Looking closer, I see San Diego as hailing port, and I think they might be fishing something other than tuna, as the mesh looks possibly too small to be for tuna. That would leave mackerel, sardine/anchovy or possibly squid(?) but there seems to be some needed deck equipment either missing or not visible, for that latter one.
Even knowing what I'm looking at for the most part, I can't tell exactly what fishery it is. The purse winch appears to be forward of the structure (on the starboard side adjacent to where they are stacking the net) set a bit inboard from the starboard rail. I think I see a line or wire crossing the deck at an "overhead" height. The davits are visible on the port side, aft of the main cabin, with a guy seemingly tending them. I think I've got it figured...
I've seen large-boat purse rings. Instead of being simple a round brass ring, they are more like a giant carabiner, but with rollers built in. Something like this, but possibly sleeker, for the pic is from a patent filing, 1989, updated 1996;
Other purse rings used by those who use wire rope for purse line, have side openings, but lack rollers, so get worn out, while also chaffing the cable (wire rope)
The guy standing at the davit at the portside rail, is detaching the purse "rings" from the purse cable, as the net is dryed up. All of the bottom line of the net, or "leadline" is bunched up out-of-sight to us, along the port side of the boat, hanging in loops from the purse cable(s) and davits, so as the rest of the net is dryed up (taken out of the water, corks, webbing etc.) then the lead line too needs go along with the rest (up through the powerblock hanging from the large main boom). So as the slack is taken up, releasing some of the weight, the man at the rail unhooks each purse ring in succession. He has to, and do so before it becomes taunt between the davits and the powerblock. It looks like he's able to simply disconnect & drop one ring at a time, periodically, leaving a low arc or small amount of slack of leadline hanging off the side, possibly above the water, but if a little leadline at a time was dropped into the water, it wouldn't matter much... until they come to the bunt-end of the net, and whatever they've caught is crowded and will seek escape through any perceived opening.
Not seeing a "prick" to slide the rings upon as the net is stacked, or seeing a trough or channel to place the rings in along the bulwarks in order, with the openings kept facing for-and-aft, I'll venture a guess the channel to set the rings in is set flush into the deck. I see three inlet/inset channels running fore-and-aft, but in the photo the stacking crew seems to be crowding the one they would need, with a bit much webbing.
All of that ring placing business is so the purse line (cable) can be run back through the rings before the net is set again.