The nutritional basis of seasonal selective feeding by a marine herbivorous fish

Marine herbivorous fishes have the potential to significantly impact reef ecology through selective feeding on macroalgae. However, the nutritional drivers underlying their diet choice are often poorly understood. Here we examine diet, nutrient ingestion and assimilation in a marine herbivorous fish (Odax pullus; Labridae) endemic to temperate rocky reefs in New Zealand. Individuals were sampled across the year and across key life history stages to obtain a comprehensive insight into the nutritional consequences of feeding behaviour. Seasonal and ontogenetic changes in diet resulted in significant changes in the relative proportion of macronutrients ingested, and these changes coincided with periods of rapid somatic and reproductive tissue growth. In particular, protein and the sugar alcohol mannitol are likely to be important determinants of adult diets, and the seasonal availability of these nutrients is likely to be a key factor shaping the ecology and life history of this species.