Selenium toxicity values used to derive SSDs
Toxicity thresholds based on egg (or ovary) Se are currently available for 12 freshwater species (Table 1), although the distribution of 2 species, razorback sucker (Xyrauchen texanus) and Yellowstone cutthroat trout (Oncorhynchus clarkii bouvieri), does not include Canadian waters. The basic study design for the toxicity thresholds compiled included exposure of parent fish to dietary organic Se (either in a natural diet or a synthetic diet spiked with selenomethionine) and evaluation of deformities (e.g., craniofacial, skeletal, and finfold defects), edema, and mortality in larval offspring. The types of toxicity studies used to derive each threshold vary, and consequently it is not always possible to derive the toxicity threshold for each species by means of a consistent approach. For example, some studies were conducted in the laboratory, where the test organisms were exposed to a series of dietary organic Se concentrations, whereas in other studies, existing fish populations were naturally exposed to Se in the field at one or more exposure sites and a reference site. For the latter, Se had either previously been identified as the element responsible for observed toxicity at a site, or it was confirmed through experimental design, exposure and accumulation evaluation of other constituents, or both. Furthermore, the most sensitive endpoints in most studies with field-exposed fish were larval deformities (including craniofacial, skeletal, and finfold defects, and edema), which are diagnostic of Se exposure (Maier and Knight 1994; Lemly 1997) and is consistent with the mode of toxic action for Se (Janz et al. 2010).
Table 1. Summary of studies evaluating selenium toxicity to embryos and larvae resulting from maternal transfer.a
|Bluegill||Bryson et al. (1984)||Field||Larval mortality||Ovary||LOEC||<49||21.5|
| ||Bryson et al. (1985a)||Field||Hatchability, swim-up||Ovary||NOEC||>9.1||—|
| || ||Field||Hatchability, swim-up||Ovary||LOEC||<30||—|
| ||Bryson et al. (1985b)||Field||Hatchability, swim-up||Ovary||NOEC||>14.8||—|
| || ||Field||Hatchability, swim-up||Ovary||NOEC||>9.2||—|
| ||Gillespie and Baumann (1986)||Field||Larval edema||Ovary||LOEC||<38.6d||v|
| ||Doroshov et al. (1992)||Lab||Larval edema||Egg||EC10||21c||—|
| ||Coyle et al. (1993)||Lab||Larval mortality||Egg||EC10||22c||—|
| ||Hermanutz et al. (1996)||Mesocosm||Larval edema||Ovary||EC10||30c|| |
Holm (2002) ; Holm et al. (2003, 2005)
|Brown trout||Formation Environmental (2011a)||Field||Alevin mortality||Egg||EC10||20.8||20.8|
| || || ||Larval deformities||Egg||EC10||22.0||—|
|Westslope cutthroat trout||Kennedy et al. (2000)||Field||Larval deformities, mortality||Egg||NOEC||>21||21|
| ||Rudolph et al. (2008)||Field||Alevin mortality||Egg||EC10||17c||—|
| ||Nautilus Environmental (2011)||Field||Alevin mortality||Egg||EC10||24.8||—|
|Yellowstone cutthroat trout||Hardy et al. (2010)||Lab||Larval deformities, mortality||Egg||NOEC||>16.04||—|
| ||Formation Environmental (2011b)||Field||Alevin mortality||Egg||MATC||25||25|
|Dolly Varden||McDonald et al. (2010)||Field||Larval deformities||Egg||EC10||54||54|
|Fathead minnow||Ogle and Knight (1989)||Lab||Reproduction||Ovary||NOEC||>10.92||<23.6|
| ||Schultz and Hermanutz (1990)||Mesocosm||Larval edema, lordosis||Ovary||LOEC||<23.6d||—|
|Largemouth bass||CP&L (1997)||Lab||Larval mortality||Ovary||EC10||22||22|
|Northern pike||Muscatello et al. (2006)||Field||Larval deformities||Egg||EC10||20.4||20.4|
Holm (2002) ; Holm et al. (2003, 2005)
Hamilton et al. (2005a
|White sucker||de Rosemond et al. (2005)||Field||Larval deformities||Egg||EC13||26||26|
Table 1 summarizes the toxicity thresholds extracted from each study and identifies the final threshold used for each species in this evaluation. When both egg and ovary Se concentrations were reported in a study, preference was given to egg Se-based concentrations because it is egg Se to which fish larvae are exposed during yolk sac absorption (Janz et al. 2010). Furthermore, if multiple endpoints were reported in a given study, the most sensitive endpoint is presented in Table 1. Following the hierarchy defined in CCME (2007), preference was given to EC10 values. For brook trout and white sucker, EC10 values could not be derived due to limitations with the available toxicity data; rather, EC10 values were approximated on the basis of the EC06 and EC13 values that could be derived. The brook trout EC06 was based on a 6% increase in craniofacial deformities in larvae from parent fish collected from a Se-exposed site relative to a reference site (and may also be considered a NOEC), whereas the EC13 for white sucker was the percentage of total deformities in larvae from parent fish collected from a Se-exposed site. Given the steepness of the concentration–response curve typically observed for Se (e.g., Doroshov et al. 1992; Coyle et al. 1993; CP&L 1997; Holm et al. 2005; McDonald et al. 2010), the EC06 and EC13 values are not expected to differ greatly from the EC10. For fathead minnow and razorback sucker, effects concentrations could only be defined on the basis of NOECs, LOECs, or the MATC. In the case of fathead minnow, a NOEC (>10.92 µg/g dry weight [dw]) and LOEC (<23.6 µg/g dw) were available from separate studies. The NOEC of >10.92 µg/g dw was not used as the fathead minnow threshold because this ovary Se concentration had less than a 2% effect on reproductive endpoints including a positive effect, relative to controls, for some endpoints. The LOEC of <23.6 µg/g dw was associated with 24.6% larval edema, which is within the range of acceptable effect concentrations in the second level of the effects concentration hierarchy (i.e., EC11–25). Accordingly, an effects threshold of <23.6 µg/g dw was used for the fathead minnow. Because the ranges of razorback sucker and Yellowstone cutthroat trout do not include Canada, they were not included in the primary SSD evaluation. However, because they may be considered surrogates for other untested Canadian sucker and salmonid species, they were included in a sensitivity analysis in order to evaluate whether they would influence the 5th percentile of the SSD.
It has been suggested that coldwater species, including trout, white sucker, and northern pike, are less sensitive to dietary Se than warmwater fish species (Chapman 2007); however, egg- or ovary-based Se thresholds for these coldwater species appear to bracket thresholds for what are sometimes referred to as warmwater species, including bluegill, largemouth bass, and fathead minnow. It should also be noted that all 3 of these species are native to parts of Canada (www.fishbase.org). Moreover, reducing the number of species in the SSD unnecessarily results in greater extrapolation to the 5th percentile of the distribution (i.e., more conservative estimates are required because the sample size is smaller). Accordingly, Se toxicity thresholds for both coldwater and warmwater fish species were included in the SSD.
Selenium SSDs were developed on the basis of the species mean toxicity thresholds summarized in Table 1. In addition, sensitivity analyses were conducted to evaluate the influence of removing certain species from the SSD. For example, SSDs were developed with brook trout and white sucker removed from the data set because there was higher uncertainty associated with these toxicity thresholds. In addition, a coldwater species SSD was developed by considering only toxicity data for trout, northern pike, and white sucker. As noted above, a sensitivity analysis was also conducted with and without razorback sucker and Yellowstone cutthroat trout included in the SSD. The best-fitting distributions to the log-transformed Se toxicity threshold data were identified by Decision Tools software (Palisade Corporation 2008). This software uses the following 3 goodness-of-fit statistics to describe each distribution's fit to the raw toxicity data: 1) chi-square; 2) Komolgorov-Smirnov; and 3) Anderson-Darling. The 3 best-fitting distributions were selected for each SSD in order to evaluate whether the SSD and its associated 5th percentile were sensitive to the statistical distribution type selected.