Kelsey Webber - AIMS@JCU

Kelsey Webber

ARC Centre of Excellence Coral Reef Studies

Kelsey Webber

ARC Centre of Excellence Coral Reef Studies
Bottom up effects of coral reef macroalgae on fishes over space and time

Pursuing his childhood dream, Kelsey moved from Brisbane to Townsville to study Marine Biology at James Cook University in 2016. After completing his Bachelor’s degree, he was fortunate enough to join Distinguished Professor Geoffrey Jones’ research lab for an Honour’s project on macroalgae and fishes in Kimbe Bay, Papua New Guinea in 2019. He is now furthering his research experience in a similar field with a PhD project under the supervision of Andrew Hoey (JCU), Katharina Fabricius (AIMS) and Morgan Pratchett (JCU).

Bottom up effects of coral reef macroalgae on fishes over space and time

2021 to 2025

Project Description

This project aims to improve our limited knowledge of the effects of macroalgae on coral reef fishes. In particular, the seasonal dynamics of these relationships, the prevalence of small bodied and cryptobenthic fishes associated with macroalgae, the characteristics of macroalgae which influence their habitability and the nature of the benefits macroalgae can provide to fishes, will be assessed and tested.

Project Importance

Tropical macroalgal beds are increasingly being recognised for their value as habitat and nutrition providers for a diverse array of organisms, despite their poor reputation for being associated with reef degradation, coastal run off and pollution. They can be important settling habitats for some reef fishes, including species crucial for providing ecological functions such as predation and herbivory, as well as commercially important fisheries species. Alongside other subtidal habitats, such as coral reefs and seagrass beds, macroalgal beds also enhance seascape diversity. However, the influence of macroalgal habitats on tropical fishes are in general poorly understood, and several knowledge gaps remain. For example, unlike coral reefs, the physical structure of macroalgal beds is highly seasonal in nature, shifting between dominance by tall, structurally complex algae and lower lying understorey algae between summer and Winter. It is therefore likely that this temporal change in algal resource availability is associated with concurrent shifts in faunal communities, yet so far, few studies have examined this. This is particularly concerning given the relationship between temporal shifts in macroalgal assemblages and temperature, and the changes in ocean thermal regimes under climate change. Cryptobenthic fishes, which are highly diverse and can contribute considerably to trophic nutrient and energy flows on coral reefs, have also received minimal attention in tropical macroalgal habitats. Improving our understanding of the relationships between fishes and macroalgae is therefore crucial if we are to understand and conserve the biodiversity and ecological functions of tropical seascapes.

Project Methods

Temporal dynamics of macroalgae and associated fishes on the inshore Great Barrier Reef (GBR):
From September 2021 – September 2022, reefs within the Palm Island Group (monthly) and Keppel Islands (3-monthly) will be surveyed. At each location, 6 sites will be selected randomly on mid reef flats on leeward, fringing reefs. At each site, 3 x 50m transects, each separated by a minimum of 10m will be surveyed in a 3-stage census: 1. Large bodied fishes (greater than or equal to 10cm) within a 5m band will be identified to species and length-estimated to cm. 2. Small bodied and juvenile fishes within a 1m band, will be identified to species and length-estimated to cm. 3. 8 square quadrats, 75cm x 75cm, randomly placed on the transect line, will be used for benthic surveys. Within these, benthic substrata beneath 25 points marked by the intercepts of string attached to each side of the quadrat will be recorded, identifying macroalgae to genus level. Also within the quadrat, 10 macroalgal thalli will be measured for vertical height, and the total number of Sargassum thalli will be counted.
Cryptobenthic fishes within macroalgal beds on coral reefs
In June and December 2022, 20 clove oil stations will be sampled at Orpheus Island reef flat - 10 stations in Sargassum dominated areas, and 10 stations in rubble dominated areas. Sampling will involve enclosing a 0.5m2 with fine (2mm) mesh net weighted with a fine chain, enclosing the net with a plastic tarp, and anesthetising all fish within the net using a 4:1 ethanol-clove oil mixture sprayed beneath the tarp. After 1 minute, all fishes beneath the net will be collected, placed in an ice slurry, identified, measured and transferred to the laboratory for fixation. Tissue samples of fishes that cannot be identified using visual methods will be preserved in ethanol, for later DNA analysis. Each sampled area will also be sampled using the quadrat method described above for determining benthic cover, algal height and Sargassum density.
Differences in fish and benthic assemblage composition between seasons and between habitats will be visualised using Non-metric Multidimensional Scaling analysis and tested using PERMANOVA. Associations between fishes and benthic characteristics will be tested using generalised linear models.

Project Results

I expect to find differences in the fish communities associated with macroalgal dominated areas depending on seasonal differences in macroalgal abundance and habitat structure. Upon investigating cryptobenthic fishes in macroalgae, I expect that these assemblages may approach the diversity and abundance now known to associate with coral reef habitats and will likely be higher than unvegetated rubble habitats. I also expect fish community compositions to differ between habitats, and abundances to be higher during austral summer. Finally, I expect positive relationships to be found between the abundance of species found in macroalgal beds and the height of the algal canopy.


Climate change,
Coastal development,
Coral reefs,
Field based,
Fishing impact,
Molecular techniques,
Natural disturbance,
Ocean warming,