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Hydrogen peroxide is a drug of low regulatory priority status that is effective in treating fish and fish eggs infected by fungi. However, only limited information is available to guide fish culturists in administering hydrogen peroxide to diseased fish. Laboratory tests were conducted to determine (1) the sensitivity of brown trout Salmo trutta, lake trout Salvelinus namaycush, fathead minnow Pimephales promelas, walleye Stizostedion vitreum, channel catfish Ictalurus punctatus, and bluegill Lepomis machrochirus to hydrogen peroxide treatments; (2) the sensitivity of various life stages of rainbow trout Oncorhynchus mykiss to hydrogen peroxide treatments; and (3) the effect of water temperature on the acute toxicity of hydrogen peroxide to three fish species. Fish were exposed to hydrogen peroxide concentrations ranging from 100 to 5,000 μL/L (ppm) for 15‐min or 45‐min treatments every other day for four consecutive treatments to determine the sensitivity of various species and life stages of fish. Except for walleye, most species of fish tested (≤2 g) tolerated hydrogen peroxide of 1,000 μL/L or greater. Walleyes were sensitive to hydrogen peroxide concentrations as low as 100 μL/L. A correlation was found between the toxicity of hydrogen peroxide and the life stages of rainbow trout; larger fish were more sensitive. Generally, the toxicity of hydrogen peroxide increased for all species as water temperature increased. The results of these experiments demonstrate that it is important to consider the effects of species, life stage, and water temperature when conducting hydrogen peroxide treatments.

We determined the acute toxicity of field‐grade formulations of atrazine, alachlor, and a 50:50 mixture of the chemicals to early and late larval stages of northern leopard frogs (Rana pipiens) and American toads (Bufo americanus). We conducted identical tests with rainbow trout (Onchorhynchus mykiss) and channel catfish (Ictalurus punctatus) to compare their sensitivity as bioindicators. The acute toxicity (96‐h median lethal concentrations [LC50s]) of herbicides to amphibian larvae ranged from 47.6 mg/L for early‐stage R. pipiens exposed to atrazine to 3.3 mg/L for late‐stage B. americanus exposed to alachlor. The toxicity of a 50:50 mixture of atrazine and alachlor after a 96‐h exposure was greater than additive (chemical synergy present) for most species and life stages tested, with 96‐h LC50s as low as 1.5 mg/L for late‐stage B. americanus larvae. Older amphibian larvae appeared to be more sensitive than younger larvae for both species and chemicals tested. Overall, rainbow trout and channel catfish appeared to be less sensitive than amphibian larvae. The 96‐h LC50s ranged from 23.8 mg/L for channel catfish exposed to atrazine to 9.1 mg/L for rainbow trout exposed to alachlor. A noteworthy sublethal effect observed in amphibian larvae exposed to atrazine was edema, probably caused by renal disfunction. Chronic predicted no‐observed‐effect concentrations (PNOECs) for lethality after a 30‐d exposure were estimated from mortality data at 3, 6, 24, 48, 72, and 96 h and found to be approximately 10‐ to 20‐fold lower than 96‐h LC50s. The calculated PNOECs are close enough to concentrations actually found in the environment to warrant concern and further tests. Additional research on environmental concentrations, chronic effects, and the effects of chemical synergy is needed before definitive conclusions can be drawn. Chemical synergy and life‐stage sensitivity should be addressed to properly assess the toxicity of herbicides to nontarget organisms.

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