What strange things have commercial fishermen found in their nets?
Many things, from unexploded ordinance to what certainly looked like a Plesiosaur, which was trawled by a Japanese fishing ship. They tried to keep it but it became so stinky that they finally took photos and disposed of it. I think it has been postulated by some skeptics as a rotting Basking Shark. However I am not inclined to agree though I'm not convinced of the Plesiosaur theory either. I'm firmly astraddle the fence.
Perhaps the most famous and fascinating discovery by a commercial fishing boat was the discovery in 1938, off the coast of Madagascar, of a 127 pound Coelacanth. A fish found in the fossil record but long believed to be extinct. Today of course we know differently and even study living Coelacanth in their natural habitat.
Treasure: A US trawler fishing for butterfish in the Gulf of Mrxico hit a snag and when the net was freed - and pulled aboard it appeared that there was a pile of black rocks - in the net - which on closer examination turned out to be newly minted Mexican silver coins that were part of a large (lost in a hurricane) shipment from Mexico to the - then Mexican territories - in what was Spanish - "New Mexico," and is now Texas, USA.