The reasons why herbivorous fishes have distributions skewed towards the tropics have been debated for decades. The hypotheses include limited time for evolution of herbivory, a lack of consistent food, and a physiological constraint related to latitude. I hypothesize that temperature constrains the ability of herbivorous fishes to digest algae, therefore limiting their ability to survive and reproduce in cold waters. To test this hypothesis, I will investigate the sources of nutrition for an herbivorous fish, the opaleye Girella nigricans, throughout a latitude and temperature gradient with the use of stable carbon and nitrogen isotope analyses. I will also study how temperature affects microbial digestion in the gut of opaleye by culturing cellulose degrading intestinal bacteria from G. nigricans at various temperatures. Additionally, I will investigate the role temperature plays in affecting the ability of opaleye to control algal abundance with the use of field cages to enclose herbivorous fish in different thermal regimes in the vicinity of a power plant discharge. The results of these studies will help me determine (1) what factors limit herbivorous fishes to the tropics and (2) the effect herbivorous fishes may have on benthic algal assemblages in response to global climate change.