The most fair and effective counterclaim to the argument that people should avoid fish due to a higher risk of skin cancer would be:
"Large fish such as grouper and bluefin tuna are known to have elevated levels of mercury, a toxic metal that can cause a wide range of health problems."
This counterclaim effectively addresses a specific health risk associated with eating certain types of fish, which complements the original argument about health risks rather than focusing solely on ethical concerns or treatment of workers. It provides a clear, relevant reason to consider the complexities of fish consumption beyond the risk of skin cancer.