The Nutritional Ecology of the New Zealand Butterfish Odax pullus

Abstract

This thesis examined the nutritional ecology of a single species of marine herbivorous fish, the New Zealand butterfish Odax pullus (Labridae). Its aim was to establish whether this species could provide any evidence for a temperature constraint on digestion in an ectothermic herbivore foraging at high latitudes. Specimens of wild O. pullus were sampled across three locations within New Zealand, which covered a latitudinal range of 11° and encompassed a year-round temperature range of 10-22°C. Diet was determined through examination of ingested material and also through analysis of stable isotopes (¹⁵N and ¹³C) in diet and consumer tissues. The nutrient content of material in the foregut of captured fish was quantified on the basis of ash, total hydrolysable amino acids (THAA), total lipid, and hydrolysable neutral sugars (including the sugar-alcohol mannitol). Assimilation efficiencies were calculated and, subsequently, digestion was also considered in terms of the pattern of changing nutrient concentrations along the gut. Finally, digestive activity in the hindgut was characterised using a metabolite profiling approach. An ontogenetic change in the diet of juvenile O. pullus was observed at all locations. During early development, fish switched from an omnivorous diet of epiphytic rhodophyte algae and epiphytic invertebrates to an herbivorous diet dominated by phaeophyte algae. Diet switching coincided with a decrease in ingested protein and lipid concentrations and a corresponding increase in the ingested concentration of crude carbohydrate. However, even on an herbivorous diet, juveniles ingested a higher protein to carbohydrate ratio than adult fish. Differences in the size at which the ontogenetic diet switch occurred were observed across locations. Since these differences also coincided with variation in the availability of dietary algae, they could not be interpreted as evidence for a temperature constraint on the development of herbivory in this species. Seasonal effects on the diet and nutrition of adult fish were examined at a single location (36°S), and significant variation in diet across the year was found for both sexes. Variation in ingested nutrient content was significant for female fish; where a high protein to mannitol intake was related to changes in nutritional demand associated with spawning activity. Nutrient assimilation efficiencies did not vary significantly across seasons, except for mannitol, which was utilised with greater efficiency during the colder half of the year. Across locations, diet analysis revealed that adult O. pullus were exclusively herbivorous during the months that sampling took place (December - February), and stable isotope analysis supported the conclusion that fish at all locations were herbivorous year-round. Ingested concentrations of protein were consistent across locations. Furthermore, mannitol was consistently present in the diet at greater concentrations than any other sugar measured and was utilised with near 100% efficiency. Comparisons of digestive activity and nutrient assimilation along the gut indicated similar levels of hindgut microbial activity at all locations. On the basis of these results it was concluded that hindgut fermentation of mannitol made a significant contribution to the nutrition of O.pullus throughout the majority of its natural species range. This study therefore provides no evidence to suggest that the digestion of algal material by a marine ectothermic herbivore is constrained by decreasing temperatures associated with latitude.