Each day at 4pm, the trawlers come back, alive with giant bass, mackerel and squirming eels, at the end of a food chain that links family dinner tables to poisons in the sea.
Besides mercury which can damage the brains of fetuses and young children and can affect healthy adults, there are PCBs, dioxins and flame retardants with unknown long-term effects.
It is the same from ancient Mediterranean towns like Sete to big city docks in Asia, America's Gulf ports, or harbors in seemingly pristine Nordic waters. Industrial waste permeates every ocean.
Although rich in omega-3 fatty acids vital to the heart and brain, many fish contain toxins that build up over time in the human body.
And as this paradox worsens, scientists express alarm at what they call inadequate government warnings, lax attitudes toward fishing industries, and insufficient data to assess the risks.
The problem is that authorities are caught between wanting to inform the public while not damaging consumer confidence in a healthy food source, says Sandrine Blanchemanche, a sociologist with France's prestigious National Institute for Agronomic Studies.
"People overreact to these things, so you have to be careful," she said. "You don't want large numbers giving up the benefits of fish while you damage the whole fishing sector for no reason."
But marine biologists, toxicologists and physicians interviewed on three continents share an all but unanimous view: Better public knowledge is essential.
Jane Hightower, a San Francisco internist whose 2002 study of mercury in her patients brought the issue to wide public attention, said she is still uncovering what she calls shocking new evidence.
"We are just starting to realize as physicians the effects of this chemical soup we live in," she said. "We really have to ask, why are we poisoning ourselves?"
She called some areas especially troubling because of contamination trapped by ocean currents.
"The Mediterranean is a toilet that no one has bothered to flush," Hightower said.
The crisis transcends borders. Three-quarters of fish eaten in America and Europe are imported, often from countries with no controls. Authorities on both sides of the Atlantic make only minimal spot checks.
"Pollution is a worldwide problem, and our fish comes from around the world," said Kate Mahaffey, toxins expert at the US Environmental Protection Agency (EPA). "No one is immune."
With contaminants in fish, she warned, "there is a very narrow range between levels with no effects, subtle effects and severe effects."
At its extreme, Mahaffey said, mercury poisoning causes an illness similar to cerebral palsy.
"As we find out more and more about mercury, we see health effects that have not been taken into account," she said. Studies continue into the impact of PCBs and dioxins, she added. "We just don't know."
Specialists accuse commercial fishing interests of minimizing the threat and using their political clout to oppose broader studies or warning labels.
Industry associations routinely reject such charges. But most of the world's catch comes from independent small fishermen and trawlers which easily evade international control.
"Information is scarce, and there is little interest in getting more," said Sergi Tudela, a Worldwide Fund for Wildlife (WWF) fisheries expert based in Barcelona, Spain.