapter 7).Sediment samples were taken at about 4-weekly intervals from the same sites, at the same time, as the fish. Concentrations of lead and cadmium were higher in samples from the River Ecclesbourne. Although concentrations of both metals fluctuated with sampling time, there was no consistent trend at any of the sites. (chapter 7).Part IV describes the development of a mathematical model which incorporates both laboratory and field studies (chapter 7). This model predicted adequately the range of cadmium levels found in stone loach in the field. Stone loach take up cadmium from water, food and sediment. The relative importance of these sources differed between the three sites because of differences in measured concentrations. For the R. Ecclesbourne. younger fish took up relatively more cadmium from water than did older fish; the latter received relatively more cadmium from sediment. The model predicted that consistent lower temperatures caused the metal burden to be lower during the winter period and consistent higher temperatures had the opposite effect.The results of the work presented clearly show the effect of body size on metabolic rate. The model that relates body weight (W) of fish to a dependant variable (M), e.g. cadmium burden, includes an exponent (y) as follows: yM= α* WThe values of the exponent y were comparable for respiration rate, food consumption on different rations and for cadmium burden during and after dietary exposure, during exposure to sediment and in field data. However, the value obtained from water-borne studies was lower. This suggested that, for similar values of a, water was less important as a source of cadmium than food and sediment; comparison of observed cadmium burdens during exposure in the laboratory with field data confirmed this. The values of the exponent for the lead burden obtained from field data and exposure to sediment, were lower compared to the value derived from respiration studies. This suggests that uptake and loss of lead is less affected by metabolic processes than is cadmium. Body burden of lead on exposure to sediment was comparable with that in loach from the River Ecclesbourne, suggesting that most of the lead is taken up from the sediment.Cadmium burden is higher during the winter than during the summer. This is caused by the greater effect of temperature on rate of intake than on rate of loss. The relative contribution of cadmium in water, food and sediment to cadmium burden of loach depends on environmental conditions, such as cadmium concentration in these sources, temperature and body weight. There is virtually no uptake of cadmium from water during the winter; most cadmium originates at this time from food. Particles with which most of the lead is associated and which are a prime source of lead for stone loach, are brought into suspension at higher flow rates and, therefore, exposure increased.The data in this study, compared with the published literature, suggest that the fish's handling of cadmium depends on the duration of exposure; after short-term exposure, the fish eliminate cadmium rapidly while, after longterm exposure, cadmium is not easily lost. This phenomenon may be explained by the time of exposure required for the induction of cadmiumbinding proteins. There is also some evidence that the type of exposure affects the rates of loss, presumably in parallel with the fate of cadmium within the fish. Based on a one-compartment model, the cadmium derived from food seems to remain longer in the body than cadmium originating from water and sediment. There is evidence that stone loach get rid of cadmium rapidly after short term exposure to both dietary and water-borne cadmium. This corroborates the observed fluctuations of cadmium burden in stone loach from the field.The importance of different routes of uptake of both lead and cadmium differs between fish species. Water is often regarded as the sole source of metal although there is evidence that diet can contribute significantly to the body burden of fish. Bottom feeders, in particular, seem able to acquire some metal from sediment. Moreover, concentrations in the different environmental compartments determine their relative contributions to the body burden.