There is a word play.

> `Но на происки и бредни` — `For intrigues and ravings`.

> `Сети есть у нас, и бредни` — `We have nets and drag-nets`.

In the first line `бредни` (from `бред` — *delirium*) means *silly twaddle, ravings*, while in the second line it's about fisherman tackles: [сеть][1] (*net*) and [бредень][2] (*drag-net*), which figurative can be used by authors of the letter to fight with that fraud.


  [1]: https://ru.wikipedia.org/wiki/%D0%A0%D1%8B%D0%B1%D0%BE%D0%BB%D0%BE%D0%B2%D0%BD%D0%B0%D1%8F_%D1%81%D0%B5%D1%82%D1%8C
  [2]: https://ru.wikipedia.org/wiki/%D0%91%D1%80%D0%B5%D0%B4%D0%B5%D0%BD%D1%8C