Histological Effects Of Repeated Hydrogen Peroxide Exposure To Branchial Tissues Among Freshwater Fishes     Mark P. Gaikowski1, Christine L. Densmore2, Jeffery J. Rach1, William H. Gingerich1, and Vicki S. Blazer2   1U.S.G.S., Upper Midwest Environmental Sciences Center, 2630 Fanta Reed Road, La Crosse, WI  54603; 2U.S.G.S., Leetown Science Center, National Fish Health Research Laboratory, 1700 Leetown Road, Kearneysville, WV  25430     Therapeutants are not abundant that may be safely, effectively, and legally used to treat freshwater fish, and efforts to develop other such agents for use in aquaculture are increasingly needed.  Hydrogen peroxide is one example.  Currently a therapeutic compound of Low Regulatory Priority status as an anti-fungal agent, hydrogen peroxide has also been effective when used as an antibacterial and anti-parasitic agent.  This study, a component of a larger toxicity study, describes the histological changes in gill tissue associated with sub-lethal hydrogen peroxide exposures on fry and fingerling fishes representing eight families of freshwater fishes cultured in U.S. public aquaculture.   Fry and fingerlings were exposed to different concentrations (47-298 mg/L) of hydrogen peroxide as a 1-hour bath every other day for three days.  Four days after the last exposure, branchial tissue samples were taken for histological analysis from treated and control fish from each species/life stage group. Gill changes/lesions observed included lamellar fusion, lamellar clubbing, epithelial lifting, increased numbers of lamellar chloride cells, mucous cell hyperplasia, epithelial cell hyperplasia and hypertrophy, telangectasia, and epithelial blebbing. Lesions were quantified, and lesion frequency was assessed on a per lamellae basis.  Slightly increased frequencies of potential branchial lesions were observed in association with hydrogen peroxide treatment at one or both life stages examined for all fish species with the exception of two of the salmonid species, lake trout and Atlantic salmon.  Conversely, markedly higher frequencies of branchial lesions (>0.10), including most notably lamellar fusion and epithelial lifting, were observed in northern pike and pallid sturgeon.  The majority of histological changes recorded for the majority of species/life stage groups, however, were seen at a low frequency that would likely cause little to no disruption in respiratory function.