A hundred years ago, fish lived longer and grew bigger – they continued to breed, engineered ecosystems through predation and were more resilient to ocean changes such as marine heatwaves.
Today these larger, long-lived fish are gone, and it's affecting fish populations, marine ecosystems, and ultimately our own food security.
Pew Marine Fellow Dr Asta Audzijonyte is a researcher at the University of Tasmania's Institute of Marine and Antarctic Studies (IMAS). She's investigating the impacts of human activity and a changing climate on fish size, what that means for fisheries productivity and marine ecosystems, and how we can bring big fish back.
"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," Dr Audzijonyte said.
"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," Dr Audzijonyte said.
"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."
Recreational fishing can have a big impact on reducing fish sizes in both coastal and inland areas, where it is not uncommon for it to account for half or more of the total catch. One example is the iconic Tasmanian sand flathead fishery, where nearly all catch is from the recreational fishery.
A recent study in Europe showed that recreational fishing increased during the COVID-19 pandemic and remained higher even after lockdowns ended. It is likely similar changes happened in Australia, but detailed estimates are lacking.
"We have the technology today to find and catch the biggest fish, and to fish to depths of 400 metres where once big fish could find refuge," Dr Audzijonyte said.
"Research also shows that anglers are more likely to target active fish – and those are the ones that can often cope better with climate change."
Dr Audzijonyte said bringing big fish back can happen within 10 to 20 years, because fish grow relatively quickly, but everyone must be involved.
"We can restore big fish and we can have healthy marine ecosystems, but this has to be shared work between anglers, community, scientists and managers, to decide how we want our oceans to look in the future and take action."
Dr Audzijonyte emphasises the importance of creating more Marine Protected Areas to help boost fish populations and biodiversity in coastal areas.
"These areas are a place where big fish can find refuge, small fish have the chance to grow and mature, genetic diversity is protected, and ecosystems can really thrive," she said.
"The data being gathered on fish size diversity will inform more sustainable management of coastal fisheries and ecosystems.
"We're learning that the full structure of a population is needed for climate resilience, and that protecting fish size diversity is vital for healthy oceans. So while reversing climate change is hard, restoring and protecting fish size diversity is one thing that we certainly can do – and we need to do it now."
Big fish…on the cards
To share the science about marine ecosystems and sustainable recreational fishing in a fun and engaging way, Dr Audzijonyte's team has developed a Big Fish card game and will be crowdfunding through Kickstarter from 15 September to get the game out into the community – in Tasmania and around the world.
"We want the game to be in the hands of as many people as possible, to take it to schools and run educational events," she said.
IMAS Technical Officer Matthew Teston is the game's main developer and said it is designed for players to have fun, while discovering for themselves how marine ecosystems and fish populations work.
"The players must find the best way to collect lots of points through fishing, but at the same time maintain a healthy fishing spot. Meanwhile, random natural events and other players can either help or undermine the best of plans," Matt said.
Cover image by Rick Stuart-Smith
