The main diet of fish comprises small crustaceans, or shelled creatures. Depending on their size, fish also eat other fish, aquatic animals, and insects. Algae, sea grass, and plankton also form part of the diet of many wild fish.
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Wild fish have diets that can range from eating smaller species of fish, insects, plants and other forms of marine life. There will always be a supply of food for wild fish to consume. You can find more information here: http://www.dfo-mpo.gc.ca/aquaculture/faq-eng.htm
Wild fish eat smaller fish that are in the oceans and seas. They also eat seaweed, kelp, plankton, and microscopic organisms. Some wild fish also eat shrimp and crab.
Forage fish, also called prey fish or bait fish, are small pelagic fish which are preyed on by larger predators for food. Predators include other larger fish, seabirds and marine mammals. Typical ocean forage fish feed near the base of the food chain on plankton...