Overfishing has caused Tasmania's favourite fish to be classified as "depleted", with researchers labelling the findings of their study "depressing".
Sand flathead is the most popular fish for recreational fishers in Tasmania, accounting for about 70 per cent of recreationally-caught species.
The Institute for Marine and Antarctic Studies (IMAS) annual Tasmanian Scalefish Fishery Assessment looked at 22 of the state's most important fish species and determined which stocks have been decreasing.
Lead researcher Dr Nils Krueck said the assessment found the biomass and reproductive potential of sand flathead populations in most of the state's regions — especially in south and south-east — had declined below critical levels.
"The most interesting and depressing finding this year is that sand flathead has been classified as depleted for the first time," said Dr Krueck.
The report blamed recreational over-fishing for impacting the stock of sand flatheads in Tasmanian waters, especially in areas including the D'Entrecasteaux Channel, Frederick Henry-Norfolk Bay and Great Oyster Bay.
Changes to size and bag limits were introduced in 2015, and were initially believed to be supporting stock recovery.
But evaluation of commercial and recreational catch data found that while smaller sand flatheads were abundant, the low number of legally-sized and sexually mature fish was cause for concern.
"[It's] a risk because we're not certain anymore that there are enough spawning biomass left to actually make sure that this population can replenish itself and recover for long-term sustainability," said Dr Krueck.
Due to heavy fishing pressure, most of the fish that grow to adult size are generally caught within a short time period.
"Fish that reach sexual maturity before reaching the minimum size limit produce substantially fewer eggs than larger fish, and their offspring tend to have a reduced chance of survival," wrote Dr Kate Fraser, co-author of the report.
"It's also important to note that females grow larger than males, so receive less protection from size limits. Their protection relies heavily on catch controls."
The data found that 12 species were assessed and classified as sustainable.
But now the focus is on how to improve scalefish stock assessment which will help inform what can be done to better manage the species.
"There certainly is a need to consider future research and monitoring needs to find the best ways to recover these depleted stocks."
Jane Gallichan, chief executive of the Tasmanian Association for Recreational Fishing (TARFish), said she was hearing anecdotal evidence from fishers that flathead are getting more difficult to find.
"Sometimes there's a lot of fish but they're small fish, they're not making the size limit," she said.
"So we're definitely seeing that some fishers are saying they're harder to catch."
Ms Gallichan said recreational fishers relied on the government to have management settings in place that would protect the fishery.
"I believe all fishers want to fish sustainably and want to do the right thing, but to do that they need information."
She said TARFish had called on the state government to commit $5 million over five years to develop a recovery plan, deliver fisher education and research the impacts of climate change and habitat loss on sand flathead.
In a statement, a spokesperson for the Department of Natural Resources and Environment Tasmania (NRE Tas) said there was a short-term improvement in flathead stock levels following size limit changes and a reduction in the bag limit in 2015.
"To ensure the future of the species, a range of actions will be implemented including education programs, research and consideration of potential management options to support recovery of the fishery," the statement said.
It said an education program would be rolled out over summer to inform recreational fishers, tackle shops and community groups about stock challenges.
In a statement, the Minister for Primary Industries and Water Jo Palmer said all budget submissions would be "carefully considered" and that the Tasmanian government had committed funding to NRE Tas's upcoming education and information program "Flathead for the Future".
