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Factors determining the uptake of persistent pollutants in an eel population (Anguila anguilla L.)
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Environmental Pollution 69 ( 1991) 39-50 .c
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Factors Determining the Uptake of Persistent Pollutants in an Eel Population (Anguilla angui//a L.) Per Larsson, Stellan Hamrin & Lennart Okla Limnology, Department of Ecology, Box 65, S-221 00 Lund, Sweden (Received 18 April 1990; revised version received 3 August 1990; accepted 7 August 1990)
A BS TRA C T The distribution of persistent pollutants m an eel population from a eutrophic lake of southern Scandinavia was examined. The origin of PCBs, DDT, DDE and lindane found in the fish was most likely the atmosphere. The most important factors for uptake of the chlorinated hydrocarbons was age (exposure time), growth rate andfat content. The life cycle of the eel is unique with a stage in freshwater when energy reserves (fat stored in muscular tissue) and lipophilic pollutants are accumulated. This stage is followed by a long migration to the spawning areas in the Sargasso Sea when pollutants are released from the fat deposits. These two stages followed by a once-in-alifetime spawning behaviour, makes the eel especially vulnerable to persistent pollutants. The effects of persistent pollutants combined with the eel's unusual life cycle may explain the decline in the eel population in northern Europe in recent decades.
INTRODUCTION The European eel (Anguilla anguilla L.) differs from most freshwater fish species in a number of ways. Being anadromous, with a larval stage in the sea (Leptocephalus larvae or glass eel, for about two years), a juvenile stage in fresh water (yellow eel, for 10-15 years) and an adult spawning phase again in the sea (silver eel), the life cycle o f eel is unique a m o n g European fishes. Especially during its freshwater stage, the eel is exposed to a variety of 39 Environ. Pollut. 0269-7491/91/£03'50 ~) 1991 Elsevier Science Publishers Ltd, England. Printed in Great Britain
40
Per Larsson, Stellan Hamrin, Lennart Okla
anthropogenic substances. As the fish is highly tolerant to poor water quality, such as low oxygen content and high amounts of waste products (Tesch, 1977; Kruse et aL, 1983; Ferrando et al., 1987), and may survive several years without food (Bo&ius & Bo&ius, 1985), the eel may inhabit waters influenced by high industrial activity (like the River Elbe; Kruse et aL, 1983). One important factor for the uptake of lipophilic, persistent pollutants (e.g. polychlorinated biphenyls (PCBs), DDT, y-hexachlorocyclohexane (lindane)) in fish is the amount of fat. Fishes that store fat around the fibres of the muscle tissue, such as the eel and Atlantic salmon (Salmo salar), contain high amounts of pollutants. A strong correlation between PCB and lipid content was observed for fishes in the Hudson River, USA (Brown et aL, 1985). Lipid-dependent pollutant uptake is consistent with the known lipophilic character of persistent chlorinated hydrocarbons. The eel has high amounts of fat compared with other freshwater fishes (Henderson & Tocher, 1987). Fat comprises up to 40% of its total body weight (fresh weight), and is mainly stored in the muscle and, to a lesser extent, in the liver and around the viscera (Henderson & Tocher, 1987). Pike (Esox lucius) contains below 2% fat in the muscle, which is a typical value for many North European freshwater species, e.g. burbot (Lota Iota), pike perch (Stizostedion lucioperca), perch (Percafluviatilis), and the cyprinids, roach (Rutilus rutilus) and bream (Abramis brama). Northern pike has no major fat deposits (Medford & Mackay, 1978), though the liver has higher values of fat (4-11%) than the muscle tissues, a strategy even more pronounced in the burbot. Cyprinids have high amounts of fat in tissue surrounding the viscera. Fat deposits accumulate lipophilic pollutants to a high degree (Yoshida et al., 1973). The fat content of fishes varies during the season, mainly due to the gonad cycle (Medford & Mackay, 1978). In the spawning season, fish invest energy in germinal tissue and especially so in females. Thus, in most species, fat and lipophilic pollutants are lost by the females during spawning, thereby lowering the fat content and the amount of pollutants. For males this factor is less pronounced. In the eel, these intraspecific variations are small, as the fish migrate when sexually mature and because the freshwater habitats are strongly dominated by juvenile females. Consequently, other factors affecting the uptake of persistent pollutants are more easily isolated. In addition to ecological factors, such as niche, habitat, age and growth rate, the uptake and concentration of persistent pollutants in fish is affected by the chemical properties of the pollutant. Several attempts have been made to relate uptake to the lipophilicity of the persistent compound, i.e. the octanol/water partition coefficient (e.g. Neely et al., 1974; Kenaga, 1980). In general there is a positive, linear relationship between uptake and the
Uptake of persistent pollutants in an eel population
41
partition coefficient, though this relationship has been questioned theoretically (Sagiura et al., 1978; Gobas et al., 1986) and shown to be insufficient to explain uptake of some persistent pollutants in the field (Swackhamer & Hites, 1988). Also, structural properties of the compounds are important in the uptake process (Kanazawa, 1982). As pollutants are taken up directly via the gills and from food, the relative importance of these two uptake routes for fish in the field has been under debate (e.g. Spigarelli et al., 1983). However, recent investigations recognize both routes as important, and the question is rather restricted to the exposure situation (Larsson, 1990). Direct uptake from water may predominate in early life stages (fry), whereas uptake from food may predominate for longer periods in adult fish. The pollutant distribution within fish populations is seldom studied. More commonly, different species are studied to investigate the effect of trophic level and, thereby, biomagnification. Factors such as age, growth rate and physical status (i.e. fat content) of individual fish are seldom related to uptake of different pollutants. The effects of such factors are more easily studied in the eel than in other freshwater species of fish, due to the high amounts of pollutants (more easy to detect) and to the fact that spawning behaviour is absent in the freshwater phase. The aim of this study was to examine the effect of these ecological factors on the pollutant distribution within an eel population in a eutrophic lake of southern Scandinavia.
MATERIALS A N D METHODS Eels in their freshwater phase (yellow eel) were captured by trawling in a eutrophic lake in southern Scandinavia (55 ° 53' N, 13 ° 30' E) in the summer of 1988. The lake area is 39.6 km 2, the mean depth 4-7 m and the theoretical water exchange 12 months. The catchment area is 347 km 2, with woods in the north and agricultural land in the south. Total phosphorus in the lake water is about 100 #g litre- i and heavy blooms of blue-green algae (mainly M i c r o c y s t i s ) occur frequently. The weight (to the nearest gram) and length (to nearest mm) were determined and the eels were frozen ( - 20°C). Sacculus otoliths were removed from the fish, rubbed free of connective tissue and used for age determination (V611estad, 1985). The instantaneous rate of increase in weight (G,) for a given age group (growth ring, t) was backcalculated from otoliths according to: G, : (L~ - L~_ I)/L~
where L t = length from otolith nucleus to growth ring t. The otolith length 3 was linearly related to weight of the eel (p < O.OO1). The two years (approximately) spent in transit from the Sargasso Sea to
42
Per Larsson, Stellan Hamrin, Lennart Okla
the Swedish coast (Tesch, 1977) was neglected and, therefore, eels in their first year after metamorphosis in freshwater were regarded as 0 +. F r o m each eel, a segment of 15-20 g was taken from the tail. The sample was weighed, homogenized in 30ml acetone and 30ml of n-hexane was added. The homogenate was then treated in an ultrasonic bath for 10 min and washed with 60 ml of NaC1 (2%). The hexane phase, containing fat and lipophilic pollutants, was separated and evaporated to dryness in a waterbath (70°C). Ten ml of acetone was added to remove traces of water, and the extract was again evaporated in the water bath. The material remaining after evaporation was weighed on a balance of the nearest 0.1 mg and regarded as fat. The fat was dissolved in hexane to obtain a 10% solution (w/w), a 1-ml subsample of the aliquot was repeatedly treated with concentrated H2SO4 (with 30% fuming H2SO 4 added), until no colour was observed in the H2SO 4 phase (revealing that no oxidation occurred). The cleaned-up extract was analyzed for persistent pollutants by gas chromatography. The pollutants were analyzed by capillary gas chromatography/ECD. A 30-m DB 5 quartz capillary was used. The injection technique was oncolumn (Okla & Wes6n, 1984) and H 2 (1 ml m i n - 1) was used as a carrier gas and N 2 (30mlmin -1) as the make-up gas. The oven temperature was programmed as follows: initially 1 min at 50°C, increase by 25°C rain- 1 to 220°C, hold for 25min, then directly to 260°C. PCB congeners were identified after Duinker and Hillebrand (1983) and numbered according to IUPAC (Ballschmiter & Zell, 1980). Clophen A 60 was used as a standard substance. Besides PCBs, p,p-DDT, p,p-DDE, p,p-DDD and lindane were identified. All concentrations of pollutants were log-transformed, due to the skewed distribution of their values (Newton, 1988). Pearson product m o m e n t correlation test was used if not otherwise stated.
RESULTS The age of the eels was significantly correlated to weight and length (r = 0"78 and 0"83, p <0.01). The amount of fat in the muscle (on fresh weight basis) increased linearly from 5 to 28% up to a weight of approximately 400g (linear regression, n = 52, r 2 = 0"29, p = 0.001), which corresponded to a length of 55 cm and an age of 11 years. After this point the relationship was broken. Twenty-five PCB congeners, p,p-DDT, p,p-DDE, p,p-DDD and lindane were always detected in the eels. Concentrations of EPCB ranged from 209 to 6582 ng g-1 extractable fat (22 to 1071 n g g - 1 fresh weight). The most abundant D D T c o m p o u n d was p,p-DDE, which reached concentrations of
Uptake of persistent pollutants in an eel population
43
Atmospheric fallout :
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Fig. 1. The compositional distribution (capillary gas chromatograms) of persistent pollutants in atmospheric fallout and in eels. The chromatograms are representative of four samples of atmospheric fallout and of 52 eels from a lake of southern Scandinavia. PCB congeners are numbered after Ballschmiter and Zell (1980). Note that p,p-DDT is metabolized to p,p-DDE in the fish.
16-965ngg -~ extractable fat (4-356ngg -1 fresh weight) and lindane concentrations were 3-57 ng g- ~ extractable fat (0.4-5 ng g- 1 flesh weight). The same persistent pollutants, and the same proportion of persistent pollutants, were recorded in eel as in atmospheric fallout from southern Sweden (Fig. 1; Larsson & Okla, 1989). Of the lipophilic persistent pollutants in the eel p,p-DDE constituted 22%, p,p-DDD 6% and p,p-DDT 1%. The dominant PCB congeners were PCB~ s3 (2,2',4,4',5,5'-hexaCB) 11%, PCBa 38 (2,2',3,4,4'5-hexaCB) 11% and PCB~8 o (2,2',3,4,4',5,5'-heptaCB) 7%. All the different pollutants in eel were significantly correlated to each other (both on extractable fat and flesh weight basis, p < 0.01) with the exception oflindane. This correlation meant, for instance, that the concentration of the
44
Per Larsson, Stellan Hamrin, Lennart Okla 10000
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Relationship between the age of eel and the concentration of EPCB (ng g - ~ fresh weight) or p,p-DDE (ng g - ~ fresh weight) in the muscle tissue.
tetrachlorobiphenyl PCB95 was correlated with the concentration of p,pD D T in the eel muscle. Concentrations of EPCB, p,p-DDE and p,p-DDD (based on extractable fat as well as fresh weight) were significantly correlated with age, length and weight of the eel (Fig. 2, p < 0.01). p,p-DDT showed a less significant correlation (p < 0-05). There were no significant differences in the relative proportion of individual PCB congeners (PCB congener/YPCB) with age of the eels (p > 0"1). Lindane was inversely correlated to the three parameters (p < 0"01) on extractable fat basis and showed a less significant correlation when calculations were based on fresh weight (p < 0"05). The instantaneous rate of increase in weight for eels decreased with age, from 0"8 in one-year-old fish to below 0.2 at age 12 (Fig. 3). The concentrations of EPCB and D D E on a fresh weight basis were inversely correlated with the instantaneous growth rate (Fig. 3, p < 0.01). PCB concentrations slowly increased up to a weight of 400 g and an age of 12 years, whereafter the increase was faster. The same relationships were recorded forp,p-DDE. Based on extractable fat, the highest increase in PCB and D D E concentrations was recorded for eels age 9 or older, and the PCB concentration was inversely correlated with fat content (p < 0.01).
Uptake of persistent pollutants in an eel population
45
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Fig. 3. (top): The inverse correlation between the instantaneous rate of increase in weight of eels during the last year (year before catch) and the concentration ofp,p-DDE (ng g - 1 fresh weight) in the muscle tissue. (bottom): The instantaneous rate of increase in weight of eels in Lake Ringsj6n at different ages (growth rings). At an age of one year the eel almost doubles its weight, while at an age of 15 less than 10% of the biomass is renewed.
DISCUSSION The similarity in the matrix of persistent pollutants in atmospheric fallout and in the eel population suggests that the atmosphere was the source of the pollutants. This conclusion was further supported by the fact that concentrations of pollutants in the eel were correlated to each other, with the exception of lindane. This indicated the same source for the various pollutants. The lack of a correlation with lindane was probably due to its metabolism in the eel and this will be discussed below. Furthermore, the pollutant pattern (the relative pollutant distribution of PCB congeners, p,p-DDE, etc.) in the fish did not change within the population despite age and, thereby, exposure time. Atmospheric deposition of the pollutants, therefore, was probably the dominant source during the lifespan of 7- to 16-year-old eels. The atmosphere has been shown to be an important route of transport
46
Per Larsson, Stellan Hamrin, Lennart Okla
to fish in remote areas, e.g. for a variety of organochlorine compounds (like PCBs, DDT, 7-HCH and hexachlorobenzene) to northern pike (Esox lucius) in Canadian lakes (Johnson et al., 1988). In a Siskiwit Lake, situated on an island in Lake Superior, Swackhamer and Hites (1988) found a wide range of chlorinated hydrocarbons in lake trout (Salvenlinus namaycush) and whitefish (Coregonus culpeaformis neohantoniensus). No source other than the atmosphere was possible, and concentration must have been the result of long-range transport to this remote island lake. Concentrations of EPCB and DDT and its metabolites increased with age (length, weight). The relationships were most distinct for calculations made on a fresh weight basis. Fish obtain persistent pollutants from water and from food (e.g. Spigarelli et al., 1983). Uptake from water is a fast process--a matter of hours or days depending on the pollutant's chemical properties (Pizza & O'Connor, 1983). On the other hand, uptakefrom food is a slower process and hydrophobic pollutants may accumulate over years (Bengtsson, 1980; Spigarelli et al., 1983). As a result, exposure time (age of the fish) is a governing parameter in the uptake process. In the limited number of studies regarding uptake of chlorinated hydrocarbons in fish populations, pollutant concentrations generally increase with age (e.g. Wszolek & Lisk, 1979; Swackhamer & Hites, 1988). An age-dependent food chain model for PCBs in lake trout (Salvelinus namaycush) has been developed by Thomann and Conolly (1984). The model successfully explained the uptake of the pollutants in older age-classes of trout, but failed for fishes up to 3 + years. Sensitivity tests of the model indicated that the growth rate of the fish was a significant parameter. The authors concluded that 99% of the PCBs in trout were derived from food. Commonly, the number of fish in pollution investigations is small and age is seldom determined, but is approximated by size (e.g. Wszolek & Lisk, 1979; Swackhamer & Hites, 1988; Connel, 1987). Furthermore, sex of the fish interferes with the results, as females and males are not usually separated. Sex has been shown to be important to explain concentrations of pollutants in other animals, as found by Aguilar and Borrell (1988) for fin whales (Balaenoptera physalus), where EPCB and E D D T were positively correlated to age in males, but negatively correlated in females. The relationship in females was due to elimination of the compounds to their offspring during pregnancy and lactation. A similar elimination of pollutants occurs via roe in fish. Germinal tissue of the female can reach 15% of total body weight in sexually mature fishes (Medford & Mackay, 1978). Since the eel does not spawn in freshwater, this process can be excluded and the age-pollutant relationship is more easily determined. Concentrations oflindane showed an inverse relationship with age in eels. The most probable explanation for this phenomenon was an increased metabolism with age. Lindane has been shown to be metabolized and
Uptake of persistent pollutants in an eel population
47
eliminated to a higher extent than DDT in fish (Murty, 1986). The efficiency of this process generally increases with age (Rand & Petrocelli, 1985), as detoxifying enzyme systems develop and are induced from juvenile to adult fishes. A similar inverse relationship for lindane, as in our study, was recorded for male ringed seal (Phoca hispida, Muir et aL, 1988) in the Canadian Arctic, while the concentrations of EPCB and E D D T were positively correlated with age. The largest increase in ZPCB and E D D T uptake (ng g- i fresh weight) was recorded for eels aged 12 years and older. From 6 to 12 years, uptake of these pollutants was lower and increased gradually. The increase in pollutant uptake after age 12 could be a result of lowered growth rate. The instantaneous growth rate (Gt) of fish generally decreases with age. For example V611estad and Jonsson (1988) found that Gt for the eel decreased from 1"41 in the first growth season to 0.2 in the seventh. Similarly, G, of the eels in Lake Ringsj6n decreased with age. This resulted in a dilution of pollutants in the growing biomass in younger year-classes and a small and continuous increase in pollutant levels. After the age of 12, G, stabilized around 0.1 and, as a result, concentrations of pollutants started to increase faster. Growth rate of fish has been hypothezised and modelled to be an important factor for levels of persistent pollutants in fish (Thomann, 1989). The increasing fat content with age/weight of the eel has implications on pollutant levels calculated on an extractable fat basis. This fat content-size relationship of the eel was demonstrated by Lovern already in 1938 (Lovern, 1938). The pollutants were 'diluted' in the increasing fat amounts of eels up to 400g, as shown by the inverse relationship between fat content and pollutant levels. Consequently, the age-dependent increase in EPCB and E D D T concentrations was not as clear as when calculations were based on fresh weight, though the correlations were significant. In addition to exposure time, a shift in diet with age may explain the increase in persistent pollutants in older fish. Young eels consume mostly zoobenthos. As eels grow, fish constitute a larger portion of the diet (Tesch, 1977). Concentrations of persistent pollutants generally are higher in fish compared to zoobenthos. Eel headform (width/length) has been related to food choice, i.e. broad-headed individuals feed mainly on fish and large macroinvertebrates and narrow-headed on smaller zoobenthos (Trybom, 1893; T6rlitz, 1922; Tesch, 1977). We found a significant, positive correlation between headform and levels of ZPCB and DDE, indicating a higher proportion of fish in the diet of the broad-headed eels. However, headform was also positively correlated with age and thus the hypothesis is still open to question. Unlike many other fishes, anguillid eels spawn only once in their life. This means that lipophilic pollutants are not eliminated seasonally through the
48
Per Larsson, Stellan Hamrin, Lennart Okla
roe, thereby decreasing exposure to these xenobiotics. Accumulation of pollutants thus continues for the whole lifetime not only in males as in most animals, but also in females. Eels contain large a m o u n t s of fat, and so levels of pollutants are higher than for most fishes. In addition, eels are relatively insensitive to inferior water quality, and are often exposed to high levels of persistent pollutants in industrial regions. Combining these facts with the long migration, using muscular fat deposits as energy reserves, eels seem to be a species highly exposed to pollutants, As the lipid deposits are depleted, the lipophilic pollutants are released into the bloodstream and can be transferred to vital organs and germinal tissue. Such situations have been shown to be deleterious for birds; as birds starve, persistent pollutants are released from their fat deposits to poison the animal (Krom, 1986). A similar situation could be the case for migrating eels, and m a y be one reason for diminishing catches in Europe in recent decades. REFERENCES Aguilar, A. & Borrell, A. (1988). Age- and sex-related changes in organochlorine compound levels in fin whales (Balaenopter physalus) from the eastern North Atlantic. Marine Environmental Research, 25, 195-211. Ballschmiter, K. & Zell, M. (1980). Analysis of polychlorinated biphenyls (PCB) by glass capillary gas chromatography. Fresenius Z. Analytische Chemie, 302, 20-31. Bengtsson, B.-E. (1980). Long-term effects of PCBs (Clophen A 50) on growth, reproduction and swimming performance in the minnow, Phoxinus phoxinus. Water Research, 14, 681 7. BoEtius, I. & BoEtius, J. (1985). Lipid and protein content in Anguilla anguilla during growth and starvation. Dana, 4, 1-18. Brown, M. P., Werner, M. B., Sloan, R. J. & Simpson, K. W. (1985). Polychlorinated biphenyls in the Hudson River. Environmental Science and Technology, 19, 656-61. Connel, D. W. (1987). Age to PCB concentration relationship with the striped bass (Morone saxatilis) in the Hudson River and Long Island Sound. Chemosphere, 16, 1469-74. Duinker, J. C. & Hillebrand, M. T. J. (1983). Composition of PCB mixtures in biotic and abiotic marine compartments (Dutch Wadden Sea). Bulletin of Environmental Contamination and Toxicology, 31, 25-32. Ferrando, M. D., Moliner-Andreu, E., Almar, M. M., Cebrian, C. & Nunez, A. (1987). Acute toxicity of organochlorine pesticides to the European eel, Anguilla anguilla: the dependency on exposure time and temperature. Bulletin of Environmental Contamination and Toxicology, 39, 365-9. Gobas, F. A., Opperhuizen, A. & Hutzinger, O. (1986). Bioconcentration of hydrophobic chemicals in fish: relationship with membrane permeation. Environmental Toxicology and Chemistry, 5, 637~46. Henderson, R. J. & Tocher, D. R. (1987). The lipid composition and biochemistry of freshwater fish. Progress in Lipid Research, 26, 281-347.
Uptake of persistent pollutants in an eel population
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Johnson, M. G., Kelso, J. R. M. & George, S. E. (1988). Loadings of organochlorine contaminants and trace elements to two Ontario lake systems and their concentration in fish. Canadian Journal of Fisheries and Aquatic Sciences, 45, 170-8. Kanazawa, J. (1982). Relationship between the molecular weights of pesticides and their bioconcentration by fish. Experientia, 38, 1045-6. Kenaga, E. E. (1980). Correlation of bioconcentration factors of chemicals in aquatic and terrestrial organisms with their physical and chemical properties. Environmental Science and Technology, 14, 553-6. Krom, M. D. (1986). An evaluation of the concept of assimilative capacity as applied to marine waters. Ambio, 15, 208-14. Kruse, V. R., Boek, K. & Wolf, M. (1983). Der gehait an organochlor-pestiziden und polychlorierten biphenylen in Elbeaalen. Archivfiir Lebensmittelhygiene, 34, 81-6. Larsson, P. (1990). Sediment as a source for PCBs to a river system. Canadian Journal of Fisheries and Aquatic Sciences, 47, 746-54. Larsson, P. & Okla, L. (1989). Atmospheric transport of persistent pollutants to Sweden in 1985 compared to 1973. Atmospheric Environment, 23, 1699-711. Lovern, J. A. (1938). Fat metabolism in fishes. XIII: Factors influencing the composition of depot fat of fishes. Biochemical Journal, 32, 1214-24. Medford, B. A. & Mackay, W. C. (1978). Protein and lipid content of gonads, liver, and muscle of northern pike (Esox lucius) in relation to gonad growth. Canadian Journal of Fisheries and Aquatic Sciences, 35, 213-19. Muir, D. C. G., Norstrom, R. J. & Simon, M. (1988). Organochlorine contaminants in Arctic marine food chains: Accumulation of specific polychlorinated biphenyls and chlordane-related compounds. Environmental Science and Technology, 22, 1071-9. Murty, A. S. (1986). Toxicity of pesticides tofish. Volume 1. CRC Press, Boca Raton, Florida, 178 pp. Neely, W. B., Branson, D. R. & Blau, G. E. (1974). Partitioning coefficients to measure bioconcentration potential of organic chemicals in fish. Environmental Science and Technology, 8, 1113-15. Newton, I. (1988). Determination of critical pollutant levels in wild populations, with examples from organochlorine insecticides in birds of prey. Environmental Pollution, 55, 29-40. Okla, L. & Wes6n, C. (1984). A simple on-column injector for capillary gas chromatography. Journal of Chromatography, 299, 420-3. Pizza, J. C. & O'Connor, J. M. (1983). PCB dynamics in Hudson River striped bass. II Accumulation from dietary sources. Aquatic Toxicology, 3, 313-27. Rand, G. M. & Petrocelli, S. R. (1985). Fundamentals of Aquatic Toxicology. Hemisphere Publishing Corporation, New York, p. 542. Sagiura, K., Ito, N., Matsumoto, N., Mihara, Y., Murata, K., Tsukakoshi, Y. & Goto, M. (1978). Accumulation of polychlorinated biphenyls in fish: limitations of 'correlation between partitioning coefficients and accumulation factors'. Chemosphere, 9, 731-6. Spigarelli, S. A., Thommes, M. M. & Prepejchal, W. (1983). Thermal and metabolic factors affecting PCB uptake by adult brown trout. Environmental Science and Technology, 17, 88-94. Swackhamer, D. & Hites, R. A. (1988). Occurrence and bioaccumulation of
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Per Larsson, Stellan Hamrin, Lennart Okla
organochlorine compounds in fishes from Siskiwit Lake, Isle Royale, Lake Superior. Environmental Science and Technology, 22, 543-8. Tesch, F.-W. (1977). The eel. Biology and management of aguillid eels. Chapman and Hall Ltd, Edinburgh, 434 pp. Thomann, R. V. (1989). Bioaccumulation model of organic chemical distribution in aquatic food chains. Environmental Science and Technology, 23, 699-707. Thomann, R. V. & Conolly, J. P. (1984). Model of PCB in Lake Michigan lake trout food chain. Environmental Science and Technology, 18, 65-71. T6rlitz, M. (1922). Anatomische und entwicklungsgeschichtliche Beitr~ige zur Artfrage unseres Flussaalen. Zeitschrift Fisch., 21, 1-48. Trybom, F. (1893). Ringsji~n. Malm6hus Liin. Dess naturf~rhhllanden och fiske. Meddelande fr~n Kongl. Lantbruksstyrelsen Nr 4, 47 pp. (in Swedish). V611estad, L. A. (1985). Age determination and growth of yellow eels, Anguilla anguilla (L.), from a brackish water, Norway. Journal ofFish Biology, 26, 521-5. V611estad, L. A. & Jonsson, B. (1988). A 13-year study of the population dynamics and growth of the European eel Anguilla anguilla in a Norwegian river: Evidence for density-dependent mortality, and development of a model for predicting yield. Journal of Animal Ecology, 57, 983-97. Wszolek, P. C. & Lisk, D. J. (1979). Persistence of polychlorinated biphenyls and 1,1-dichloro-2,2-bis (p-chlorophenyl)ethylene (p,p'-DDE) with age in lake trout after 8 years. Environmental Science and Technology, 13, 1269-71. Yoshida, T., Takashima, F. & Watabe, T. (1973). Distribution of ~14~PCB in carp. Ambio, 2, 111-13.
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Environmental Pollution 69 ( 1991) 39-50 .c
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Factors Determining the Uptake of Persistent Pollutants in an Eel Population (Anguilla angui//a L.) Per Larsson, Stellan Hamrin & Lennart Okla Limnology, Department of Ecology, Box 65, S-221 00 Lund, Sweden (Received 18 April 1990; revised version received 3 August 1990; accepted 7 August 1990)
A BS TRA C T The distribution of persistent pollutants m an eel population from a eutrophic lake of southern Scandinavia was examined. The origin of PCBs, DDT, DDE and lindane found in the fish was most likely the atmosphere. The most important factors for uptake of the chlorinated hydrocarbons was age (exposure time), growth rate andfat content. The life cycle of the eel is unique with a stage in freshwater when energy reserves (fat stored in muscular tissue) and lipophilic pollutants are accumulated. This stage is followed by a long migration to the spawning areas in the Sargasso Sea when pollutants are released from the fat deposits. These two stages followed by a once-in-alifetime spawning behaviour, makes the eel especially vulnerable to persistent pollutants. The effects of persistent pollutants combined with the eel's unusual life cycle may explain the decline in the eel population in northern Europe in recent decades.
INTRODUCTION The European eel (Anguilla anguilla L.) differs from most freshwater fish species in a number of ways. Being anadromous, with a larval stage in the sea (Leptocephalus larvae or glass eel, for about two years), a juvenile stage in fresh water (yellow eel, for 10-15 years) and an adult spawning phase again in the sea (silver eel), the life cycle o f eel is unique a m o n g European fishes. Especially during its freshwater stage, the eel is exposed to a variety of 39 Environ. Pollut. 0269-7491/91/£03'50 ~) 1991 Elsevier Science Publishers Ltd, England. Printed in Great Britain
40
Per Larsson, Stellan Hamrin, Lennart Okla
anthropogenic substances. As the fish is highly tolerant to poor water quality, such as low oxygen content and high amounts of waste products (Tesch, 1977; Kruse et aL, 1983; Ferrando et al., 1987), and may survive several years without food (Bo&ius & Bo&ius, 1985), the eel may inhabit waters influenced by high industrial activity (like the River Elbe; Kruse et aL, 1983). One important factor for the uptake of lipophilic, persistent pollutants (e.g. polychlorinated biphenyls (PCBs), DDT, y-hexachlorocyclohexane (lindane)) in fish is the amount of fat. Fishes that store fat around the fibres of the muscle tissue, such as the eel and Atlantic salmon (Salmo salar), contain high amounts of pollutants. A strong correlation between PCB and lipid content was observed for fishes in the Hudson River, USA (Brown et aL, 1985). Lipid-dependent pollutant uptake is consistent with the known lipophilic character of persistent chlorinated hydrocarbons. The eel has high amounts of fat compared with other freshwater fishes (Henderson & Tocher, 1987). Fat comprises up to 40% of its total body weight (fresh weight), and is mainly stored in the muscle and, to a lesser extent, in the liver and around the viscera (Henderson & Tocher, 1987). Pike (Esox lucius) contains below 2% fat in the muscle, which is a typical value for many North European freshwater species, e.g. burbot (Lota Iota), pike perch (Stizostedion lucioperca), perch (Percafluviatilis), and the cyprinids, roach (Rutilus rutilus) and bream (Abramis brama). Northern pike has no major fat deposits (Medford & Mackay, 1978), though the liver has higher values of fat (4-11%) than the muscle tissues, a strategy even more pronounced in the burbot. Cyprinids have high amounts of fat in tissue surrounding the viscera. Fat deposits accumulate lipophilic pollutants to a high degree (Yoshida et al., 1973). The fat content of fishes varies during the season, mainly due to the gonad cycle (Medford & Mackay, 1978). In the spawning season, fish invest energy in germinal tissue and especially so in females. Thus, in most species, fat and lipophilic pollutants are lost by the females during spawning, thereby lowering the fat content and the amount of pollutants. For males this factor is less pronounced. In the eel, these intraspecific variations are small, as the fish migrate when sexually mature and because the freshwater habitats are strongly dominated by juvenile females. Consequently, other factors affecting the uptake of persistent pollutants are more easily isolated. In addition to ecological factors, such as niche, habitat, age and growth rate, the uptake and concentration of persistent pollutants in fish is affected by the chemical properties of the pollutant. Several attempts have been made to relate uptake to the lipophilicity of the persistent compound, i.e. the octanol/water partition coefficient (e.g. Neely et al., 1974; Kenaga, 1980). In general there is a positive, linear relationship between uptake and the
Uptake of persistent pollutants in an eel population
41
partition coefficient, though this relationship has been questioned theoretically (Sagiura et al., 1978; Gobas et al., 1986) and shown to be insufficient to explain uptake of some persistent pollutants in the field (Swackhamer & Hites, 1988). Also, structural properties of the compounds are important in the uptake process (Kanazawa, 1982). As pollutants are taken up directly via the gills and from food, the relative importance of these two uptake routes for fish in the field has been under debate (e.g. Spigarelli et al., 1983). However, recent investigations recognize both routes as important, and the question is rather restricted to the exposure situation (Larsson, 1990). Direct uptake from water may predominate in early life stages (fry), whereas uptake from food may predominate for longer periods in adult fish. The pollutant distribution within fish populations is seldom studied. More commonly, different species are studied to investigate the effect of trophic level and, thereby, biomagnification. Factors such as age, growth rate and physical status (i.e. fat content) of individual fish are seldom related to uptake of different pollutants. The effects of such factors are more easily studied in the eel than in other freshwater species of fish, due to the high amounts of pollutants (more easy to detect) and to the fact that spawning behaviour is absent in the freshwater phase. The aim of this study was to examine the effect of these ecological factors on the pollutant distribution within an eel population in a eutrophic lake of southern Scandinavia.
MATERIALS A N D METHODS Eels in their freshwater phase (yellow eel) were captured by trawling in a eutrophic lake in southern Scandinavia (55 ° 53' N, 13 ° 30' E) in the summer of 1988. The lake area is 39.6 km 2, the mean depth 4-7 m and the theoretical water exchange 12 months. The catchment area is 347 km 2, with woods in the north and agricultural land in the south. Total phosphorus in the lake water is about 100 #g litre- i and heavy blooms of blue-green algae (mainly M i c r o c y s t i s ) occur frequently. The weight (to the nearest gram) and length (to nearest mm) were determined and the eels were frozen ( - 20°C). Sacculus otoliths were removed from the fish, rubbed free of connective tissue and used for age determination (V611estad, 1985). The instantaneous rate of increase in weight (G,) for a given age group (growth ring, t) was backcalculated from otoliths according to: G, : (L~ - L~_ I)/L~
where L t = length from otolith nucleus to growth ring t. The otolith length 3 was linearly related to weight of the eel (p < O.OO1). The two years (approximately) spent in transit from the Sargasso Sea to
42
Per Larsson, Stellan Hamrin, Lennart Okla
the Swedish coast (Tesch, 1977) was neglected and, therefore, eels in their first year after metamorphosis in freshwater were regarded as 0 +. F r o m each eel, a segment of 15-20 g was taken from the tail. The sample was weighed, homogenized in 30ml acetone and 30ml of n-hexane was added. The homogenate was then treated in an ultrasonic bath for 10 min and washed with 60 ml of NaC1 (2%). The hexane phase, containing fat and lipophilic pollutants, was separated and evaporated to dryness in a waterbath (70°C). Ten ml of acetone was added to remove traces of water, and the extract was again evaporated in the water bath. The material remaining after evaporation was weighed on a balance of the nearest 0.1 mg and regarded as fat. The fat was dissolved in hexane to obtain a 10% solution (w/w), a 1-ml subsample of the aliquot was repeatedly treated with concentrated H2SO4 (with 30% fuming H2SO 4 added), until no colour was observed in the H2SO 4 phase (revealing that no oxidation occurred). The cleaned-up extract was analyzed for persistent pollutants by gas chromatography. The pollutants were analyzed by capillary gas chromatography/ECD. A 30-m DB 5 quartz capillary was used. The injection technique was oncolumn (Okla & Wes6n, 1984) and H 2 (1 ml m i n - 1) was used as a carrier gas and N 2 (30mlmin -1) as the make-up gas. The oven temperature was programmed as follows: initially 1 min at 50°C, increase by 25°C rain- 1 to 220°C, hold for 25min, then directly to 260°C. PCB congeners were identified after Duinker and Hillebrand (1983) and numbered according to IUPAC (Ballschmiter & Zell, 1980). Clophen A 60 was used as a standard substance. Besides PCBs, p,p-DDT, p,p-DDE, p,p-DDD and lindane were identified. All concentrations of pollutants were log-transformed, due to the skewed distribution of their values (Newton, 1988). Pearson product m o m e n t correlation test was used if not otherwise stated.
RESULTS The age of the eels was significantly correlated to weight and length (r = 0"78 and 0"83, p <0.01). The amount of fat in the muscle (on fresh weight basis) increased linearly from 5 to 28% up to a weight of approximately 400g (linear regression, n = 52, r 2 = 0"29, p = 0.001), which corresponded to a length of 55 cm and an age of 11 years. After this point the relationship was broken. Twenty-five PCB congeners, p,p-DDT, p,p-DDE, p,p-DDD and lindane were always detected in the eels. Concentrations of EPCB ranged from 209 to 6582 ng g-1 extractable fat (22 to 1071 n g g - 1 fresh weight). The most abundant D D T c o m p o u n d was p,p-DDE, which reached concentrations of
Uptake of persistent pollutants in an eel population
43
Atmospheric fallout :
i
:
:
:
! • :
::
!
i
i
i
anguilla )
~Oa~LMO
~
=or~',ct
Fig. 1. The compositional distribution (capillary gas chromatograms) of persistent pollutants in atmospheric fallout and in eels. The chromatograms are representative of four samples of atmospheric fallout and of 52 eels from a lake of southern Scandinavia. PCB congeners are numbered after Ballschmiter and Zell (1980). Note that p,p-DDT is metabolized to p,p-DDE in the fish.
16-965ngg -~ extractable fat (4-356ngg -1 fresh weight) and lindane concentrations were 3-57 ng g- ~ extractable fat (0.4-5 ng g- 1 flesh weight). The same persistent pollutants, and the same proportion of persistent pollutants, were recorded in eel as in atmospheric fallout from southern Sweden (Fig. 1; Larsson & Okla, 1989). Of the lipophilic persistent pollutants in the eel p,p-DDE constituted 22%, p,p-DDD 6% and p,p-DDT 1%. The dominant PCB congeners were PCB~ s3 (2,2',4,4',5,5'-hexaCB) 11%, PCBa 38 (2,2',3,4,4'5-hexaCB) 11% and PCB~8 o (2,2',3,4,4',5,5'-heptaCB) 7%. All the different pollutants in eel were significantly correlated to each other (both on extractable fat and flesh weight basis, p < 0.01) with the exception oflindane. This correlation meant, for instance, that the concentration of the
44
Per Larsson, Stellan Hamrin, Lennart Okla 10000
,
,
1000
rn
13-
'
100
i
10
"
I
'
!
I
10
15
1000
20
|
I
LU
100 I •
Q_
I
10
1
5
I
|
10
15
20
Age Fig. 2.
Relationship between the age of eel and the concentration of EPCB (ng g - ~ fresh weight) or p,p-DDE (ng g - ~ fresh weight) in the muscle tissue.
tetrachlorobiphenyl PCB95 was correlated with the concentration of p,pD D T in the eel muscle. Concentrations of EPCB, p,p-DDE and p,p-DDD (based on extractable fat as well as fresh weight) were significantly correlated with age, length and weight of the eel (Fig. 2, p < 0.01). p,p-DDT showed a less significant correlation (p < 0-05). There were no significant differences in the relative proportion of individual PCB congeners (PCB congener/YPCB) with age of the eels (p > 0"1). Lindane was inversely correlated to the three parameters (p < 0"01) on extractable fat basis and showed a less significant correlation when calculations were based on fresh weight (p < 0"05). The instantaneous rate of increase in weight for eels decreased with age, from 0"8 in one-year-old fish to below 0.2 at age 12 (Fig. 3). The concentrations of EPCB and D D E on a fresh weight basis were inversely correlated with the instantaneous growth rate (Fig. 3, p < 0.01). PCB concentrations slowly increased up to a weight of 400 g and an age of 12 years, whereafter the increase was faster. The same relationships were recorded forp,p-DDE. Based on extractable fat, the highest increase in PCB and D D E concentrations was recorded for eels age 9 or older, and the PCB concentration was inversely correlated with fat content (p < 0.01).
Uptake of persistent pollutants in an eel population
45
4OO
300" u,l o o
200" []
100"
m
-..4 ..'fall
0 O0
Instantaneous growlh rate of last year
i O0 S
10
15
20
25
30
Age (rings)
Fig. 3. (top): The inverse correlation between the instantaneous rate of increase in weight of eels during the last year (year before catch) and the concentration ofp,p-DDE (ng g - 1 fresh weight) in the muscle tissue. (bottom): The instantaneous rate of increase in weight of eels in Lake Ringsj6n at different ages (growth rings). At an age of one year the eel almost doubles its weight, while at an age of 15 less than 10% of the biomass is renewed.
DISCUSSION The similarity in the matrix of persistent pollutants in atmospheric fallout and in the eel population suggests that the atmosphere was the source of the pollutants. This conclusion was further supported by the fact that concentrations of pollutants in the eel were correlated to each other, with the exception of lindane. This indicated the same source for the various pollutants. The lack of a correlation with lindane was probably due to its metabolism in the eel and this will be discussed below. Furthermore, the pollutant pattern (the relative pollutant distribution of PCB congeners, p,p-DDE, etc.) in the fish did not change within the population despite age and, thereby, exposure time. Atmospheric deposition of the pollutants, therefore, was probably the dominant source during the lifespan of 7- to 16-year-old eels. The atmosphere has been shown to be an important route of transport
46
Per Larsson, Stellan Hamrin, Lennart Okla
to fish in remote areas, e.g. for a variety of organochlorine compounds (like PCBs, DDT, 7-HCH and hexachlorobenzene) to northern pike (Esox lucius) in Canadian lakes (Johnson et al., 1988). In a Siskiwit Lake, situated on an island in Lake Superior, Swackhamer and Hites (1988) found a wide range of chlorinated hydrocarbons in lake trout (Salvenlinus namaycush) and whitefish (Coregonus culpeaformis neohantoniensus). No source other than the atmosphere was possible, and concentration must have been the result of long-range transport to this remote island lake. Concentrations of EPCB and DDT and its metabolites increased with age (length, weight). The relationships were most distinct for calculations made on a fresh weight basis. Fish obtain persistent pollutants from water and from food (e.g. Spigarelli et al., 1983). Uptake from water is a fast process--a matter of hours or days depending on the pollutant's chemical properties (Pizza & O'Connor, 1983). On the other hand, uptakefrom food is a slower process and hydrophobic pollutants may accumulate over years (Bengtsson, 1980; Spigarelli et al., 1983). As a result, exposure time (age of the fish) is a governing parameter in the uptake process. In the limited number of studies regarding uptake of chlorinated hydrocarbons in fish populations, pollutant concentrations generally increase with age (e.g. Wszolek & Lisk, 1979; Swackhamer & Hites, 1988). An age-dependent food chain model for PCBs in lake trout (Salvelinus namaycush) has been developed by Thomann and Conolly (1984). The model successfully explained the uptake of the pollutants in older age-classes of trout, but failed for fishes up to 3 + years. Sensitivity tests of the model indicated that the growth rate of the fish was a significant parameter. The authors concluded that 99% of the PCBs in trout were derived from food. Commonly, the number of fish in pollution investigations is small and age is seldom determined, but is approximated by size (e.g. Wszolek & Lisk, 1979; Swackhamer & Hites, 1988; Connel, 1987). Furthermore, sex of the fish interferes with the results, as females and males are not usually separated. Sex has been shown to be important to explain concentrations of pollutants in other animals, as found by Aguilar and Borrell (1988) for fin whales (Balaenoptera physalus), where EPCB and E D D T were positively correlated to age in males, but negatively correlated in females. The relationship in females was due to elimination of the compounds to their offspring during pregnancy and lactation. A similar elimination of pollutants occurs via roe in fish. Germinal tissue of the female can reach 15% of total body weight in sexually mature fishes (Medford & Mackay, 1978). Since the eel does not spawn in freshwater, this process can be excluded and the age-pollutant relationship is more easily determined. Concentrations oflindane showed an inverse relationship with age in eels. The most probable explanation for this phenomenon was an increased metabolism with age. Lindane has been shown to be metabolized and
Uptake of persistent pollutants in an eel population
47
eliminated to a higher extent than DDT in fish (Murty, 1986). The efficiency of this process generally increases with age (Rand & Petrocelli, 1985), as detoxifying enzyme systems develop and are induced from juvenile to adult fishes. A similar inverse relationship for lindane, as in our study, was recorded for male ringed seal (Phoca hispida, Muir et aL, 1988) in the Canadian Arctic, while the concentrations of EPCB and E D D T were positively correlated with age. The largest increase in ZPCB and E D D T uptake (ng g- i fresh weight) was recorded for eels aged 12 years and older. From 6 to 12 years, uptake of these pollutants was lower and increased gradually. The increase in pollutant uptake after age 12 could be a result of lowered growth rate. The instantaneous growth rate (Gt) of fish generally decreases with age. For example V611estad and Jonsson (1988) found that Gt for the eel decreased from 1"41 in the first growth season to 0.2 in the seventh. Similarly, G, of the eels in Lake Ringsj6n decreased with age. This resulted in a dilution of pollutants in the growing biomass in younger year-classes and a small and continuous increase in pollutant levels. After the age of 12, G, stabilized around 0.1 and, as a result, concentrations of pollutants started to increase faster. Growth rate of fish has been hypothezised and modelled to be an important factor for levels of persistent pollutants in fish (Thomann, 1989). The increasing fat content with age/weight of the eel has implications on pollutant levels calculated on an extractable fat basis. This fat content-size relationship of the eel was demonstrated by Lovern already in 1938 (Lovern, 1938). The pollutants were 'diluted' in the increasing fat amounts of eels up to 400g, as shown by the inverse relationship between fat content and pollutant levels. Consequently, the age-dependent increase in EPCB and E D D T concentrations was not as clear as when calculations were based on fresh weight, though the correlations were significant. In addition to exposure time, a shift in diet with age may explain the increase in persistent pollutants in older fish. Young eels consume mostly zoobenthos. As eels grow, fish constitute a larger portion of the diet (Tesch, 1977). Concentrations of persistent pollutants generally are higher in fish compared to zoobenthos. Eel headform (width/length) has been related to food choice, i.e. broad-headed individuals feed mainly on fish and large macroinvertebrates and narrow-headed on smaller zoobenthos (Trybom, 1893; T6rlitz, 1922; Tesch, 1977). We found a significant, positive correlation between headform and levels of ZPCB and DDE, indicating a higher proportion of fish in the diet of the broad-headed eels. However, headform was also positively correlated with age and thus the hypothesis is still open to question. Unlike many other fishes, anguillid eels spawn only once in their life. This means that lipophilic pollutants are not eliminated seasonally through the
48
Per Larsson, Stellan Hamrin, Lennart Okla
roe, thereby decreasing exposure to these xenobiotics. Accumulation of pollutants thus continues for the whole lifetime not only in males as in most animals, but also in females. Eels contain large a m o u n t s of fat, and so levels of pollutants are higher than for most fishes. In addition, eels are relatively insensitive to inferior water quality, and are often exposed to high levels of persistent pollutants in industrial regions. Combining these facts with the long migration, using muscular fat deposits as energy reserves, eels seem to be a species highly exposed to pollutants, As the lipid deposits are depleted, the lipophilic pollutants are released into the bloodstream and can be transferred to vital organs and germinal tissue. Such situations have been shown to be deleterious for birds; as birds starve, persistent pollutants are released from their fat deposits to poison the animal (Krom, 1986). A similar situation could be the case for migrating eels, and m a y be one reason for diminishing catches in Europe in recent decades. REFERENCES Aguilar, A. & Borrell, A. (1988). Age- and sex-related changes in organochlorine compound levels in fin whales (Balaenopter physalus) from the eastern North Atlantic. Marine Environmental Research, 25, 195-211. Ballschmiter, K. & Zell, M. (1980). Analysis of polychlorinated biphenyls (PCB) by glass capillary gas chromatography. Fresenius Z. Analytische Chemie, 302, 20-31. Bengtsson, B.-E. (1980). Long-term effects of PCBs (Clophen A 50) on growth, reproduction and swimming performance in the minnow, Phoxinus phoxinus. Water Research, 14, 681 7. BoEtius, I. & BoEtius, J. (1985). Lipid and protein content in Anguilla anguilla during growth and starvation. Dana, 4, 1-18. Brown, M. P., Werner, M. B., Sloan, R. J. & Simpson, K. W. (1985). Polychlorinated biphenyls in the Hudson River. Environmental Science and Technology, 19, 656-61. Connel, D. W. (1987). Age to PCB concentration relationship with the striped bass (Morone saxatilis) in the Hudson River and Long Island Sound. Chemosphere, 16, 1469-74. Duinker, J. C. & Hillebrand, M. T. J. (1983). Composition of PCB mixtures in biotic and abiotic marine compartments (Dutch Wadden Sea). Bulletin of Environmental Contamination and Toxicology, 31, 25-32. Ferrando, M. D., Moliner-Andreu, E., Almar, M. M., Cebrian, C. & Nunez, A. (1987). Acute toxicity of organochlorine pesticides to the European eel, Anguilla anguilla: the dependency on exposure time and temperature. Bulletin of Environmental Contamination and Toxicology, 39, 365-9. Gobas, F. A., Opperhuizen, A. & Hutzinger, O. (1986). Bioconcentration of hydrophobic chemicals in fish: relationship with membrane permeation. Environmental Toxicology and Chemistry, 5, 637~46. Henderson, R. J. & Tocher, D. R. (1987). The lipid composition and biochemistry of freshwater fish. Progress in Lipid Research, 26, 281-347.
Uptake of persistent pollutants in an eel population
49
Johnson, M. G., Kelso, J. R. M. & George, S. E. (1988). Loadings of organochlorine contaminants and trace elements to two Ontario lake systems and their concentration in fish. Canadian Journal of Fisheries and Aquatic Sciences, 45, 170-8. Kanazawa, J. (1982). Relationship between the molecular weights of pesticides and their bioconcentration by fish. Experientia, 38, 1045-6. Kenaga, E. E. (1980). Correlation of bioconcentration factors of chemicals in aquatic and terrestrial organisms with their physical and chemical properties. Environmental Science and Technology, 14, 553-6. Krom, M. D. (1986). An evaluation of the concept of assimilative capacity as applied to marine waters. Ambio, 15, 208-14. Kruse, V. R., Boek, K. & Wolf, M. (1983). Der gehait an organochlor-pestiziden und polychlorierten biphenylen in Elbeaalen. Archivfiir Lebensmittelhygiene, 34, 81-6. Larsson, P. (1990). Sediment as a source for PCBs to a river system. Canadian Journal of Fisheries and Aquatic Sciences, 47, 746-54. Larsson, P. & Okla, L. (1989). Atmospheric transport of persistent pollutants to Sweden in 1985 compared to 1973. Atmospheric Environment, 23, 1699-711. Lovern, J. A. (1938). Fat metabolism in fishes. XIII: Factors influencing the composition of depot fat of fishes. Biochemical Journal, 32, 1214-24. Medford, B. A. & Mackay, W. C. (1978). Protein and lipid content of gonads, liver, and muscle of northern pike (Esox lucius) in relation to gonad growth. Canadian Journal of Fisheries and Aquatic Sciences, 35, 213-19. Muir, D. C. G., Norstrom, R. J. & Simon, M. (1988). Organochlorine contaminants in Arctic marine food chains: Accumulation of specific polychlorinated biphenyls and chlordane-related compounds. Environmental Science and Technology, 22, 1071-9. Murty, A. S. (1986). Toxicity of pesticides tofish. Volume 1. CRC Press, Boca Raton, Florida, 178 pp. Neely, W. B., Branson, D. R. & Blau, G. E. (1974). Partitioning coefficients to measure bioconcentration potential of organic chemicals in fish. Environmental Science and Technology, 8, 1113-15. Newton, I. (1988). Determination of critical pollutant levels in wild populations, with examples from organochlorine insecticides in birds of prey. Environmental Pollution, 55, 29-40. Okla, L. & Wes6n, C. (1984). A simple on-column injector for capillary gas chromatography. Journal of Chromatography, 299, 420-3. Pizza, J. C. & O'Connor, J. M. (1983). PCB dynamics in Hudson River striped bass. II Accumulation from dietary sources. Aquatic Toxicology, 3, 313-27. Rand, G. M. & Petrocelli, S. R. (1985). Fundamentals of Aquatic Toxicology. Hemisphere Publishing Corporation, New York, p. 542. Sagiura, K., Ito, N., Matsumoto, N., Mihara, Y., Murata, K., Tsukakoshi, Y. & Goto, M. (1978). Accumulation of polychlorinated biphenyls in fish: limitations of 'correlation between partitioning coefficients and accumulation factors'. Chemosphere, 9, 731-6. Spigarelli, S. A., Thommes, M. M. & Prepejchal, W. (1983). Thermal and metabolic factors affecting PCB uptake by adult brown trout. Environmental Science and Technology, 17, 88-94. Swackhamer, D. & Hites, R. A. (1988). Occurrence and bioaccumulation of
50
Per Larsson, Stellan Hamrin, Lennart Okla
organochlorine compounds in fishes from Siskiwit Lake, Isle Royale, Lake Superior. Environmental Science and Technology, 22, 543-8. Tesch, F.-W. (1977). The eel. Biology and management of aguillid eels. Chapman and Hall Ltd, Edinburgh, 434 pp. Thomann, R. V. (1989). Bioaccumulation model of organic chemical distribution in aquatic food chains. Environmental Science and Technology, 23, 699-707. Thomann, R. V. & Conolly, J. P. (1984). Model of PCB in Lake Michigan lake trout food chain. Environmental Science and Technology, 18, 65-71. T6rlitz, M. (1922). Anatomische und entwicklungsgeschichtliche Beitr~ige zur Artfrage unseres Flussaalen. Zeitschrift Fisch., 21, 1-48. Trybom, F. (1893). Ringsji~n. Malm6hus Liin. Dess naturf~rhhllanden och fiske. Meddelande fr~n Kongl. Lantbruksstyrelsen Nr 4, 47 pp. (in Swedish). V611estad, L. A. (1985). Age determination and growth of yellow eels, Anguilla anguilla (L.), from a brackish water, Norway. Journal ofFish Biology, 26, 521-5. V611estad, L. A. & Jonsson, B. (1988). A 13-year study of the population dynamics and growth of the European eel Anguilla anguilla in a Norwegian river: Evidence for density-dependent mortality, and development of a model for predicting yield. Journal of Animal Ecology, 57, 983-97. Wszolek, P. C. & Lisk, D. J. (1979). Persistence of polychlorinated biphenyls and 1,1-dichloro-2,2-bis (p-chlorophenyl)ethylene (p,p'-DDE) with age in lake trout after 8 years. Environmental Science and Technology, 13, 1269-71. Yoshida, T., Takashima, F. & Watabe, T. (1973). Distribution of ~14~PCB in carp. Ambio, 2, 111-13.