When it comes to complicated scientific questions, there's lots going on. There are lots of different stressors, so there are a few ways to answer this.
One is that this is one stressor and there is evidence of some risk, so if you remove it, you're making a precautionary decision. The other example of the precautionary principle comes down to just looking at individual pieces of evidence. As I often see in my work, DFO won't assess some of the diseases we study as disease agents because there's not a causal relationship with diseases in a population. Sometimes the bar is set so high by DFO that with the level of evidence they require, it's almost impossible to know if something is having a population effect with 100% certainty.
That's why the precautionary principle is so important. You won't always have that level of certainty, but we have to make important decisions about populations that in some cases are in severe states of decline.