44.4.1 Aluminum
Dietary-aluminum studies with fish showed that aluminum accumulated in fish, but apparently did not result in toxic effects.126–128 Interestingly, Poston126 reported no effects on growth, survival, or feed conversion in Atlantic salmon fed up to 2000 àg/g dietary aluminum. These studies contrast with waterborne studies that showed that, under acid rain conditions (i.e., low pH), certain aluminum species can be toxic to fish, although they did not seem to accumulate substantially.128 Poston126 reported that trace amounts of aluminum had some nutritional benefits to Atlantic salmon, thus suggesting that aluminum may be an essential inorganic element and most likely metabolically regulated.
44.4.2 Arsenic
Dietary inorganic arsenic compounds (arsenic trioxide [AT] and disodium arsenate heptahydrate [DSA]) were more toxic to rainbow trout than organic arsenic compounds (dimethylarsinic acid [DMA] and arsanilic acid [AA]).129 The NOEC (no-observed-effect concentration) for inorganic arsenic was between 1 and 137 àg/g for DSA, between 1 and 180 àg/g for AT, and at least 1497 àg/g for DMA and AA. In a more detailed dietary-arsenic study with rainbow trout, Cockell et al.102 reported adverse effects in fry at 33 àg/g of pentavalent arsenic as DSA. In another dietary arsenic study with rainbow trout exposed to trivalent arsenic (sodium arsenite), Oladimeji et al.130 reported adverse effects at 20 and 30 àg/g, thus suggesting that the dietary toxicity of trivalent and pentavalent forms of arsenic were similar. However, in aquatic ecosystems, arsenic does seem to biomagnify in invertebrates but not in fish.42 Chen and Folt37 reported that in the arsenic- and lead- contaminated Aberjona Watershed of Massachusetts, arsenic accumulated in lower food-chain components such as algae and zooplankton, but not in upper trophic levels including fish. Woolson131 also reported that although arsenic was bioconcentrated from water to aquatic plants, it was not biomagnified to invertebrates or fish. Although arsenic bioaccumulated in plants and zooplank- ton,37,42 no biological effects were measured in fish.37,42,131 Furthermore, Chen and Folt37 noted that although arsenic was bioaccumulated in lower trophic levels from the arsenic-contaminated site, fish from that site had tissue arsenic concentrations similar to reference areas.
44.4.3 Cadmium
There is a controversy as to the importance of exposure route in the accumulation of cadmium in fish. Studies have reported that (1) a greater proportion of cadmium was accumulated from food than from water,19,22,132 (2) about equal amounts of cadmium were accumulated from diet or water exposures,133 or (3) more cadmium was accumulated from water than dietary exposures.134–136 Several studies have reported cadmium concentrations in food that were related to adverse effects.
Handy137 reported that dietary cadmium at 10 mg/g caused increased mortality of subadult rainbow trout in a 28-day study. Hatakeyama and Yasuno138 reported that the reproduction of the guppy (Poecilia reticulata) was adversely affected, i.e., the cumulative number of fry produced was reduced
by exposure to 210 àg/g in midge used as a food source. Kumada et al.135 reported that 100 àg/g cadmium as cadmium stearate in the diet caused liver and kidney damage in rainbow trout. Sublethal dietary concentration of 10 àg/fish/day fed to tilapia (Oreochromis mossambicus) resulted in disruption of plasma calcium, magnesium, and phosphate concentrations, which was ameliorated somewhat by high waterborne calcium concentrations.139 Crespo et al.140 reported that sublethal exposure of rainbow trout to dietary cadmium concentrations of 5 àg/g resulted in morphological disorders in the middle and posterior intestine, which the authors concluded would impair intestinal absorption of nutrients. Lundebye et al.141 also reported intestine damage in Atlantic salmon exposed to dietary cadmium. In a comparison of the toxicity of cadmium and copper to rainbow trout, Handy137 reported that dietary cadmium was more toxic than dietary copper, which is an essential element to animals.
In the extensive review of cadmium in food webs by Kay142 and the review by Handy,30 the authors cited several studies that reported cadmium concentrations to be generally higher in aquatic invertebrates than in fish, thus suggesting that cadmium was not biomagnified through the food web. Nevertheless, the authors concluded that food, as a source of cadmium, might be relatively more important in nature than implied from laboratory studies. Dallinger and Kautzky22 and Dallinger et al.23 also concluded that dietary exposure of fish to cadmium was especially important in aquatic ecosystems where waterborne cadmium concentrations were low, but sediment concen- trations could cycle cadmium into the food chain.
Harrison and Curtis143 compared natural foods to commercial diets fortified with cadmium and reported that five times more cadmium was accumulated by fish from natural foods than from a fortified commercial diet. This observation is the opposite of Merlini et al.144 and Pentreath,145 who reported that zinc was more efficiently accumulated from a prepared diet than from a natural food.
This difference between cadmium and zinc accumulation from different types of foods may be due to the fact that zinc is an essential element to animals, whereas cadmium is not. Likewise, fish mortality in dietary cadmium exposures may not be due exclusively to direct dietary cadmium toxicity because investigators have reported disruption of copper and zinc metabolism that may have contributed to the observed mortality.137,146
A concern raised in dietary-cadmium studies reviewed by Kay142 was that a major problem in cadmium-treated artificial diets was the use of soluble chloride and nitrate forms of cadmium, which could rapidly leach from the diet, thus resulting in a partial aqueous cadmium exposure. He further noted that rapid-exchange flow-through exposure systems and immediate removal of uneaten food were only partial solutions to the problem of waterborne exposure. These two concerns — soluble trace elements added to diets and rapid flushing or rapid removal — apply to any study where a trace element or other toxic is added to a prepared diet.
44.4.4 Cesium
Aoyama et al.147 reported that predator fish accumulate cesium from prey fish. The cesium concentration in predator fish increased with ration size, but altering the feeding interval did not influence the accumulation of the trace element. Fish growth, however, diluted the body residue when the dietary concentration of cesium was constant.
44.4.5 Chromium
One study investigated the effects of dietary chromium in fish using trivalent chromium.148 Rainbow trout did not accumulate chromium in carcass, fin, vertebrae, skin, gill, muscle, or intestine, even at the highest concentration tested (8.2 àg/g). Nevertheless, chromium apparently caused reduced growth at this concentration. These results were consistent with those of Patrick and Loutit53 and Dallinger and Kautzky,22 who investigated aquatic ecosystems contaminated with a mixture of inorganic elements and reported that chromium in the food chain accumulated less in fish than
other elements such as cadmium, copper, lead, and zinc. Part of the reason for a lack of adverse effects from chromium may be due to it’s metabolic regulation as an essential element in animals.
44.4.6 Cobalt
Baudin and Fritsch149 investigated the relative contribution of cobalt from food and water with common carp (Cyprinus carpio) in a 63-day exposure. They concluded that the uptake and accu- mulation of cobalt, which is an essential element for plants and animals, was greater from water exposure than from food. However, the exposure concentrations were very low (0.2 àg/L in water and 0.13 àg/g in food), which resulted in low accumulation in fish (total cobalt 0.0033 àg/g). Suzuki et al.54 reported that cobalt accumulated about equally in fish exposed by either waterborne or dietary routes of exposure.
44.4.7 Copper
The dietary concentration of copper that was toxic to rainbow trout as reported by Lanno et al.150 was 664 àg/g. This elevated toxic concentration, relative to the lower toxic dietary concen- trations of arsenic, cadmium, and mercury that cause adverse effects in fish, may be due, in part, to copper being an essential inorganic element to fish and other animals. Miller et al.151 reported that rainbow trout exposed to dietary copper as high as 684 àg/g showed no adverse effects on survival, growth, condition factor, or food conversion efficiency, but they noted that diet seemed to be the dominant source of copper in their water and dietary exposure study. Similarly, Handy et al.152 reported no effects on survival or growth of rainbow trout exposed to 490 àg/g of dietary copper, but lipids were reduced in liver, and swimming activity was altered. Berntssen et al.115 reported adverse effects on growth and whole-body energy stores of protein and glycogen in Atlantic salmon fed 500 àg/g or greater copper in the diet. Lundebye et al.141 also reported that growth was reduced in Atlantic salmon exposed to dietary copper at concentrations of 700 àg/g or greater. Most investigators hypothesized that biochemical regulation of copper prevents toxic effects at low concentrations (i.e., < 100 àg/g) because copper is an essential element to fish and other animals.
In a comparison of the toxicity of copper and cadmium to rainbow trout, Handy137 reported that dietary copper was less toxic than dietary cadmium. This differential toxicity probably reflects the regulation of copper and the limited regulation of cadmium via metallothionein.137,146
44.4.8 Lead
Hodson et al.153 reported that dietary lead was not accumulated by rainbow trout, although the highest concentration tested in the diet was only 1 àg/g. Chen and Folt37 reported that elevated lead concentrations in the arsenic- and lead-contaminated Aberjona Watershed of Massachusetts were accumulated in lower food-chain components, such as algae and zooplankton, but not in upper trophic levels including fish. Leland and McNurney154 also reported that the highest lead concen- trations were in periphyton and macrophytes, and lead was not biomagnified through the aquatic food chain. They showed that lead accumulation in fish was lowest in piscivores, intermediate in predators of macroinvertebrates, and highest in grazers or detritus feeders. Patrick and Loutit53 and Dallinger and Kautzky22 investigated aquatic ecosystems contaminated with a mixture of inorganic elements and reported that lead was accumulated in fish through the food chain, but to a lesser extent than other important elements such as copper and zinc. Nevertheless, a dietary lead concen- tration of 10 àg/g fed to rainbow trout caused morphological disorders in the middle and posterior intestine and altered chlorine and sodium fluxes and sodium–potassium ATPase activity.140 Vighi15 reported that more lead was accumulated via the food chain than from water in an algae-daphnia- guppy food-chain exposure. However, the accumulation factors showed high accumulation in algae, lower concentrations in daphnia, and still lower in fish. Exposure of rainbow trout to very elevated
dietary lead (7 mg/g) caused a 12% incidence of black-tail, a well-established symptom of lead toxicosis in fish.155
44.4.9 Manganese
Manganese accumulates in fish primarily from dietary sources.54,145 Dallinger and Kautzky22 studied a trace-element-contaminated (copper, cadmium, chromium, lead, manganese, nickel, zinc) river ecosystem and reported that rainbow trout accumulated a substantial amount of manganese from dietary sources. No one has reported adverse effects in fish associated with dietary manganese exposure.22,54,145
44.4.10 Mercury
Mercury has no known essential function in vertebrate organisms.156 Diet was the primary route of methylmercury uptake by fish and methylmercury the dominant mercury form in fish.156
For example, Phillips and Buhler13 reported that 70% of dietary methylmercury was assimilated fish, whereas only 10% of waterborne methylmercury was assimilated. Handy30 and Wiener and Spry156 noted that the trophic transfer of mercury to fish has received a substantial amount of attention because of mercury poisoning in humans from consumption of contaminated fish. How- ever, there is limited information on dietary mercury toxicity to fish. Dietary mercury exposure of fish to about 20–40 àg/g resulted in adverse effects such as gastrointestinal damage and inhibition of enzymes in the gut.30 Zhou and Wong157 reported a fourfold difference in mercury accumulation from the diet in six species of fish due to differences in feeding behavior.
44.4.11 Vanadium
One study investigated the toxicity of vanadium in the diet to fish.158 Vanadium concentrations of 10.2 àg/g and greater in the diet of rainbow trout reduced growth and feeding responses. It was concluded that rainbow trout were extremely sensitive to dietary vanadium, and dietary vanadium toxicity was similar to that of dietary selenium toxicity. In contrast, waterborne exposure of fish to vanadium in freshwater ecosystems does not seem to be of major concern.159 Hamilton et al.84 suggested that elevated vanadium residues in aquatic invertebrates fed to larval endangered razor- back suckers in an on-site toxicity test contributed to the mortality of larvae. Vanadium is one of the least-studied trace elements from a dietary-toxicity standpoint but could potentially be a very important dietary contaminant. Most research with dietary vanadium has been done to determine its essentiality.160
44.4.12 Uranium
One dietary study has been conducted with uranium in which lake whitefish (Coregonus clupeaformis) exposed for 100 days to 100 àg/g of dietary uranium had concentration- and duration- dependent histopathologies in liver and posterior kidney.161,162 Those effects were chemotoxic rather than radiotoxic because α-radiation rates were negligible.
44.4.13 Zinc
In a comparison of waterborne vs. dietary uptake of zinc, Merlini et al.144 reported a two- to fourfold greater uptake in pumpkinseed (Lepomis gibbosus) of zinc from food than from water.
Likewise, Willis and Sunda17 compared waterborne vs. dietary uptake in western mosquitofish and spot (Leiostomus xanthurus) and reported that 78 to 82% of the accumulated zinc came from the dietary uptake. Investigations have reported that zinc was more efficiently accumulated from
surficially incorporated prepared diets than from zinc incorporated into live foods.144,145 However, others have reported that zinc proteinate (i.e., a chelated amino acid complex of zinc) accumulated to greater concentrations in fish than did dietary zinc supplemented as unbound zinc as zinc sulfate.163–165 Wekell et al.163 reported that exposure of rainbow trout to up to 1.7 mg/g dietary zinc did not cause effects on growth or survival, which was similar to Brafield and Koodie,165 who found no effects in common carp exposed to up to 5 mg/g dietary zinc. These results were probably due to the fact that zinc is an essential element to animals and is efficiently regulated in fish from a dietary exposure route.16 However, exposure of rainbow trout to very elevated dietary zinc (8.2 mg/g) has been reported to reduce growth, which may have been due to feed avoidance.155 In general, waterborne exposure to zinc seems to lead to greater toxic effects than from dietary exposure.18,166 Elevated concentrations of calcium or phosphorus in the diet can cause a functional zinc deficiency in fish.30 Similarly, Satoh et al.167 reported that fishmeal diets deficient in zinc reduced growth of rainbow trout.
44.4.14 Mixtures of Inorganic Elements
It is rare in the aquatic environment that one inorganic contaminant stands out alone as the sole source of pollutant stress on fish, with the possible exceptions of mercury and selenium. Rather, a mixture of contaminants is often present at generally low concentrations in water but elevated concentrations in sediment and food organisms.22,23,53 Woodward, Farag, and coworkers have reported two good examples of mixtures of elements contaminating food organisms and the resulting adverse effects in fish in the Clark Fork River of Montana168–172 and the Coeur d’Alene River of Idaho.170,173,174 Both rivers were Superfund sites contaminated by wastes from mining activities and have depressed trout populations. The Clark Fork River is contaminated dominantly by copper and the Coeur d’Alene River by lead.
Food organisms from the Clark Fork River contained elevated concentrations of arsenic, cad- mium, copper, lead, and zinc and when fed to brown trout (Salmo trutta) or rainbow trout resulted in adverse physiological, metabolic, behavioral, and histopathological effects.168–172 Woodward et al.171 reported that dietary exposure to the mixture of trace elements was more important than waterborne exposure in causing reduced growth and survival of fish. One interesting aspect of these studies was that the benthic invertebrates collected from the Clark Fork River and used in the experimental diets were either homogenized and refrozen for later feeding or pasteurized and supplemented with vitamins and minerals. Fish growth was reduced in fish fed either the unsupple- mented diet or the supplemented diet. However, survival was reduced in fish fed the unsupplemented diet but not in those fed the supplemented diet, which suggested that either pasteurization (elimi- nated diseases in the wild caught invertebrates) or addition of vitamins and minerals (which may have produced healthier fish) may have enabled the fish to better withstand stresses from the dietary inorganic elements. A large number of studies were conducted as part of the Clark Fork River investigations including chemistry studies of terrestrial and aquatic abiotic and biotic components;
ecological and population studies of terrestrial and aquatic organisms; and in situ, laboratory, and field studies of organisms. The weight of evidence in the risk assessment gave great weight to the findings in the fish studies with dietary exposure to inorganic elements.175
One interesting study conducted as part of the Clark Fork River investigation was a dietary study with nauplii of brine shrimp. Nauplii were enriched with cadmium, copper, lead, and zinc, individually and as a mixture including arsenic, then fed to rainbow trout fry for 60 days starting at 11-days post swim-up.176 Fish exposures also included simultaneous exposure to a mixture of waterborne elements at sublethal concentrations. Nauplii were exposed to aqueous element con- centrations for about 24 h before hatch and about 24 h post hatch to achieve residues close to those in invertebrates from the Clark Fork River; then they were fed to fish. No adverse effects on survival or growth of fish were observed. In citing this study, Woodward et al.172 noted that the duration of the brine shrimp exposure was short; thus, the elements were probably attached to the
external surfaces of the nauplii and in the free form instead of incorporated protein, as in the studies by Woodward et al.171,172 and Farag et al.168,169 They also noted that ionic inorganic elements were not absorbed as efficiently in the gut and may not be as toxic as elements bound to proteins.143,165,170,177 Farag et al.170 reported that inorganic elements incorporated naturally into invertebrates collected from the Clark Fork River were processed differently during digestion by cutthroat trout (Oncorhynchus clarki) than those from diets made up of brine shrimp nauplii exposed to a mixture of elements in the laboratory for ~24 h, as was done by Mount et al.176 They cautioned that the results from studies that incorporate inorganic elements into live foods as part of a dietary exposure may result in toxicological effects different than those using natural foods with elevated inorganic elements.
Results similar to those in the Clark Fork River studies were found in the Coeur d’Alene River studies, where the elements of concern included arsenic, cadmium, copper, lead, mercury, and zinc.170,173,174 In general, inorganic elements were highest in sediments and aufwuchs, intermediate in invertebrates, and lowest in fish. Although the elements did not biomagnify, they were bioavail- able and did biotransfer to fish. Fish fed diets incorporating benthic invertebrates collected from contaminated sites in the Coeur d’Alene River had reduced survival, growth, and feeding activity and increased histopathological abnormalities.174 One interesting aspect reported by Farag et al.173 was that smaller invertebrates accumulated greater concentrations of elements than larger inverte- brates, which the authors hypothesized would expose early life stages of fish to larger doses of elements than adults.