Compare Chinese and Indian myopia frequencies. One country loves fish the other is mostly vegetarian.
China's myopia rate is 31%: 400 million of its 1.3 billion people are myopic. The prevalence of myopia in high school in China is 77%, and in college is more than 80%.
A study indicated that the prevalence of myopia among urban children in India of aged 5 to 15 increased from 4.44% in 1999 to 21.15% in 2019.
Check also the fish consumption per capita. In China it’s over 30kg per year and in India around 6kg per year. If anything, less fish seems better for the eyes. And with an N of over 2B.
In populations that depend on a very heavy vegetarian diet, which is comprise primarily of shortchange, polyunsaturated fatty acids, they might have other issues not related to myopia when they go off of their traditional diet.
Myopia in my opinion is a result of the industrialization of food.