University of Tasmania study unveils decline in big fish populations, need for marine protection efforts

By Aishwarya Singhal 

A century ago, fish populations consisted of larger, longer-living species that contributed significantly to marine ecosystems through predation and resilience against environmental changes, such as marine heatwaves. Today, however, the number of these large fish has sharply decreased, affecting not only fish populations and marine ecosystems but also global food security.

Dr. Asta Audzijonyte, a Pew Marine Fellow at the University of Tasmania's Institute for Marine and Antarctic Studies (IMAS), is leading research into the impacts of human activities and climate change on fish sizes. Her work focuses on understanding how declining fish sizes influence fisheries productivity and ecosystem health and exploring potential solutions for bringing back larger fish populations.

“There’s a perception that the ocean is full of big fish because we still catch them with better and better gear, but historical photos, human memories, and data show they’re much smaller than they used to be. The size structure of marine fish populations has changed. This is mostly because we caught and are still catching all the biggest fish – so many fish populations consist only of younger, smaller fish. And that is putting entire populations at risk, changing the natural balance in the ocean and to some degree also taking fun away from fishing.”

She highlighted that the structural composition of marine fish populations has shifted dramatically, largely due to the selective harvesting of the largest fish. Consequently, many fish populations are now made up primarily of younger, smaller individuals, which destabilizes marine ecosystems and poses a risk to population sustainability. “The natural balance in the ocean has been altered, and to some degree, this even reduces the enjoyment of fishing,” she added.

Recreational fishing has proven to be a significant factor in reducing fish sizes in both coastal and inland areas, sometimes accounting for half or more of the total catch. In Tasmania, for instance, the popular sand flathead fishery relies almost entirely on recreational fishing.

A recent European study revealed that recreational fishing activity increased during the COVID-19 pandemic and has remained elevated even after restrictions were lifted, a trend likely mirrored in Australia though exact figures are not yet available.

“Today’s technology allows us to find and capture large fish even in deep waters, once considered safe havens,” Dr. Audzijonyte remarked. “Studies also show that anglers tend to target active fish—those most likely to adapt to climate change.”

According to Dr. Audzijonyte, restoring populations of large fish could be achieved within a 10 to 20-year timeframe, given the relatively rapid growth rate of fish, but it requires a collective effort.

“Recovering big fish populations and fostering healthy marine ecosystems will require cooperation among anglers, communities, scientists, and policymakers. Together, we can envision the future of our oceans and take necessary actions,” she noted.

A strong advocate for Marine Protected Areas, Dr. Audzijonyte emphasizes their role in promoting fish population growth, enhancing biodiversity, and preserving genetic diversity. “These protected areas offer a refuge for larger fish and provide smaller fish with the chance to grow and mature, allowing ecosystems to thrive,” she said.

Through ongoing data collection on fish size diversity, her team is contributing valuable insights to support sustainable management practices in coastal fisheries and ecosystems. Dr. Audzijonyte underscores the importance of protecting fish size diversity to ensure marine resilience to climate change. While reversing climate change is complex, preserving and enhancing fish size diversity is an attainable and essential step toward healthier oceans.

A New Educational Tool – Big Fish Card Game

To engage communities in marine science and sustainable recreational fishing, Dr. Audzijonyte’s team has developed an educational card game, “Big Fish.” Launching on Kickstarter on September 15, the game aims to reach a global audience and foster environmental stewardship.

“We hope this game will be played widely, not only as a fun activity but as an educational tool in schools and community events,” Dr. Audzijonyte said.

IMAS Technical Officer Matthew Teston, the lead developer of the game, explained, “Players compete to maximize their points while maintaining a healthy fishing environment. Random events and competition add a strategic layer to the game, illustrating the challenges marine ecosystems face.”

By promoting awareness and understanding of marine conservation through interactive learning, “Big Fish” aspires to inspire future generations to take an active role in protecting our oceans.