And China is signatory to the Convention on International Trade in Endangered Species (CITES), which lists both species under Appendix I, prohibiting commercial trade (which at any rate does not threaten the vaquita). Enforcement of existing protection is therefore part of the problem.
But I would argue demand for fish overall contributes to the likely extinction of the vaquita, because, of course, other kinds of fishing activity and netting also put the vaquita at risk, even if the gear used is not so likely to catch vaquitas as the nets used for totoabas. Their mesh is the same size as the vaquita's head.
Totoaba themselves are sometimes imported into the US as "white bass," as well as smuggled illegally into the US, and they are at any rate caught unintentionally (or not) in nets set for other, legally caught species. Can we really blame fishers from impoverished regions for being part of supplying the rest of the world's insatiable appetite for fish?
And then there is our demand for both water and electricity. Historically, the flow of water from the Colorado River maintained perfect conditions for the totoaba's spawning grounds. But, since the completion of the Hoover and Glen Canyon dams, the diversion of fresh water to maintain crops, lawns, golf courses, and other demands - including for cities like Las Vegas and Phoenix, placed in hot deserts where no city should be - at best only the tiniest dribble of polluted fresh water now enters the Sea of Cortez, which means it is too salty for baby totoabas. It is ironic, too, that as climate change seems to lead to extended extreme draught in the US southwest, our demand for water, in part to fight the threat of massive brush and forest fires, comes when there is the least amount available.