Several early dietary studies with selenite and rainbow trout (Oncorhynchus mykiss) were published in the early 1980s by Canadian researchers.55–59 These investigations were not apparently connected to the fish population disappearances in Belews Lake, North Carolina, which were
believed to be due to selenium toxicity.31 This same group of researchers also conducted two waterborne studies with selenite and rainbow trout to complement their dietary studies.14,60 These early selenium studies explored dietary requirements, elimination and uptake rates from water and diet sources, the minimum dietary requirement for rainbow trout (between 0.15 and 0.38 àg/g in dry feed) for maximal storage, half-life period, influence of dietary carbohydrate, and toxic con- centrations in water and diet. It was shown that plasma glutathione peroxidase homeostasis was maintained at up to 1.25 àg/g dry-feed activity; toxicity occurred at 13 àg/g dry feed, but the authors speculated that dietary concentrations in excess of 3 àg/g in dry feed over long time periods might be toxic; liver and kidney were the primary tissues of storage; and excess dietary carbohydrate enhanced dietary selenium toxicity in rainbow trout. Channel catfish (Ictalurus punctutus) have similar responses to dietary selenium (requirement between 0.1 and 0.5 àg/g and toxicity at 15 àg/g) to those of rainbow trout.61
Two other early investigations of dietary selenite toxicity to rainbow trout were conducted by the Colorado Division of Wildlife.62,63 Their investigations were prompted, in part, by two field investigations in Colorado and Wyoming that concluded selenium toxicity was occurring in fish via the food chain.64,65 Barnhart64 was the first publication to suggest that selenium in the food chain was causing fishery problems in Sweitzer Lake in western Colorado. It is interesting to note that these publications and those of the Hilton group55–59 did not mention selenium problems in Belews Lake, North Carolina, which was occurring during the same time period. Goettl and Davies63 reported that dietary selenium toxicity occurred between 5 and 10 àg/g dry diet, which is remarkably close to that reported by the Hilton/Hodson group and also to later dietary studies conducted with selenomethionine and other fish species in the late 1980s.
Sandholm et al.66 were the first to show that selenium accumulation in fish was greater from dietary sources such as phytoplankton or zooplankton than from water. They also showed that there was little difference in fish accumulation of selenite or selenomethionine in the food chain. Besser et al.67 further reported that selenate and selenite were accumulated in fish primarily via the food chain, whereas selenomethionine was accumulated via both aqueous and food chain uptake.
Kleinow and Brooks68 reported that selenite and selenate were efficiently absorbed from the gastrointestinal tract of fish, thus resulting in a high assimilation efficiency.
In general, dietary studies with selenomethionine have reported that toxic responses in fish were similar to those in fish fed diets containing naturally incorporated selenium compounds such as fishmeal made from western mosquitofish (Gambusia affinis)69 or zooplankton.70 However, Bell and Cowey71 reported that the digestibility and availability of selenium to Atlantic salmon (Salmo salar) were the least for fishmeal (source not given) and followed the order from greatest to least:
selenomethionine > selenite > selenocystine > fishmeal. The comparability of selenium-laden fishmeal diet and a selenomethionine-fortified diet in three studies with chinook salmon (Onco- rhynchus tshawytscha)69 compared to differential digestibility reported in Atlantic salmon71 may reflect species differences or fishmeal differences. In the study by Hamilton et al.,69 the authors noted in both their freshwater and brackish-water studies with chinook salmon that fish growth was significantly reduced at lower concentrations and in shorter exposure periods in fish fed the diet made with the western mosquitofish fishmeal compared to the selenomethionine diet, which con- tained a comparable amount of clean fishmeal from western mosquitofish. They suggested that the slightly greater toxic effect in fish fed the fishmeal diet could have been caused by three factors:
(1) additional toxic elements accumulated in the western mosquitofish inhabiting the San Luis Drain, (boron, chromium, and strontium); (2) other forms of organoselenium such as selenocystine present in the western mosquitofish; or (3) differential uptake, distribution, or elimination of the protein- bound organoselenium in the fish fed the western mosquitofish fishmeal diet compared to fish fed the free amino acid selenomethionine diet that contained a comparable amount of clean fishmeal.
Due to the similarity between selenomethionine and naturally selenium-laden food organisms, selenomethionine-fortified diets have been used in studies with bluegill (Lepomis macrochirus) to
determine toxic effects on reproduction.72–74 The study by Woock et al.72 incorporated a waterborne exposure to 10 àg/L selenite and reported that larvae survival was reduced in the adult dietary exposure to 13 àg/g diet. The Cleveland et al.73 study with juvenile fish reported effects at some of the lowest whole-body selenium residues (~4–5 àg/g, wet weight) that have been linked to adverse effects in fish. The Coyle et al.74 study incorporated a 10 àg/L waterborne selenium exposure and reported adverse effects on fry when adults were exposed to 33 àg/g in the diet. Ogle and Knight75 conducted a reproduction study with adult fathead minnow (Pimephales promelas) and progeny exposed to a dietary mixture of selenate, selenite, and selenomethionine. They reported reduced growth of adults exposed to 20 àg/g selenium after 56, 70, 84, and 96 days of exposure, but no effects on progeny.
As was reported with selenomethionine, Bryson et al.70 reported that selenocystine incorporated into a fish food diet also produced adverse effects comparable to selenium-laden zooplankton.
Bryson et al.70 and Woock et al.72 reported that diets incorporating selenite were not as toxic to fish as diets incorporating selenomethionine. This finding may be related to those of Lorentzen et al.76 who reported that at low dietary selenium concentrations (~1–2 àg/g), dietary selenite was accu- mulated primarily in liver, whereas dietary selenomethionine accumulated primarily in whole body and muscle. This differentiation of storage compartments for different selenium compounds may influence the toxic effects observed.
Studies conducted in experimental outdoor streams at the U.S. Environmental Protection Agency’s Monticello Research Station have demonstrated adverse effects on reproduction of fathead minnow and bluegill exposed for about a year to a selenite concentration of 10 àg/L.77–79 Although these studies did not measure selenium concentrations in the food chain, selenium was rapidly accumulated during the growing season (i.e., May–September) in sediments and emergent, floating, and submerged aquatic plants in wetlands that comprised a portion of the experimental streams.80 Consequently, accumulation of selenium in food organisms no doubt occurred and contributed to a dietary exposure of the fish.
Several dietary selenium studies have been conducted with food organisms collected from selenium-contaminated environments. Woock81 demonstrated in a cage study with golden shiners (Notemigonus crysoleucas) that fish in cages with access to bottom sediments accumulated more selenium than fish held in cages suspended about 1.5 m above the sediments. This study showed that effects in fish were linked to selenium exposure via sediment, benthic organisms, detritus, or a combination of sediment compartments. A similar finding was reported by Barnhart64 who reported that “numerous species of game fish” lived at least 4 months when held in a livebox, which limited access to food organisms and sediment, but fish lived less than 2 months when released in selenium-contaminated Sweitzer Lake, Colorado. The highly toxic nature of benthic organisms from selenium-contaminated Belews Lake, North Carolina was shown by Finley82 in an experiment where bluegill-fed Hexagenia nymphs died in 17 to 44 days. In another study, selenium- contaminated red shiner (Notropos lutrensis) collected from Belews Lake, North Carolina were highly toxic to striped bass (Morone saxatilis).83 In a series of experiments with the endangered razorback sucker (Xyrauchen texanus), larvae were fed selenium-laden food organisms from sites in the upper Colorado River and the Green River, both in the upper Colorado River basin.84–86 Larvae consistently showed adverse effects at 4–5 àg/g in the diet (zooplankton collected from backwater areas).
Another important aspect influencing the toxic effects in fish resulting from dietary selenium exposure is season.87 The toxicity to bluegill of combined low dietary (5.1 àg/g) and low waterborne (4.8 àg/L) selenium at low water temperature (4ºC) resulted in significantly increased mortality of fish.88 The combination of a stress-related elevated energy demand from selenium exposure and reductions in feeding due to cold temperature and short photoperiod led to a severe depletion of stored body lipid and an energetic drain that resulted in the death of about a third of the fish tested.
Heinz and Fitzgerald89 also suggested that stress from winter conditions might have increased the harmful effects of dietary selenium in adult mallards (Anas platyrhynchos).