Various causes have been identified .
Various causes have been hypothesized , including an increase in ultraviolet radiation affecting the spawn on the surface of ponds , chemical contamination from pesticides and fertilizers , and parasites such as the trematode Ribeiroia ondatrae .
Various causes have been identified or hypothesized , including an increase in ultraviolet radiation affecting the spawn on the surface of ponds , chemical contamination from pesticides and fertilizers , and parasites such as the trematode Ribeiroia ondatrae .