Molybdic acid

Long-term toxicity to fish

Description of key information

Relevant fish studies were identified for the freshwater environment, and using bounded and most relevant endpoints/end parameters, species-specific NOECs could be determined for the fathead minnow Pimephales promelas and the rainbow trout Oncorhynchus mykiss. Test results were obtained in experiments that followed international accepted guidance, and were based on measured levels of dissolved molybdenum. For P. promelas and O. mykiss a NOEC of 60.2 mg/L and 43.2 mg/L, respectively.

For marine environment, a reliable 28d-EC10 of 84.1 mg Mo/L has been reported for the sheepshead minnow Cyprinodon variegatus.

Additional information

Freshwater compartment

No reliable NOEC-values could be identified for the goldfish Carassius auratus (Birge, 1978). Therefore this species is not taken into account in the effects assessment of molybdenum for the aquatic compartment.

 

Four 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 to 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.

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. 

  

Marine compartment

Two reliable studies have been identified for the marine environment, and these for the sheepshead minnow Cyprinodon variegatus (Parametrix, 2009) and Menidia beryllina (Dinehart, 2013).

For the Menidia beryllina, reliable (K1) 37d-NOEC values (28d post-hatch) were derived for standard length and blotted weight. The 37d-NOEC for these endpoints was 139 mg Mo/L.

For the Cyprinodon variegatus, 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.