Chemicals that seep out of cigarette butts can be acutely toxic to fish and micro-organisms.
Micevska, T., et al. "Variation in, and Causes of, Toxicity of Cigarette Butts to a Cladoceran and Microtox." Archives of Environmental Contamination and Toxicology. 50(2). Feb. 2006: 205-12. Web.http://www.springerlink.com/content/p433v47q31282841/
Register, Kathleen M. "Cigarette Butts as Litter—Toxic as Well as Ugly." Clean Virginia Waterways. Farmville, VA. Web.http://www.longwood.edu/cleanva/ciglitterarticle.htm
Slaughter, Elli, et al."Toxicity of cigarette butts, and their chemical components, to marine and freshwater fish." Tobacco Control. 2011. Web.http://tobaccocontrol.bmj.com/content/20/Suppl_1/i25.full