Transshipment at sea vs port landing: how enforcement tracks each
A catch that comes across the deck gets to shore one of two ways. Either the vessel steams into a declared port and offloads at the quay, or it hands the catch to a carrier vessel out on the water and the carrier takes it from there. Both routes end up as fish on a plate somewhere, but the paper trail behind them, and the watch desk work needed to verify it, are almost nothing alike.
Port landing: the paper trail is built in
A landing at a port state has checkpoints baked into the process. The vessel files a prior notification, port authorities log the arrival, customs and the competent authority reconcile the landed weight against the logbook, and in most RFMOs there's a port inspection scheme that can pull the vessel aside before the catch even leaves the dock. None of that is foolproof. Logbooks get fudged, weights get estimated generously, and a port with weak capacity waves most vessels through on a quick glance.
But the structure exists. An analyst reconciling a landing has documents to pull: the port call record, the inspection report if one was done, the catch certificate, the logbook. The job is checking whether those documents agree with each other, not proving the landing happened at all.
Transshipment at sea: no dock, no customs officer, no default record
A ship-to-ship transfer offshore has none of that scaffolding unless the operator chooses to report it. There's no port authority logging the event because there's no port. If both vessels are flagged to a state that requires at-sea transshipment authorization and both are carrying observers or VMS that reports the rendezvous, you get a record. If either piece is missing, the transfer leaves no official trace at all. The fish still moves from the catching vessel to the carrier, the carrier still steams off toward a port somewhere else, often under a different flag than where the catch originated, and the only thing left behind is whatever a watch desk happened to notice at the time.
This is the gap that lets a dark fleet operate. A vessel can fish without reporting its catch, meet a carrier mid-ocean out of camera and patrol range, and the carrier arrives at a legitimate-looking port with a load that has no clean chain of custody back to the water it came from.
What a watch desk actually looks for
Since the paperwork can't be trusted to generate itself, analysts fall back on behavioral indicators. Two vessels converging to a dead stop, holding position relative to each other for hours instead of minutes, well outside any known port or anchorage. Speed profiles that drop to near zero and sit there, rather than the usual pattern of a vessel slowing briefly to check gear. A carrier vessel's track showing repeat rendezvous points with different fishing vessels over a season, a pattern that looks nothing like normal transit. AIS gaps that bracket the suspected meeting window, since an operator trying to hide the transfer often kills the transponder for the hours it takes, then switches it back on once the carrier is underway again.
None of those signals confirm a transshipment on their own. A vessel sitting still could be riding out weather or repairing gear. An AIS gap could be a hardware fault. What makes the pattern worth flagging is the combination: a gap, in open water, near another vessel's track, at a time that doesn't match any routine maintenance stop. That's the kind of correlation a desk has to build by hand when the only inputs are AIS history and whatever satellite pass happened to catch the area that day.
Illegal Fishing Detection was built around that specific gap. It runs daily optical passes against the AIS record and surfaces vessels that show up on the water with no broadcast to match, the same signature an at-sea transfer produces when an operator goes dark for the handoff. The output is a daily list an analyst can work from, not a replacement for the judgment call on whether to flag it as a transshipment.
If your desk is still building that AIS-gap list by hand every morning, it's worth seeing what a daily unmatched-detection feed looks like for your draw area.