Well-thought, and thanks for the recent follow! I agree with your sentiment in this article, but disagree with your solution. I like the market-based approach that you touched on. Educating people on the health benefits of plant-based proteins is a great way to reduce consumption in other areas. Where we disagree is on the use of taxation, which punishes an entire industry for the actions of a few. And, by extension, it raises the price of goods sold, which then punishes consumers, notably those with less economic power who must then consume less healthy foods that fit into their tight budgets.
I would also suggest looking at externalities and unintended consequences. For example, if you were successful in reducing the demand for fish, one possible outcome would be a rise in the demand for beef. This can result in more CO2 produced by land farming and livestock. It may also decrease the overall health of people who consume meat, where previously they may have benefited from Omega 3 & 5 fatty acids found in fish. These people then have more health problems, live shorter lives, and produce less.
You mentioned that much of this waste comes from 10 rivers. These rivers are likely in nations that would not participate in such a system of taxation. The result is that people in countries like the US are now paying for the behaviors of people in countries like China and India.
Here's another direction to consider, which requires no force or taxation.
1) Buy from, and invest in companies like 4ocean. There's a market for products produced from reclaimed materials. This company was smart enough to hire fishermen to spend part of their time collecting ocean waste, which is then repurposed as jewelry that they sell on their website. There's space for more companies like this.
2) Support non-profits and other organizations that improve sanitation infrastructure in countries like India. I don't know enough about the Gates Foundation to have an opinion on their overall organization. But I do like that they have invested heavily in developing plumbing alternatives in such countries. There's probably a better example for plastic waste, as their solution is to reduce fecal sludge. But the concept is the same—find ways to improve conditions at the source (i.e., the locations of those 10 rivers), and the output will be less pollution with the added benefit of an improved quality of life for local populations.
Like I said, I admire your desire to improve conditions in the ocean. I just feel like there's a more voluntary way to reduce the problem.