The research is pretty clear on the degree of pinniped predation. It’s not speculation.
The presence of nets in the river does not explain the large population of harbour seals in the estuary area. If you want to see for yourself just hop onto a harbour air sea plane flight on a calm day and look down. Lots of seals to be seen.
Netting that is out of control doesn’t help matters as well. We just need to look beyond the obvious shiny penny.