From weed to reef fish: Invertebrates living on seaweed power the middle of marine food webs

ACRS Research Award

Trophic flows | Epifauna | Ningaloo Reef

From weed to reef fish: Invertebrates living on seaweed power the middle of marine food webs

By Yi-Yang (Alex) Chen Australian National University

Reef fishes that feed on the invertebrates associated with seaweed (called epifaunal invertebrates) are key mid-food web consumers, facilitating the transfer of energy to higher levels of the food web. These food web links are a critical part of marine ecosystem functioning and coastal fishery production. Quantifying existing flows between middle-order reef fish and their invertebrate prey will assist in better predicting fishery production in response to environmental disturbances including climate change. To date, there are few estimates of the dietary preferences, including prey selectivity of fishes which consume invertebrates (called invertivorous fishes). We also currently have little information on the amount of epifaunal invertebrates that are consumed by invertivorous fishes, and how this linkage may drive potential fluctuations in the stock of reef fishes that feed within seasonally-variable habitats such as seaweed meadows. My PhD aims to quantify the production of tropical seaweeds and their associated epifaunal invertebrates that reef fishes can access, and to apply models to predict the consequences for reef fish populations in response to temporal and spatial changes in seaweed meadows. My study region was Coral Bay located in Ningaloo Marine Park, Western Australia. The shallow waters in Coral Bay are dominated by corals and the canopy-forming seaweed, Sargassum. I collected samples of Sargassum and its associated epifauna from sites across the bay. Back at the laboratory I painstakingly sorted and counted all the epifauna that had been living on the Sargassum to estimate the daily production of these critical marine communities. I also conducted underwater visual surveys to record the foraging behaviour and biomass of reef fishes within the Sargassum meadows at these same sites. Finally, I set up a series of cages within my sites to exclude fishes from parts of the Sargassum meadow for a period of two weeks. At the end of the two weeks, I collected samples of Sargassum from full-cages and controlled cages. Moving forward, I will quantify the difference in epifaunal communities under fish predation pressure in order to calculate how much production

“Understanding and quantifying the energy flows between reef fishes and their prey can help us better predict fishery production and how seaweed meadows will respond to future environmental disturbances.”

Above: Counting and identifying the epifaunal invertebrates extracted from seaweed canopies. Epifaunal invertebrates (dominated by gastropods and crustaceans) collected from Sargassum seaweeds. © Yi-Yang Chen

from these invertebrates is being consumed and converted into fish biomass at higher levels in the food web.

I found that the invertivorous reef fishes of Coral Bay demonstrate strong microhabitat preferences when foraging within Sargassum meadows and could be grouped into seaweed canopy foragers, opensubstrate foragers and generalists. Actual levels of production by epifaunal communities within Sargassum meadows are strongly influenced by the canopy structure, and are sensitive to changes in canopy height and percent cover. Importantly, these results suggest that die-backs of Sargassum associated with marine heatwave events will have strong impacts not just on availability of seaweeds for herbivores, but also on availability of invertebrates for invertivorous fishes. Future work will include quantification of the energy flows from epifaunal invertebrates to invertivorous fishes by comparing the overall level of epifaunal production within Sargassum meadows exposed to predation with those where invertivores are excluded.

Above: Conducting underwater visual surveys to record seaweed meadow structure at my study site (Coral Bay, Ningaloo Marine Park, Western Australia). © Yi-Yang Chen

Acknowledgements

2020 has been an extremely tough year for all students and I really appreciate the funding support from the ACRS which has enabled me to continue with data collection and analysis that will form a substantive part of my PhD thesis. This support will allow me to complete my PhD to schedule, despite COVIDassociated interruptions. I would also like to acknowledge the financial support of The Ministry of Education Taiwan, The Australian National University, the Australian Society for Fish Biology, and The Linnean Society of NSW. Finally, thanks to my supervisors, Dr. Rebecca Fox and Prof. Michael Jennions for their helpful suggestions and warm encouragement.