People eat fish and people eat tomatoes. Often they eat them together. After all, tomato is an ingredient in many of the best fish recipes, like bouillabaisse. So at least some people are not bothered by a fish gene in a tomato.
For these people the critics talk about a more scary example. They claim that genetic engineers have transferred a gene for a scorpion toxin into corn. They ask, ``Do you want a scorpion gene in your cornflakes?''
Although genetic engineers have used the gene for scorpion toxin, they didn't transfer it into a plant. Instead it has been used to genetically engineer a virus that parasitizes a plant pest, the cabbage looper. If it works, farmers could protect existing plant varieties from cabbage loopers without use of chemicals. The technology is presently undergoing field trials in England.
As nearly as I have been able to determine, the story about corn with a scorpion gene is a complete fabrication, but it has been widely spread. It seems to have originated with the Natural Law Party, one of the many organizations that campaign against genetic engineering.