Macroalgae are recognised as important nursery habitat, yet little is known about their contribution to juvenile fish diets. This project examined the significance of macroalgae and associated infauna, to the diets of juvenile fish in the Ningaloo lagoon, focussing on the contribution to juvenile diets and whether this varies seasonally.
Juvenile fish were collected from macroalgal beds in the Ningaloo lagoon during February and July, 2015. Stomach contents from 164 fish, representing 11 species, were identified to 37 taxa in 14 categories, and the percent volume of items was recorded. Multivariate analyses quantified similarities in the stomach contents to define 3 trophic groups (herbivore, zoobenthivore and carnivore). The variation in specialisation or generalisation of prey items was also assessed between species and seasons.
Macroalgae represented 34.2% of Naso fageni and 41.1% of Siganus fuscescens stomach contents in February, which increased in July as fish became larger (37.7% and 58.2% respectively). Specifically, Sargassum content increased by 10-fold in Siganus guts collected during winter, even though Sargassum biomass typically declines at this time. Infauna groups associated with macroalgal beds were the dominant components in the guts of juvenile Lethrinidae (Lethrinus atkinsoni, Lethrinus nebulosus), Lutjanidae (Lutjanus kasmira, Lutjanus quinquelineatus), and Mullidae (Parupeneus barberinoides, Parupeneus spilurus, Upeneus sp.). Intra-specific variation analyses found that smaller fish caught in February had a narrower trophic width and a more specialised feeding strategy than larger-bodied individuals of the same species in July.
These results highlight the importance of macroalgal beds to diets of the selected juvenile fish species and their contribution to trophic flows in the Ningaloo lagoon. These findings enhance our understanding of variation in diet with ontogeny, improve our understanding of fish diets and provided fundamental information to build an Ecopath with Ecosim food-web model to explore the importance of species interactions in the Ningaloo ecosystem.