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How butterfish "buck the system"

7 March 2005

University of Auckland research aims to find out how butterfish "buck the system"

Finding out how New Zealand butterfish "buck the system" is the aim of a study by researchers at The University of Auckland's School of Biological Sciences in the Faculty of Science.

Dr David Raubenheimer, a nutritional ecologist, and Dr Kendall Clements, an expert on marine herbivorous fishes, are the recipients of a Marsden Fund grant to investigate the foraging and nutrient intake of wild and captive butterfish.

Their 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. There's 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," says Dr Raubenheimer.

"But not only do butterfish survive here, flying in the face of received dogma, they also grow bigger and are at higher population densities the further south you go and the colder the water becomes. We aim to find out how they buck the system."

The butterfish is found in shallow water around the coasts of New Zealand and its offshore islands where it feeds largely on kelp. It is caught in small commercial quantities by set net, mainly in the Cook Strait. It is most plentiful around Stewart Island and lives up to at least 25 years of age. At the Three Kings Islands off Cape Reinga the butterfish is replaced by the closely-related blue-finned butterfish, Odax cyanoallix.

While the project is primarily aimed at better understanding of the nutritional biology of the butterfish and furthering knowledge about biological systems, Dr Raubenheimer says there are potential implications for commercial fish farming.

"The oceans are pretty stretched at the moment in terms of how sustainable fishing is as an industry, so the alternative is aquaculture. The problem with most fish aquaculture is that carnivorous fish are farmed, which means they are fed basically on fish meal further depleting the ocean's resources.

"But the butterfish is an edible, herbivorous fish which could be farmed much more efficiently and sustainably. It would be hopelessly inefficient for our pastoral farmers to farm carnivores, but we do it all the time with fish."

Dr Clements says the project is a "marriage" of his and Dr Raubenheimer's expertise. Dr Raubenheimer joined the Faculty of Science's School of Biological Sciences earlier this year from Oxford University. Professor Howard Choat from James Cook University, Queensland, is also a collaborator on the project.

"I've worked for nearly 20 years on the biology of these fish. We have a good idea of what they eat and how they digest it. But with this grant, we are looking not so much from a microbiological viewpoint as we have done previously, instead we are bringing in David's expertise as a world authority on how animals organise their nutrient budgets.

"Herbivorous fish sit at the base of the food chain, they feed on plants which are primary producers and almost nothing is known about the way seaweed is converted into fish flesh. We don't know what the processes are, why fish eat some seaweed and not others. At a simplistic level, this would be a huge advantage in terms of aquaculture. But these fish occur naturally on our reefs and if we want to understand the way our reef ecosystems function, then it is important to know about these things."

Using theoretical modeling techniques that Dr Raubenheimer has developed to study nutrition in insects and other animals, the research will examine food selection patterns of the butterfish in the wild in northern, central and southern New Zealand, and analyse the chemical composition of the nutrients gained. Dr Raubenheimer has recently used the same modeling techniques to derive a new theory for the relationship between food selection and the global increase in obesity and obesity-related diseases in humans.

In the laboratory, the chemical composition of nutrients will be manipulated to analyse selection patterns of captive fish. Field and laboratory outcomes will be compared to get "a very strong approach for reading the nutritional foraging and processing" of these fish, say the researchers.

The study will also look at growth patterns in each marine region to determine where the butterfish does best and if water temperature has an effect on their growth and at what stage of their life cycle.

ENDS

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