Purse seiner or longliner? Reading hull and deck gear in satellite imagery
When a detection pops up with no AIS tag, the first question on the watch desk is usually "what is it," not just "where is it." Gear type is the fastest way to narrow that down, and at 0.5-2 m resolution you've got enough to work with if you know what you're looking at.
Hull shape and proportions
Purse seiners tend to read short and beamy on an overhead frame. A tuna seiner in the 50-80 m class often has a hull that looks almost square from above compared to a longliner of similar length, because the beam-to-length ratio runs higher. That's the net. A purse seine and its skiff take up deck space, and the hull is built to carry the weight of a full set without rolling the boat over.
Longliners run the other way: long and narrow, with a lower, flatter profile forward. A lot of the distance-water longline fleet sits in the 25-45 m range, and from altitude the hull looks almost like a pencil next to a seiner's stubbier outline. If you're eyeballing aspect ratio and the vessel looks noticeably longer than it is wide, lean longliner before you lean seiner.
Size class alone won't close the call. A small seiner working inshore tuna and a mid-size longliner can land in the same length bracket. Hull shape is the tiebreaker, and deck gear is the confirmation.
Deck gear and stern layout
This is where the two gear types stop looking anything alike.
A purse seiner's stern is built around the power block and net reel, usually a visible drum or block structure hanging off the stern quarter, with open deck space running most of the vessel's length for net stacking. Larger tuna seiners often carry a crow's nest forward (a spotting platform) and sometimes a helicopter pad, both of which show up as distinct shapes against the deck once you're used to spotting them. The skiff, when it's not in the water, sits on a ramp or cradle at the stern and reads as a small rectangular block.
A longliner's deck tells a completely different story. Line haulers and reels sit along one side rather than the stern, the deck is broken up into bait-and-haul stations rather than open net space, and many carry visible ice holds or refrigeration housing that run most of the vessel's midsection. There's no net reel, no power block, no skiff ramp. If the stern is clean and the gear is strung along the rail instead of stacked aft, you're looking at a longliner.
Drifting gillnetters and trawlers complicate the picture further; gillnetters often show a similarly clean deck to longliners but without the haul stations, and trawlers carry visible net drums at the stern that can be mistaken for a purse seiner's at a glance. At 0.5-2 m GSD you're working from shapes and shadows, which keeps a single frame ambiguous on its own. Cross-check against a second pass if you can get one, and treat the gear-type call as a working hypothesis for the duty officer, not a courtroom finding.
Why gear type matters for the dark-fleet question
None of this replaces a track history or a boarding. What it does is tell the watch desk what kind of vessel they're dealing with before anyone commits a patrol asset, which matters because seiners and longliners behave differently when they go dark. A seiner that drops off AIS mid-set is a different enforcement picture than a longliner that's gone quiet during a long haul, and knowing which one you're looking at shapes the next move.
That's the gap this kind of correlation work is built to close: a daily list of unmatched optical detections, each one carrying a position, a size class, and a timestamp, pulled against the AIS record so the gear-type read isn't the only thing an analyst has to go on. If gear-type calls on dark detections are part of your daily workload, see how Illegal Fishing Detection builds the unmatched-vessel list your desk works from.
Run a few known vessels through this before you trust it on an unknown one. Pull imagery of a seiner and a longliner you can already identify from AIS, compare the stern and the hull proportions side by side, and build your own eye for the two shapes before you lean on them for a dark-fleet call.