Finding out how New Zealand butterfish âbuck the systemâ is the aim of a study by researchers at The University of Aucklandâs School of Biological Sciences in the Faculty of Science. Research aims to find out how butterfish, also known as green-bone, defy conventional thinking that cold-blooded, herbivorous fish only thrive in warm, ocean environments. Like cows, many seaweed-eating fishes employ symbioses with micro-organisms in the gut to ferment plant foods and extract the nutrients. Butterfish are cold-blooded, live in cold water and feed mainly on kelp. Thereâs the general belief that these animals should not be able to meet their nutritional needs from these plants because the microbes needed to break them down require a relatively high temperature to function. But not only do butterfish survive here, they also grow bigger and are at higher population densities the further south you go and the colder the water becomes. The butterfish is found in shallow water around the coasts of New Zealand and its offshore islands where it feeds largely on kelp.