2020托福听力练习：人工养殖鳟鱼以满足食用需求（下载）

　　Half of all fish people eat worldwide now come from fish farms. So farms need to do more to keep up with demand.

　　"If we look to the future, at today's per capita fish intake around the world we would need to double aquaculture production."

　　Ron Hardy is the University of Idaho's Director of Aquaculture Research. He presented his research at the recent International Symposium on Fish Nutrition and Feeding in Sun Valley, Idaho, which he also chaired.

　　In the wild, rainbow trout eat insects and other, smaller fish. But Hardy says there aren't enough little fish to feed larger fish in the wild and still meet market demand as the human population increases. So he's used selective breeding to create strains of farmed fish that get by on food that's less expensive than little fish—feed made from soybeans, corn and wheat.

　　Some of the farmed fish really thrive: 16 years ago, Hardy had to wait a year for a one-pound trout. These days, his efforts yield trout up to four times as large in the same amount of time.

　　"So, it would be kind of like if you were going to breed dogs. So we've got everything from Rottweilers to little Scotties or whatever."

　　But the farmed fish are not completely vegetarian.

　　"Soybeans don't have skeletons, they don't have bones and the bones in fish meal are a major source of minerals for the fish, for example."

　　And much like humans, fish diets require omega-3 fatty acids, not found in terrestrial plants. So Hardy has to add a little fish-oil back into the plant-based feed. For those of us without waders, this kind of aquaculture is our best shot at a fish fry.