They argue that attorney general Bill Lockyer's case against them rests on flimsy evidence, with only a tenuous connection to tuna.
According to the US Tuna Foundation, Lockyer's science is based on studies of people in Denmark who ate pilot whale meat and blubber, which are both very high in naturally occurring mercury, in addition to chemicals like PCBs.
The level of mercury in both pilot whale meat and blubber are exponentially higher than that found in tuna.
"What we've learned from the first full week in this trial is that Lockyer's case would be much more appropriate for whale meat than canned tuna," said David Burney, executive director of the US Tuna Foundation.
"Warning labels would cause real harm as people would be discouraged from eating a food that science has time and again shown to have overwhelming benefits for people of all ages. The Attorney General has one study underpinning his entire case, and it involves whales, not tuna."The association believes that if Lockyer is successful, it would counter a flood of recent studies that show seafood consumption should be encouraged. It has consistently argued that warnings - such as a Proposition 65-like label - would cause more harm than good by needlessly scaring consumers from seafood.
"I'm confident the court will recognize that science and nutrition are on our side," said Forrest Hainline, attorney for Bumblebee, Starkist and Chicken of the Sea, all members of the US Tuna Foundation.
"For starters, canned tuna has already been proven to be safe and in no way violates Proposition 65. In addition, what the Attorney General is proposing directly conflicts with federal law.
"The FDA created a targeted and nuanced warning intended to convey the benefits of seafood, which they believed to be a necessary aspect of the advisory."
All types of fish contain some amounts of mercury. For most people this is not a cause for concern, but a build-up in the blood stream can lead to reproductive problems in women and affect the development of the nervous system in children.
The FDA therefore advises that people eat up to 12 ounces (two average meals) a week of fish and shellfish that are lower in mercury, such as canned light tuna, salmon, pollock, and catfish.
As albacore tuna and fresh tuna steaks contain more mercury than light tuna, the agency says that consumption of this variety should be limited to 6 ounces per week.
"The federal guidelines were developed to ensure that everyone, including pregnant women, have the best possible advice about seafood and nutrition," said Burney.