Why are freshwater fish more likely to be a source of tapeworm infection than marine fish?

This is not true. Marine fish are more likely to be a source of tapeworm infection humans than freshwater fish. Fish tapeworm (Diphyllobothrium latum) is a parasite found in raw or undercooked fish, especially those from freshwater lakes and rivers. However, there are also several species of tapeworm that can be transmitted to humans by consuming raw or undercooked marine fish, such as tuna, salmon, and mackerel.