UPTAKE FROM WATER, DEPURATION AND TIS S U E

D I S T R I B U T I O N OF ll~ IN A FRESHWATER FISH,

Cyprinus carpio L.

J. P. BAUDIN*, J. G A R N I E R - L A P L A C E , and A. LAMBRECHTS

Institut de Protection et de S~rets NuclOaire, DOpartement de Protection de l'Environnement et des Installations, Laboratoire de RadioOcologie des Eaux Continentales, IPSN/CEA, 13108 Saint-Pal-Lez-

Durance Cedex, France

(Received February 25, 1992; accepted March 11, 1993)

Abstract. Radionuclide accumulation was studied on a group of 20 fish maintained at 20 • 1 ~ in spring water containing 30 Bq mL -1 of 110mAg. Because of a very significant radionuclide adsorption onto available surfaces and in order to simulate chronic exposure, the water was completely renewed three times a week. Accumulation of 110mAg was slow, with an estimated time to maximum contamination level of 180 days. The concentration factor, calculated from the ratio of the integrals of the curves representing the radionuclide concentration in filtered water and in whole fish reached a maximum value of 106 (w.w.). After 41 days exposure, 10 fish were placed in non-labelled water, renewed daily, to follow 110mAg deputation. A two-compartment exponential model was fitted to the depuration data. The corresponding radionuclide half-lives were relatively short, 0.8 (Tbl) and 30 days (Tb2), and after 6 weeks the fish retained about 30% of their initial 110mAg content. The liver and digestive tract retained most ll0mAg. Although they only represented respectively about 4 and 10% of the total body mass, these two organs accounted for 30-40% of the total radionuclide body burden.

1. Introduction

For several years radioecological field studies have detected ll~ resulting from radioactive low level liquid effluents released into the natural environment from pressurized water reactors under normal operationing conditions (Picat et al., 1986; Foulquier et al., 1991). For instance, in the Rhone river, 11~ has been measured in fish living downstream from such nuclear facilities (Lambrechts et al., 1988). Since May 1986 its presence in freshwater ecosystems has been related to radioactive fallout due to the Chernobyl reactor accident (Van Dam, 1986). Subsequently, ll~ appeared in areas remote from any nuclear facilities (Petersen et al., 1986; Descamps and Baudin-Jaulent, 1988) and its detection frequencies increased in fish from the Rhone river, in zones subject to liquid waste discharges (Lambrechts et al., 1988). Despite its radioecological importance, l l0mAg has been poorly investigated, as shown by literature reviews (Phillips and Russo, 1978; Coughtrey and Thorne, 1983). Poston and Klopfer (1986) do not mention ll~ in their bibliographical review of the concentration factor for 27 radionuclides in various freshwater and sea fish.

Because of this lack of data, a laboratory research program on H~ was undertaken focused to a large extent on the freshwater fish Cyprinus earpio. This

* C.N.R.S. Researcher.

Water, Air, and Soil Pollution 72: 129-141, 1994. �9 1994 Kluwer Academic Publishers. Printed in the Netherlands.

130 J. P. BAUDIN ET AL.

TABLE I

Main chemical characteristics of spring water used in the experiments (expressed as mg L 1)

Cations Anions

C a 2§ 97.2 H C O 3 - 350.0 M g z+ 19.5 C1- 17.0

N a + 8.9 SO42- 19.0

K + 0.9 N O 2 - <0.05

N H 4 + <0.1 N O 3 - 8.00 - - PO4 3- <0.10

species, selected for its wide geographic distribution and its omnivorous feeding habits (Muus and Dahlstom, 1968), has been the subject of many radioecological studies with several radionuclides (Foulquier et al., 1972; Baudin, 1983; Lambrechts, 1984; Baudin and Fritsch, 1988; Baudin et al., 1981, 1989). In this paper, results from experiments conducted to evaluate 11~ uptake from water and elimination by Cyprinus carpio are reported. This study is part of the development of an explicative and predictive model of 11~ transfer among the different compartments of an experimental freshwater ecosystem. The 11~ accumulation from water was investigated during a 6-week period. After this exposure phase the individuals were transferred to non-radioactive water to follow depuration of the radionuclide. The 11~ distribution in the tissues after uptake and excretion are also discussed.

2. Material and Methods

The experiment was carried out on the leather carp which is a variety of Cyprinus carpio almost devoid of scales. The study used an homogeneous group of 20 individuals approximately 6 months old and of an mean initial body mass of 1.46g (SD 0.10). The fish were placed in an aquarium containing 5 L of aerated spring water (Table I) at a constant temperature of 20 • 1 ~ This temperature was chosen because it is the lower limit of the thermal optimum of carp (D'Aubenton and Spillmann, 1976). During the 41-day exposure, the animals were fed 5 days out of 7 with non-radioactive field captured midge larvae. The ll~ which is an /3 and a 7 emitter, was added as 11~ solution (0.06% of v~ the only form available from the French Office of Ionizing Radiation. Held in dissolved form in 0.1 N NH4OH, this complex can react when diluted to yield a pool of stable soluble compounds such as AgCN, AgCN(OH)-, and Ag(NH3) + (Amin and Rumeau, 1977). Because of radionuclide adsorption onto all available surfaces, it was impossible to maintain a constant radioactivity level in the water with only a single dose. So, to simulate chronic exposure conditions, the water, labelled with 30 Bq mL-1 of 11~ was completely renewed three times a week. In the contaminant solution the 11~ specific activity was 7.2 • 104 Bq txg -1 Ag and on every water

11~ RETENTION BY A FRESHWATER FISH 131

renewing 0.41 Isg L -1 of silver was added. In freshwater, silver concentration range between 0.15 to 0.50 gg L -l (Turekian and Scott, 1965; Coughtrey and Thorne, 1983). The radioactivity of the water was monitored several times a week by measurement of 10 mL aliquots filtered through a 0.45 gm cellulose membrane. The ~l~ uptake kinetics was followed by radioactivity measurements in 10 to 20 individuals selected at random.

At the end of the accumulation phase (41 days), after 60 hr of fasting, 10 fish were dissected to determine the 11~ concentration and distribution in the tissues. The remaining fish were placed in non-radioactive water, changed daily, to study radionuclide deputation. Periodically, all individuals were monitored for 11~ content and after 42 days depuration the tissue distribution of the residual radionuclide was measured.

In the two cases (accumulation and depuration), for the study of the ll~ tissue distribution the same organs of the 10 dissected fish were placed together in a single counting tube in order to improve the radioactivity measurement. Gills were counted with cartilage attached and all bone were used, except the head.

Radioactivity was measured using a single-channel analyser connected to a thallium-activated sodium iodine flat probe. The individuals were removed from the tank, rinsed, blotted dry with tissue, weighed and monitored for l~~ content by whole-body counting. The fish were mildly anesthetized, by placing them in aqueous solution of 0.3 g L -1 of Sandoz MS-222, to ensure repeatable positioning for the radioactivity measurements.

Radionuclide accumulation from water was estimated using the concentration factor (CF). This radioecological parameter is defined as the ratio, at any moment, of the ll~ concentration in the fish to the concentration in the filtered water. During the uptake phase, because of the cyclic variation of the radioactivity of the medium (water renewed three times a week), the concentration factor was calculated using the following equation:

CF(t) = ll0m ll0m [ Ag]carv(t)/[ Ag]0.45gm filtered water(t) :

= [1 l~ ) d t / ll~ [ g]0.45~m filtered water(t) d(t)

The numerator is the integral of the curve representing the development of the l l~ concentration in the fish, calculated by exact integration of the equation describing this development. The denominator is the integral of the curve representing the development of the radionuclide concentration in the filtered water, calculated numerically by the three points Simpson method. This integration method accounts for the fluctuations in the radioactivity level of the water liable to affect radionuclide uptake by living organisms.

During the elimination phase, the residual v~ in the fish was estimated by the retention percentage (R). This parameter was used to calculate the biological

132 J.P. I~AUD1N ET AL.

half-life, defined as the time required by an organism to eliminate half of the previously accumulated radionuclide.

Mathematical processing of experimental data assumed a model in which the organism is considered as one or several compartments characterized, for a given element, by uptake and release flux of different rates. This general theory applicable to this type of model has been the subject of various developments and applications, particularly in the field of radioecology (Goldstein and Elwood, 1971; Aoyama and Inoue, 1973; Anderson, 1983). Influx and efflux rates are typical of the element and the organism, and similar for accumulation and elimination. The number of compartments and the kinetics parameter values are determined using the depuration data, because during this phase only one radionuclide flux is involved. In this study, data related to 11~ elimination by carp gave the best fit to a two-compartment exponential model.

The standard equation describing the change in retention percentage (R) during a two-part depuration phase of a radionuclide by a given organism is:

R = C e -Bit + (1-C)e -B2t (1)

where C, B1 and Bz are the parameters to be calculated. The two biological half- lives of the radionuclide (Tbi) can be calculated in conjunction with the formula

Tbi = In 2/Bi. The change of a radioecological parameter P(t) during the two corresponding

uptake phases is expressed by the equation:

P(t) : Al(1-e -Bit) + A2(1-e -Bzt) (2)

where AI and A2 are to be calculated. By extrapolation of this model, the maximum contamination level as well as the time necessary to reach equilibrium can be estimated.

The parameters of the equations describing the temporal development of radio- ecological factors are adjusted on the basis of experimental results using a non linear regression, the Gauss-Marquardt method (Vila, 1982).

3. Results and Discussion

3.1. F I S H G R O W T H D U R I N G THE EXPERIMENT

During the 83-day experiment (41 days of uptake and 42 days of excretion), the carp body mass followed an exponential curve (m = 1.46 e ~176 with a total bio- mass increase of 95%. This weight gain is in the range observed in field studies of carp and other freshwater fish (Chappaz, 1986). These facts indicate satisfactory protein synthesis and suggest that the physiological processes were normal and thus the ll~ uptake by the fish was unlikely to be affected by metabolic disturbances.

l l °mAg RETENTION BY A F R E S H W A T E R FISH 133

90

A

3=

8 "6 u..

o =

o c 0 0

80

70

60

50

40

30

20

10

0

® =B B I

-1.149t -0.029t C F = 1 7 ( 1 - e ) + 8 9 ( 1 - e

(Coefficient determination = 0,96)

A

E o" ID

_O

E o o t- O o

28

24

20

16

12

®

r

L

r -

0 I 0 20 30

Time (days) Fig. 1. Concentration of i JOmAg in water (a) and fish (b) during the exposure phase.

40

3.2. l l °mAg EXPOSURE PHASE

During this phase, the radionuclide concentration in the water varied as a result of adsorption and periodic renewal of the contaminated medium (Figure 1 a). Despite these fluctuations, the fish contamination kinetics demonstrated continuous and regular ll°mAg accumulation with time (Table II). The change of the concentration factor (Figure lb) can be described by the equation:

134 J. P. BAUDIN ET AL.

TABLE II

Bioaccnmulation parameters for 110mAg in Cyprinus carpio

ll0mAg in the fish

Duration Fish body mass Total content Concentration (days) (g, w.w.) a (Bq) a (Bq g-1 w.w.)a

Concentration factor (w.w.) b

2 1.46• 791• 531• 20 6 1.58• 1736+656 1116+440 26 9 1.66• 628• 382• 36

15 1.70• 1575• 917• 51 19 1.57• 2711• 1814+601 57 23 1.80• 3021m493 1694• 64 27 1.87• 2282• 1224• 69 30 1.93• 2401• 1222• 73 34 1.95• 2747• 1413• 69 37 2.00• 3179• 1591• 71 41 1.93• 3225• 1681• 73

a Mean values given with their confidence intervals • t • s/v/-~,, where ' t ' is the Student variable for 95% probability, 's ' the estimated standard deviation and 'n' the number of measurements carried out (10 fish). b Calculated values: ratio of the integrals of the curves of the 11~ concentration in the fish and in the filtered water.

CF = 17(1-e -1.149t) + 89(1-e -0-~ (determination coefficient = 0.96)

By extrapolation from this relation, the maximum l l~ concentration factor (w.w.), reached after 180 d, would be 106.

It must be indicated that radionuclide dilution through growth is allowed for in the model. Considering the theory for compartmental model and that experimental results were corrected for 11~ radioactive decay, the resolution of the system of differential equations that expressed fluxes between the different compartments leads to Equation (2) where each parameters can be decomposed as follows:

B1 = Xl + G A1 = D1/(Xl + G)

B2 = X2 + G A2 = D2/(X2 + G)

where X x and X 2 are the characteristic constants of the depuration phase in com- partment 1 and 2 (ht = in 2/tbi), D1 and D2 the characteristic constants of the accumulation phase and G, expressed in d -1, the growth rate (G dt = dm/m). Therefore, in the above equation for CF, the values -1.149 and -0.029 represent

()k 1 -}- G).

The values for l l~ concentration factor are considerably higher than those obtained under chronic exposure for other radionuclides. Maximum concentration factor values are 60 for 22Na (Baudin et al., 1981), 25 for 6SZn (Baud• 1983), 11 for x37Cs (Frisch and Baud• 1984; Lambrechts, 1984), 6 for 6~ (Baudin and Fritsch, 1988) and 0.2 for 125Sb (Baudin et al., 1989). Such a difference is perfectly

l I ~ RETENTION BY A FRESHWATER FISH 135

consistent with classification of metallic elements according to their chemical and biological properties. In this respect, Nieboer and Richardson (1980) propose to divide metals into three categories as a function of their affinity with the ligands found in biological tissues. Unlike the radionuclides previously mentioned, silver becomes irreversibly bound to nitrogen and sulfur in biological systems. As a result, concentration factors for this metal are higher than the values for metals, such as Zn, Co and Cs, that do not belong to this category.

The few data found in the literature indicate that ll~ accumulation by fish differs widely, depending on the species. For marine fish, Pentreath (1977) showed that metabolization of silver taken up from water was notably slower in the plaice (Pleuronectes platessa) than in the ray (Raja clavata). In the case of freshwater fish, Colemen and Cearly (1974) showed that silver was more toxic to the largemouth bass (Micropterus salmoides) than to the bluegill (Eupomotis macrochirus). In trout (Salmo trutta), Garnier et al. (1990) report a maximum CF of 9 with equilibrium reached after 912 days. These two values are substantially far from those obtained for carp under fully comparable experimental conditions (CF-106 after 180 days).

With ll~ as with most other radionuclides, experimental investigations are necessary to interpret data from field studies. Thus, in the zones of the Rhone river receiving liquid wastes from pressurized water reactors, ll~ is frequently detected in fish (Baudin et al., 1989) but only rarely detected in the unfiltered water and then, exclusively in the particulate fraction. Consequently, the concentration factor for living organisms cannot be calculated. After the Chernobyl accident, field measurements in French freshwater ecosystems provide a significant data base. For example, in some Corsican lakes unaffected by any radioactive waste, Descamps and Baudin-Jaulent (1988) measured ll~ concentration of 2 Bq g-1 (w.w.) in trout as early as July 1986. These findings indicate very rapid uptake of the radionuclide by the fish after contamination of their ecosystem by fallout. Considering that the 11~ concentration was unmeasurable in water, this observation appears inconsistent with the experimentally determined radionuclide uptake kinetics that show a gradual contamination (Figure 2). Thus, in situ, if 11~ accumulation by fish can be attributed to uptake from water, it is likely there is also a significant uptake from food. In this regard, recent studies show the high capacity of organisms that belong to the first troppic levels of the freshwater ecosystems for I1~ accumulation, especially planktonic algae (Garnier and Baudin, 1989) as well as gammarids and midge larvae which are a common food of carp and most omnivorous and carnivorous freshwater fish in their natural biotope (Muus and Dahlstom, 1968; D'Aubenton and Spillman, 1976). For gammarids and midge larvae, the concen- tration factor (w.w.) is about 103 (Garnier-Laplace et al., 1992) and from such a value a high H~ troppic transfer factor for the fish can be expected.

3.3. ll~ DEPURATION PHASE

During the depuration phase, the 11~ total content of the fish decreased rapidly reaching 22% by 42 days (Table III). The excretion process included two phases

136 J. P. BAUDIN ET AL.

100,

0~ o~ 80

9

0 es

, " 60 o e -

E o 40

"7 <

20

0 0

Excretion (Ag-110m Total Content) :

.~, �9 Experimental data

- - C a l c u l a t e d data Rvc = 23.6 e 'O'877t + 76.4 e "0"023t

" ~ (CoefficletR determination = 0.99)

' . ~ . Tbl = O . 8 d T b 2 = 3 0 d ~_., �9

Excretion + Biological Dilution (Ag-110m Concentration): " : + Experimental data

-1.149t -0.029t . . . . Calculated data R c = 23.6 �9 + 76.4 e

(Coefficient detamlination = 0.98)

Tbl = 0.6 d Tb2 -- 23 d I I I I

10 20 30 40

Time (days) Fig. 2. Depuration of ll0mAg by carp.

TABLE III

Depuration parameters for 11~ in Cyprinus carpio

ll~ in the fish

Duration Fish body mass Total content % of initial Concentration % of initial (days) (g, w.w.) a (Bq) a total content b (Bq g-l w.w.)a concentration b

0 1.93• 3225• 100 1681• 100 1 1.99• 2727• 84.6 1379• 82.0 3 1.98• 2333• 72.3 1155• 68.7 6 2.03• 2267• 70.3 1129• 67.2

10 2.12• 1965• 60.9 943• 56.1 14 2.05• 1635• 50.7 821• 48.8 27 1.98• 1367• 42.4 703• 41.8 34 2.60• 1108• 34.4 440• 26.2 42 2.85• 1001• 31.0 365• 21.7

a Mean values given with their confidence intervals + t • s / x ~ , where ' t ' is the Student variable for 95% probability, 's ' the estimated standard deviation and 'n' the number of measurements carried out (10 fish). b Calculated from the mean values.

l l ~ RETENTION BY A FRESHWATER FISH 137

with very different elimination rates (Figure 2). Thus, in the first three days the 11~ loss was 31% while another 37% were eliminated during the following 39 days. The change in the n~ retention percentage (RTc) can be well described by the relation:

RTC = 23.6 e -0.877t + 76.4 e -0-023t (determination coefficient = 0.99).

This equation shows that the n~ fraction associated with the first compartment which corresponds to rapid elimentation is lower (23.6%) than this associated with the second (76.4%) which correspond to slow elimination. The two corresponding biological half-lives are 0.8 (Tbl) and 30 days (Tb2). The decrease of the 11~ concentration is more important than that of total content because of the radionuclide dilution due to fish weight gain. The change in the percentage of the 11~ initial concentration value (beginning of the deputation phase) can be expressed by the following equation:

R c = 23.6 e -1.149t + 76.4 e -0-029t (determination coefficient = 0.98).

The two corresponding biological half-lives are then 0.6 (Tbi) and 23 days (Tb2). For the same species and in similar radioecological experimantal studies, this

two phase depuration process was reported for several radionuclides such as 54Mn, 22Na, 65Zn, 137Cs, 6~ and 125Sb (Baudin et al., 1981; Baudin, 1983; Lambrechts, 1984; Fritsch and Baudin, 1984; Baudin and Fritsch, 1988; Baudin et al., 1989). On the contrary, it must be emphasized that the data related to n~ depuration from carp are completely different from those obtained for Salmo trutta (Granier et al., 1990). For this species, indeed, ll~ elimination by previously contaminated individuals was not demonstrated. Thus, after 28 days in non-radioactive water, the ll~ body burden of the fish was the same as at the end of the exposure phase. The radionuclide concentration decreases, however, due to biological dilution from a gain in fish body mass. Such an interspecific difference may be linked to silver metabolism and storage (Martoja et al., 1988).

3.4. 11~ TISSUE DISTRIBUTION

At the end of the exposure phase, the liver showed the highest ll~ concentration factor (CF=866). Moreover, this organ accounted for 30% of the radioactivity of the fish in toto although it represents only 4.4% of the total body mass (Table IV). Concentration of ll~ in liver has also been reported in marine species sampled in situ (Folsom and Young, 1965; Jenkins, 1969, Folsom and Hodge, 1974; Greig and Jones, 1976). Pouvreau and Amiard (1974) calculated a concentration factor of 320 in a marine fish, Blennis pholis. For the freshwater fish Salmo trutta, Garnier et al. (1990) found the highest concentration in the liver (Cf-280) which contains 70.3% of the total accumulated radionuclide. For Cyprinus carpio, the other organs for which the concentration factor is higher than for the fish in toto at the end of the exposure phase (CF=73), are the digestive tract (CF=560), kidneys (CF=299), spleen (CF=155) and air-bladder (CF=109). However, the digestive tract accounted

138 J , P. BA U D IN ET AL.

TABLE IV

ll~ distribution in carp tissues after the exposure phase (sum for the 10 dissected fish)

Tissues

ll~ in the fish

Tissues mass % of total Concentration Concentration % of whole (g, w.w.) body mass (Bq g-l w.w.) factor body content

Liver 0.76 4.4 13162 866 30.1 Digestive tract 1.69 9.8 8513 560 43.7 Kidneys 0.20 1.2 4552 299 2.7 Spleen 0.04 0.2 2354 155 0.9 Air bladder 0.12 0.7 1658 109 0.6 Skin 1.98 11.5 577 39 3.5 Bone 2.01 11.7 544 36 3.3 Gills 0.48 2.8 544 36 0.8 Head 4.77 27.8 506 33 7.3 Muscle 5.12 29.8 498 33 7.7

TABLE V

ll0mAg distribution in the carp tissues after the depuration phase (sum for the 10 dissected fish)

ll~ in the fish

Tissues Tissues mass % of total Concentration % of whole (g, w.w.) body mass (Bq g-l w.w.) body content

Liver 0.85 3.8 4404 36.3 Digestive tract 2.17 9.6 1675 35.2 Kidneys 0.41 1.8 754 3.0 Spleen 0.13 0.6 500 0.6 Air bladder 0.15 0.7 465 0.7 Skin 2.47 10.9 148 3.5 Bone 2.62 11.6 141 3.6 Gills 1.16 5.1 186 2.1 Head 5.72 25.3 162 9.0 Muscle 6.96 30.7 88 5.9

for abou t 43.7% of the total ll~ fish content a l though it represented only 9.8%

of the total body mass. All other tissues, especially muscle, were characterized by

a low radionucl ide concent ra t ion and a percentage of total radioact ivi ty lower than

their relative mass.

After 42 days of depura t ion , the rank ing of the tissues as a func t ion of the ll~

concen t ra t ion was the same as at the end of the exposure period (Table V). The

ll~ concent ra t ion decreased most in muscle and least in liver. The radionucl ide

d is t r ibut ion in the tissues was generally the same as after the accumula t ion phase,

bu t the percentage of total ll~ in the liver increased slightly (from 30% to 36%)

whereas it decreased in the digestive tract (from 44% to 35%).

l l O m A g RETENTION BY A FRESHWATER FISH 139

The ll~ distribution in tissues of carp may be explained, partly at least, by physiological mechanisms responsible for silver accumulation by animals. This metal enters a metabolic cycle characterized as a general detoxication process. Indepen- dently of the animal species, silver is requestered in a very stable chemical form AgzS, that becomes localized mainly in extracellular structures such as basement membranes, but also in macrophageous cells (Martoja et al., 1985, 1988). Before storage as a stable mineral conbination, silver binds to proteins containing a large proportion of sulphydryl groups such as metallothionein (Hodson, 1988). The last stage in the catabolic pathway of these proteins leads to storage of silver after, reaction with a sulfur ligand S 2- (Martoja et al., 1988). These mechanisms explain why liver, the most important organ for protein synthesis, shows the highest capacity for 11~ accumulation. The same reasons explain the high radioactivity level of the spleen, which is rich in macrophageous cells. The high ll~ concentration in the digestive tract, is undoubtly linked to the numerous basement membranes contained in its tissues. Storage in other tissues, such as muscle and bone, is not very significant, due to the detoxication process and the biological ligands of silver. Some aspects of silver metabolism could also explain the interspecific differences between Salmo trutta and Cyprinus carpio, reported above. The higher percentage of accumulated H~ in trout liver (70.3 versus 30.1% for carp) and an absence of depuration from this species (Garnier et al., 1990) may be due to different detoxication mechanisms, differentiated by the distribution between organic and mineral compounds and by cellular and histological storage centers.

4. Conclusion

Uptake of 11~ from water by Cyprinus carpio results in a maximum concentration factor value of about 110. This value is the highest of all those calculated for the contamination of the same species by the other major radionuclides contained in liquid wastes from pressurized water reactors such as 137Cs, 54Mn or 6~ for instance. From a radioecological and radioprotection standpoint, this high capacity of ll~ accumulation by fish shows that it is essential to carry out experimental studies in order to evaluate accurately the possibilities of its transfer in freshwater ecosystems.

The distribution of 11~ in the tissues of living organisms is not only of interest from a radioecological point of view but also provides valuable information that can contribute to knowledge of silver metabolism is fish. In this respect, the con- siderable interspecific differences noted between Cyprinus carpio and Salmo trutta need further investigation, especially as this aspect has been poorly studies.

Despite the difficulties inherent in extrapolating laboratory findings to the natural environment, results related to ll~ accumulation by carp from water represent a first stage in interpreting the field data. The results presented in this paper must therefore be extended by other experiments investigating the peculiar ecological importance of 11~ By reason of its high concentration in organisms that constitute

140 J . P . BAUDIN ET AL.

the first steps of freshwater food chains, studies of trophic transfer of ll~ are especially warranted.

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