Jerry Percy, of the Low Impact Fishers of Europe, said Wales faced some "very significant negotiations" after the vote to leave the EU.
Welsh fishing boats were banned from landing sea bass for two months this year in a bid to halt decline fish populations.
Environment Secretary Lesley Griffiths said she would "fight for Wales".
Scientists have said bass stocks are in steep decline and this year the European Union put limits on how many bass could be caught by both commercial fishermen and recreational anglers.
There are currently 419 fishing vessels registered in Wales. Of these, 137 vessels caught bass in 2015.
The limits on them depend on whether they catch with nets or rod and line, but all vessels were banned from catching bass in February and March.
Mr Percy said: "There's going to be some very significant negotiations about the management of fish stocks, and Wales really has to step up to the mark.
"The government has to be there in support of recreational and commercial fisherman, and those coastal communities that are reliant of both of them."
Lesley Griffiths told BBC Radio Wales' Eye on Wales programme: "The first minister's [Carwyn Jones] made it very clear we need to be right there at the heart of the negotiations, and we certainly will be. We have to look for the opportunities and not just the challenges.
"We don't know the full scope of the options at present and it may be some time before we do know. We'll have to wait and see what the new prime minister brings forward.
"The most important thing is that we have a seat at those negotiations and we will be fighting for the best outcome for the people of Wales."
Wales has to "step up to the mark" in Brexit talks about fish stocks, a fishermen's leader has said.