Molybdenum dioxide

Long-term toxicity to fish

Description of key information

Five relevant fish studies were identified for the freshwater environment, and using bounded and most relevant endpoints/end parameters, species-specific EC10s/NOECs could be determined for the fathead minnow Pimephales promelas, the rainbow trout Oncorhynchus mykiss and the brown trout Salmo trutta. Test results were obtained in experiments that followed international accepted guidance, and were based on measured levels of dissolved molybdenum. For P. promelas, O. mykiss and S.trutta a NOEC of 60.2 mg/L, 43.2 and 202.5 mg Mo/L, respectively, were retained for risk and hazard assessment purposes. These data points are are used for PNEC derivation as the statistical extrapolation method (i. e., species sensitivity distribution) is applied for the determination of a freshwater PNEC.

For the marine environment, two reliable long-term no-effect levels have been reported for this trophic level: a reliable 28d-EC10 of 84.1 mg Mo/L for the sheepshead minnow Cyprinodon variegatus, and a 37d-NOEC of 139 mg Mo/L for the inland silverside Menidia beryllina. These values are used in a weight of evidence approach for the derivation of a marine PNEC for molybdenum (statistical extrapolation method, using the species sensitivity distribution approach).

Additional information

Freshwater compartment

A number of unbounded NOEC-values were identified for the brown trout Cirrhinus mrigala and the trout species Oncorhynchus kisutsch and Oncorhynchus mykiss (Adhikari and Mohanty, 2012; Ennevor, 1993; McConnell, 1977b; Kennedy et al, 2019). These data were not taken into account for the derivation of a PNEC, but were considered as supporting studies when evaluating the level of protection of the PNEC for the aquatic system in general.

Five studies have been identified that report chronic effect levels for the rainbow trout Oncorhynchus mykiss. Two of these studies (Davies et al, 2005; Parametrix, 2008) produced high-quality effects data (K1-data). The first study reported NOEC-values for survival as effect parameter, whereas the second study presented NOEC- and EC10-values for survival and biomass growth. Comparable effect levels (NOECs) were found for survival in both studies. Biomass growth, however, was found to be a more sensitive effect parameter than survival. EC10-values were available based on a single test and on the combined effect-concentration relationship of two tests. Arguments can be put forward why a specific test result should be preferred over the other value. On one hand, combining two tests implies mixing the outcome of test results that were obtained with two different batches of organisms. The normalised effect-concentration relationship, however, shows a clear decrease from 0% effect to >95% effect. Consequently, the confidence interval that is associated with the combined data set is also narrower than the confidence interval around the EC10 of the second test, as only small but significant effects (10-22%) were observed at the two highest effect levels. From a conservative point of view, it was decided to use the EC10-value of 43.2 mg Mo/L for the derivation of a molybdenum PNEC.

 For the fathead minnow Pimephales promelas two reliable studies were identified: Canton et al (2006) and Parametrix (2007). K1-values were available for three endpoints, with (biomass) growth being more sensitive than survival in both studies. As EC10-values are preferred over NOEC-values for risk assessment purposes, the geometric mean of 60.2 mg Mo/L (EC10-values of 39.9 and 90.9 mg Mo/L) was selected as chronic no-effect value for P. promelas.

Lucas et al (2017) assessed the long-term effects of molybdate on the early-life stages of brown trout, Salmo trutta, following testing methods based on Environment Canada procedures. No significant effects on survival (>98%) or length (21-22 mm) were reported in any of the treatment concentrations, resulting in IC10 values of >1247 mg Mo/L for these endpoints. A significant reduction of wet weight was noted with increasing Mo-levels, resulting in an 85d-IC10 value for wet weight of 202.5 mg Mo/L. This value was considered reliable and relevant for assessing long-term effects of molybdate in the aquatic environment and was thus taken into account in the risk assessment of molybdate. 

 

Marine compartment

Two reliable studies have been identified for the marine environment. The first sudy reported on the chronic toxicity of sodium molybdate on the sheepshead minnow Cyprinodon variegatus (Parametrix, 2009). Dry weight was the most sensitive endpoint (other endpoints: embryo/larval survival, biomass). For this endpoint a reliable 28d-EC10 of 84.1 mg Mo/L has been reported.

The second study evaluated the long-term toxicity of sodium molybdate to the inland silverside Menidia beryllina (Dinehart, 2013). Standard wet weight and blotted wet weight were the most sensitive endpoints, resulting in a NOEC of 139 mg Mo/L (other endpoints: hatch start, hatch completion, egg hatchability, fry survival). The EC10 could not be calculated due to the lack of statistically significant responses. The LOEC for the most sensitive endpoints was 265 mg Mo/L.

The values of 84.1 mg Mo/L for C.variegatus and 139 mg Mo/L for M.beryllina are used in a weight of evidence approach for the derivation of a marine PNEC for molybdenum.