Fishing vessel pressure monitoring inside marine protected areas
Combining AIS vessel tracking with SAR-based dark-vessel detection gives MPA regulators spatially explicit evidence of fishing pressure that port inspections alone cannot provide, including vessels that deliberately go dark inside no-take zones.
Sensors
- Sentinel-1 SAR (ESA/Copernicus): C-band (5.405 GHz) synthetic aperture radar; IW mode delivers 10 m resolution over 250 km swaths. Revisit is 6 days at the equator for a single satellite, 3 days with both Sentinel-1A and 1B operational. Detects vessels by radar cross-section (RCS); vessels above roughly 20–30 m length are reliably detected in moderate sea states, though performance degrades in high wind and heavy rain.
- ICEYE SAR constellation: X-band (9.65 GHz) commercial SAR constellation. Spotlight mode achieves 25 cm resolution; Strip mode around 3 m. Revisit to a specific MPA can be tasked to sub-daily frequency. Higher frequency improves small-vessel discrimination relative to Sentinel-1 but is sensitive to sea-surface roughness at wind speeds above roughly 12 m/s.
- Spire Global AIS (space-based): Space-based AIS receiver constellation with global coverage. Spire reports message latency under 20 minutes for most ocean areas. AIS is mandatory for vessels over 300 GT internationally and fishing vessels over 15 m in many flag states, but compliance is uneven and deliberate deactivation inside MPAs is a documented behaviour.
- exactEarth AIS: Space-based AIS service providing historical and near-real-time vessel position data. Useful for reconstructing vessel tracks before, during and after MPA transits, enabling pattern-of-life analysis to distinguish incidental passage from sustained fishing effort.
What the boundary line cannot enforce on its own
Marine protected areas exist on paper and on charts. Whether fishing effort respects them is a different question. Port-based monitoring catches vessels when they land catch; it cannot tell you where that catch was taken. Aerial patrols are expensive and infrequent. VMS (vessel monitoring systems) covers licensed fleets, not the unlicensed ones most likely to fish inside a no-take zone.
Satellite observation changes the geometry of enforcement. A SAR image is a snapshot of every reflective object on the water surface at the moment of acquisition, regardless of whether any transponder is transmitting. Cross-referencing that snapshot against AIS positions recorded at the same time produces two lists: vessels that appear in both (cooperative, trackable) and vessels that appear only in the SAR image (dark, uncooperative, or simply below the AIS carriage threshold). For MPA regulators, the second list is the one that matters most.
How SAR distinguishes a fishing vessel from background clutter
A vessel on the ocean surface returns a much stronger radar signal than the surrounding water. The ratio between vessel RCS and the local sea clutter determines detectability. Sentinel-1's CFAR (constant false alarm rate) detectors, applied to IW-mode imagery, reliably flag vessels with RCS above roughly 20–30 dBsm, which corresponds approximately to vessels longer than 20–30 m in calm to moderate conditions. ICEYE's X-band imagery, with finer resolution, can resolve smaller targets, though the exact detection floor depends on sea state and vessel orientation.
Movement pattern provides a second discriminant. A vessel drifting with nets deployed shows a different Doppler signature and track geometry than one transiting at speed. Drift-net and longline sets produce characteristic slow-speed, curved tracks when reconstructed from successive AIS pings or repeated SAR acquisitions. Trawlers show persistent back-and-forth patterns at 2–4 knots. These signatures are not infallible. A vessel anchored inside an MPA may be genuinely sheltering from weather. Analysts must be honest about ambiguity in single-pass detections.
The limits that regulators need to understand before acting
Small non-motorised craft, canoes, and vessels under roughly 10–15 m fall below the reliable detection threshold of current spaceborne SAR in typical sea states. In many small-island and artisanal fisheries contexts, this is precisely the category of vessel most active near reef MPAs. Satellite evidence addresses industrial and semi-industrial pressure well; it addresses artisanal pressure poorly.
SAR revisit is a structural constraint. Sentinel-1 provides a 6-day single-satellite repeat at the equator. A vessel that fishes inside an MPA on day 3 and departs on day 5 may never appear in a SAR acquisition during that incursion. Commercial tasking of ICEYE or similar constellations can compress this gap to hours, but cost scales with tasking frequency. Cloud has no effect on SAR, which is an advantage over optical sensors, but high wind speeds above roughly 15–17 m/s raise the noise floor and reduce small-vessel detectability.
AIS spoofing is a documented and growing problem. Vessels can transmit false positions, replay historical tracks, or broadcast another vessel's MMSI. Space-based AIS alone cannot confirm a vessel's true location; only SAR cross-validation can expose the discrepancy. This is the analytical step that converts a suspicion into evidence.
Building a fishing-effort index inside MPA boundaries
Regulators benefit less from individual vessel detections than from a consistent, comparable metric of fishing pressure over time. A fishing-effort index aggregates SAR detections and AIS tracks within defined MPA polygons across a time series, normalising for the number of acquisitions in each period to avoid artefacts from variable revisit. The output is a density surface, typically expressed as vessel-hours per square kilometre per month, that can be compared against pre-designation baselines or adjacent unprotected zones.
This approach was applied systematically in the Pacific Islands region following the designation of large-scale MPAs, where open-access Sentinel-1 data provided the archive depth needed for multi-year trend analysis. The method is reproducible because Sentinel-1 data is freely available back to 2014, giving analysts a decade of retrospective coverage without additional data acquisition cost. For newly designated MPAs, that archive can establish whether fishing pressure predates the designation and at what intensity.
From detection to evidence: what regulators can present in an enforcement context
A single SAR detection of a dark vessel inside an MPA is suggestive, not conclusive. Enforcement-grade evidence requires a chain: the vessel was detected at coordinates X at time T by a calibrated sensor; its AIS transponder was not transmitting; its RCS and movement pattern are consistent with active fishing rather than transit; and its flag-state registration can be inferred from hull geometry or subsequent AIS reactivation outside the zone.
Satellize structures MPA monitoring outputs as time-stamped GIS layers with associated detection confidence scores, so that fisheries agencies can present spatial evidence in the format required by their domestic legal frameworks. The workflow is similar in principle to the analytics architecture used in the Kingdom of Tonga crop-estimation programme, where spatial evidence needs to be defensible to government decision-makers rather than just technically correct.
Regulators should set expectations with legal counsel early. Satellite evidence has been accepted in international fisheries tribunal proceedings, but admissibility standards vary by jurisdiction. The analytic record, including acquisition metadata, processing chain and detection methodology, must be preserved and documented from the outset.
Designing a monitoring programme that matches the threat
Not all MPAs face the same pressure. A remote high-seas MPA with documented industrial IUU activity warrants frequent commercial SAR tasking and near-real-time AIS feeds. A coastal reef MPA with predominantly artisanal pressure may be better served by periodic Sentinel-1 analysis combined with community-based reporting, since satellite methods will miss the small-vessel category most relevant there.
The practical design question is: what vessel size and behaviour does the MPA manager most need to detect, and what revisit frequency is needed to catch an incursion before the vessel departs? Answering that question determines whether open Sentinel-1 data is sufficient or whether commercial tasking is required. A tiered approach, using free Sentinel-1 for baseline monitoring and triggered commercial tasking when suspicious activity is flagged, keeps costs proportionate to the enforcement priority.
Typical figures
| SAR spatial resolution (Sentinel-1 IW mode) | 10 m ground range |
| SAR spatial resolution (ICEYE Spotlight mode) | 25 cm (commercial tasking) |
| Sentinel-1 revisit (equatorial, dual satellite) | 3–6 days; single satellite 6 days |
| ICEYE tasked revisit | Sub-daily to specific AOI with constellation scheduling |
| AIS data latency (Spire space-based) | Under 20 minutes for most ocean areas |
| Minimum detectable vessel length (Sentinel-1, moderate sea state) | Approximately 20–30 m; smaller vessels unreliable |
| SAR archive depth (Sentinel-1) | 2014 to present; freely accessible via Copernicus Data Space |
| Cloud sensitivity | None (SAR is cloud-independent) |
| Wind-speed detection limit | Small-vessel detectability degrades above approximately 15–17 m/s |
| Delivery formats | GeoTIFF vessel-density rasters, GeoJSON detection points, CSV track logs, PDF evidence reports |
Analytics Satellize can run
| Dark-vessel detection layer | CFAR ship detection on Sentinel-1 or ICEYE SAR imagery, cross-referenced against contemporaneous AIS positions to identify non-transmitting vessels | Time-stamped GeoJSON point layer with RCS estimate and AIS-match flag, delivered per acquisition |
| Fishing-effort density surface | Aggregation of SAR detections and AIS fishing-mode tracks within MPA polygon, normalised by acquisition count; expressed as vessel-hours per km² per month | Monthly GeoTIFF raster and summary statistics table for MPA management zone |
| Vessel behaviour classification | Speed and heading analysis of AIS tracks to assign fishing-mode labels (trawling 2–4 knots, drifting gear, transit); SAR Doppler and multi-pass geometry for dark vessels | Classified track GIS layer with behaviour label and confidence score |
| AIS spoofing alert | Spatial discrepancy detection between AIS-reported position and SAR-confirmed position at matched acquisition time | Incident report with acquisition metadata, position offset distance and vessel identifier where available |
| Multi-year pressure trend report | Time-series analysis of monthly effort density surfaces against MPA designation date, using Sentinel-1 archive from 2014 onwards | Annual trend report with before/after comparison charts and zone-level statistics |
| Enforcement evidence package | Documented detection chain: sensor metadata, processing parameters, detection coordinates, behaviour classification and legal-format spatial exhibit | PDF report with GIS attachments formatted for submission to fisheries authority or tribunal |
Who does the work
We can get this done for you. Satellize runs its own analyst desk and a strong science team. You do not buy a data feed and work out what it means; our people source the imagery, run the analysis described on this page, and hand you the answer with its confidence limits stated. Discuss this requirement.