They die just as fast is freshwater. It is possible, and probably has been done, to look for the tags on seal haulouts, places where birds like cormorants defecate.

Quick death in the salt could simply be response to salt; they had issues. I think the assumption, though, is that losses are primarily predation.

On the stream I mentioned the loss could be explained by one or two Great Blue Heron or a River Otter. So, trying to find the one predator might be hard.

In Oregon, they did document that a huge chunk of steelhead smolt loss in a bay occurred around a seal/cormorant congregation site. They were alive upstream of it, not present below.