Partial migration of Black Bream

2/07/2025

Thirun Gawarammana - Honours student, University of Adelaide

Project: Unmasking the impacts and consequences of environmental change on partially migrating fish populations: A combined approach using otoliths and eye lenses.

Supervisors: Bronwyn Gillanders and Patrick Reis-Santos

Black Bream

Tell us about you?

I was born and raised in Sri Lanka and grew up with a close connection to the ocean. I came to Australia to pursue my passion for marine wildlife and ecology and completed a Bachelor of Science (Marine Biology) at the University of Adelaide in 2023.

What is the main focus of your project?

My project investigated the causes and consequences of partial migration in fishes. Using black bream (Acanthopagrus butcheri) populations in the CLLMM region, this project will explore how the proportion of migrants change in a population and if changes in the environment influence partial migration, to evaluate the ecological consequences of having different movement life-histories.

What first led you to this project area?

I first learned about otoliths (fish ear bones) during one of my undergraduate lectures and I was struck by the sheer diversity of questions that can be answered using such a tiny structure. Animal behaviour has also been a long-standing interest of mine, so the possibility of using otoliths to understand why some fish behave differently to others, was instantly appealing.

What are the anticipated outcomes of the project?

The project explored the proportion of migrant variations over time, and if environmental parameters (things like- annual flow rate, temperature) influence whether fish migrate or remain estuary residents. It is also anticipated that novel approaches using fisheye lens chemistry will reveal dietary differences between migratory and non-migratory fish.

How will the project inform understanding of climate change in the CLLMM region?

1. By identifying how migratory patterns change and their key environmental drivers, the project can indicate how partial migration in this population may be affected by climate change in the CLLMM region.

2. If the project reveals that migrant fish benefit nutritionally by migrating compared to resident fish, we can anticipate the ecological consequences if climate change discourages migration in the CLLMM region.

What would you be doing if you were not a researcher?

If I was not a researcher, I would probably be pursuing some of my more artistic hobbies including writing, acting, wildlife photography and documentary filmmaking.

Are there any fun facts about your project or focus species?

1. Fish ear bones (called otoliths) are like biomineralized passports that can tell us when, where and how many times a fish migrated.

2. Like otoliths, fish eye lenses also have amazing record-keeping abilities. Fish eye lenses are gelatinous food diaries, detailing diet shifts as a fish matures from a larva to an adult, or as it moves through different food webs.

3. Black bream (Acanthopagrus butcheri) are rudimentary hermaphrodites, which means that as juveniles they possess both ovaries and testes but will eventually mature into a male or a female in time for their first spawning season.

If you are interested in Thirun’s completed thesis, we can make that available for reading. Please get in touch at nick.whiterod@goyderinstitute.org