Rabbi Rafi Yochai, who heads the kashrut-fraud department of the Chief Rabbinate, told Arutz Sheva, Thursday, that the department recently received a cod (Bakala in Hebrew) filet with crab in after "a consumer came to eat in one of the kosher dining rooms and, when they served her the young cod filet, she saw something sticking out which turned out to be part of the body of a crab."
Rabbi Yochai said it doesn't happen often but one must be aware of the possibility. He explained, "The problem is large cartons that come to large kitchens in quantities of 15 kg [33 pounds] per package. Generally, private homes usually buy individual pieces, each in a bag, so if you have a kashrut symbol on the bag, it should not be a problem. " He added that overseas, the fish themselves are tested one by one, immediately after fishing, because bo pure and impure fish come up in the nets.