PSSA: North-Western Mediterranean Sea

Background: what the PSSA does, and what it does not

A Particularly Sensitive Sea Area is an area the International Maritime Organization recognizes as needing special protection through IMO action because of its ecological, socio-economic or scientific significance and its vulnerability to damage from international shipping. The governing instrument is IMO Assembly Resolution A.982(24) of 1 December 2005, the Revised Guidelines for the Identification and Designation of Particularly Sensitive Sea Areas, as amended by Resolution MEPC.267(68) of 15 May 2015. A coastal state, or a group acting jointly, submits a documented case to the MEPC under three criterion families, ecological, social-cultural-economic, and scientific-educational, and pairs it with at least one Associated Protective Measure drawn from an existing IMO instrument.

The North-Western Mediterranean PSSA is the unusual case where the protective package is entirely recommendatory. Most PSSAs lean on a mandatory teeth-bearing measure: the Galapagos PSSA on a binding area-to-be-avoided under SOLAS Chapter V Regulation 10, the Western European Waters PSSA on the mandatory WETREP ship-reporting system under SOLAS V/11. The NW Med PSSA has no SOLAS routeing or reporting measure. Annex 4 of Resolution MEPC.380(80) states the measures are recommendatory in nature. That single design choice shapes everything about how the area works in practice, and it is the first fact a watchkeeper, a DPA or a flag surveyor needs to get right.

The distinction matters operationally. A recommendatory APM cannot be the basis for a port-state-control detention, a flag-state sanction, or a coastal-state prosecution of a transiting foreign ship. It is an IMO-blessed call on the seafarer’s professional judgment, backed by the moral weight of an MEPC resolution and by charted notation, but not by enforcement. A master who transits the area at 18 knots without slowing for a reported whale aggregation breaks no rule. The measure works only to the degree that the industry chooses to follow it, which is why the monitoring of voluntary uptake by ACCOBAMS, discussed below, is the real test of the designation rather than any enforcement record.

Geography: the basin between the French coast and the Balearics

The PSSA covers the north-western sector of the Mediterranean, the deep-water basin bounded by the southern coast of France and Monaco, the western coast of Italy down to about the latitude of central Italy, the eastern coast of Spain, and the Balearic Archipelago to the south-west. The boundary is defined in Annex 1 of the Resolution by the lines joining ten geographic points whose latitudes and longitudes are listed there. The area takes in the Ligurian Sea, the Gulf of Lion, the Provencal basin, the seas around Corsica and the northern flank of Sardinia, and the waters east and north of the Balearics.

Two features of the basin explain why it concentrates whales. The first is the Ligurian-Provencal frontal system, a persistent thermohaline front along the continental slope where wind-driven divergence and winter mixing lift nutrients into the photic zone. That upwelling drives a spring and summer bloom of the krill Meganyctiphanes norvegica, the primary prey of the Mediterranean fin whale. The second is bathymetry: the basin drops steeply to depths over 2,500 meters close inshore, so deep-diving sperm whales and the krill swarms that fin whales feed on are found within a few tens of nautical miles of the busiest ferry and merchant lanes. The animals and the ships occupy the same water.

The eBulletin summary of the proposal records the basin’s biological weight bluntly: although the north-western Mediterranean is roughly 1 percent of the world ocean surface, it holds on the order of 10 percent of recorded marine species. The PSSA encloses both Barcelona Convention SPAMIs (the Pelagos Sanctuary and the Cetacean Migration Corridor) along with the interconnecting waters, so its outer extent is larger than either protected area alone.

Pelagos Sanctuary and the Cetacean Migration Corridor

The PSSA is built on top of two pre-existing Specially Protected Areas of Mediterranean Importance (SPAMI) listed under the Protocol concerning Specially Protected Areas and Biological Diversity in the Mediterranean of Barcelona, 1995, the SPA/BD Protocol to the Barcelona Convention.

The Pelagos Sanctuary for the conservation of marine mammals covers about 87,500 square kilometers of the Ligurian Sea and the Corsican-Sardinian seas. It was created by a trilateral agreement signed by France, Italy and Monaco at Rome on 25 November 1999, entered into force in 2002, and was added to the SPAMI list in 2002. It was the first high-seas marine protected area in the Mediterranean, covering territorial waters of the three states and a substantial area of the adjacent high seas, which is part of why an IMO shipping measure layered over it had to respect the navigational freedoms that apply beyond the territorial sea.

The Cetacean Migration Corridor covers about 46,385 square kilometers off the eastern Spanish coast, north and east of the Balearic Islands. Spain designated it as a marine protected area and it was added to the SPAMI list in 2018. It is a feeding and transit area for striped dolphins, Risso’s dolphins, sperm whales and beaked whales, and a migratory route for fin whales moving between the Ligurian summer feeding grounds and wintering areas to the south and west.

The PSSA stitches these two SPAMIs and the water between them into a single IMO-recognized unit. The conservation regimes of the two SPAMIs, governed under the Barcelona Convention and administered through UNEP/MAP and its SPA/RAC regional activity center, remain in force on their own terms. The PSSA does not replace them; it adds the shipping-specific layer that the Barcelona instruments could not deliver, because regulating the navigation of foreign-flag ships is an IMO competence, not a regional-seas-convention competence.

PSSA criteria met: the highest cetacean density in the Mediterranean

The proposal documented the area against the criteria of A.982(24), and the basin satisfies several of them at once.

Under the ecological criteria, the area qualifies on uniqueness or rarity, critical habitat, dependency, productivity, and bio-geographic importance. The dominant claim is critical habitat for the Mediterranean fin whale, a genetically distinct resident subpopulation listed as Vulnerable on the IUCN Red List and separate from the North Atlantic stock. ACCOBAMS aerial survey work cited in the proposal estimated that the abundance of fin whales inside the PSSA represents roughly 67 percent of the entire Mediterranean fin-whale population, the single densest concentration in the sea. The sperm whale, the second species of concern, is present year-round; a 2017 estimate for about half the proposed PSSA put the number between 300 and 600 individuals, with higher counts in winter.

Under the social, cultural and economic criteria, the proposal pointed to whale-watching tourism, the cultural standing of the Pelagos Sanctuary, and the dependence of the regional marine economy on a functioning ecosystem. Under the scientific and educational criteria, the basin is one of the most intensively studied cetacean habitats in the world, with decades of survey data from ACCOBAMS, the Pelagos parties, and academic groups, and it serves as a reference site for ship-strike research that informs mitigation elsewhere.

The vulnerability case is the other half of an A.982(24) submission, and here it is straightforward. The same waters that concentrate whales carry one of the heaviest traffic loads in the Mediterranean, with vessel movements in the area on the order of 220,000 per year, merchant ships running at typical speeds of 14 to 20 knots and high-speed craft up to about 35 knots. High whale density plus high vessel speed in overlapping water is the textbook ship-strike risk profile.

Ship-strike science: why speed is the lever

Ship strikes are collisions between vessels and large marine animals, and for the great whales of the north-western Mediterranean they are the leading cause of human-induced mortality. The IMO acknowledged this directly in designating the PSSA. The mechanism is blunt: a fin whale resting or feeding near the surface, or surfacing to breathe, is struck by the bow or cut by the propeller of a ship it cannot avoid and the crew never sees.

Two variables drive the outcome. The first is detection, the chance the animal or the crew avoids the encounter at all; the second is lethality given a strike, which rises steeply with vessel speed. The widely cited probability-of-lethal-injury curves from large-whale strike studies show the chance of a lethal outcome climbing through an inflection around 10 to 15 knots and approaching certainty above roughly 15 knots. That is the evidence behind the 10 to 13 knot recommendation: it puts a complying ship on the lower, survivable part of the lethality curve while keeping enough way on for safe maneuvering. Slowing also buys detection time, since a slower ship covers less distance during the seconds between a lookout’s sighting and a helm or engine order.

The fin whale’s behavior makes it the hardest species to protect. Mediterranean fin whales spend long periods near the surface skim-feeding on krill, move slowly when feeding, and do not reliably flee an approaching ship. Strike risk concentrates along the high-density ferry and merchant lanes between the Gulf of Lion, the Ligurian Sea and the Tyrrhenian approaches, the same corridors mapped as risk hotspots in the peer-reviewed strike-risk literature for fin and sperm whales along the main north-western Mediterranean shipping lanes. Reducing speed is the one mitigation a transiting ship can apply unilaterally, without rerouting, which is why it anchors the APM.

The Associated Protective Measure: a recommendatory vigilance and speed zone

The whole protective content of the PSSA sits in Annex 4 of MEPC.380(80), and it is recommendatory. The measures are deemed to apply to any commercial ship and pleasure yacht of 300 gross tonnage and above transiting the area. They ask the mariner to do several specific things.

First, navigate with particular caution and maintain enhanced lookout throughout the area, and especially in any zone where large or medium cetaceans are detected or reported. Enhanced lookout includes posting dedicated observers in daylight and, where fitted, using thermal or infrared cameras to spot blows and bodies at night, when visual detection of a dark whale on a dark sea is close to impossible.

Second, reduce speed to between 10 and 13 knots where cetaceans are detected or reported, while keeping the speed needed for safe maneuvering. This is the core mitigation. It is framed as a voluntary speed reduction (VSR), the same instrument used in North American and other strike-mitigation programs, and it is explicitly capped at the lower band where the lethality curve flattens.

Third, keep a safe distance from any detected animal and avoid converging tracks. Fourth, report sightings and any collisions: pass cetacean sightings to the relevant coastal authorities so other ships can be warned, and report any strike so the case enters the data record used by the International Whaling Commission and ACCOBAMS to track mortality.

Nothing in the APM creates an area-to-be-avoided, a no-anchoring area, a traffic separation scheme, a mandatory reporting line, or compulsory pilotage. There is no SOLAS Chapter V routeing measure attached, which is the structural difference between this PSSA and almost every other one. The protective effect is the sum of many individual masters choosing to slow down and watch.

REPCET: the real-time sighting network

The sighting-and-warning element of the APM has an operational backbone in REPCET, a collaborative real-time cetacean-sighting system developed for the Pelagos Sanctuary by the French association Souffleurs d’Ecume. The name stands for the French for shared reporting of cetaceans. The idea is simple and it predates the PSSA: a ship’s crew that sights a whale logs the position through onboard software, the report is relayed by satellite to a shore server, and the sighting is pushed back out to every other equipped ship in the area within minutes, displayed on the bridge as a time-stamped position to avoid.

REPCET turns one ship’s lookout into a shared warning for the whole fleet, which addresses the central weakness of visual detection: any single bridge team can only watch its own track, and a whale spotted by a ferry an hour ahead is invisible to a tanker behind it unless the sighting is shared. Several French and Italian ferry and ro-pax operators equipped their fleets with REPCET in the years before the PSSA, partly under French regulatory encouragement. The PSSA’s reporting recommendation gives the system an IMO-level rationale and a wider intended user base, though, like the rest of the APM, REPCET participation is voluntary. Its value scales with the number of equipped ships; a network covering only a fraction of the 220,000 annual movements warns of only a fraction of the whales sighted.

Relationship to the Mediterranean MARPOL Special Area

The PSSA must be read against the Mediterranean’s MARPOL Special Area status, which is a different legal animal. A MARPOL Special Area imposes uniform discharge prohibitions on all ships in a defined sea region; a PSSA designates an area as vulnerable and attaches a tailored package of protective measures. The two regimes can and do overlap in the same water.

The Mediterranean Sea is a Special Area under several MARPOL annexes. It is an oil Special Area under MARPOL Annex I, so the overboard oil-discharge limits are stricter than the general open-sea criteria of Regulation 15. It is a Special Area under MARPOL Annex V for garbage. These discharge controls already apply across the whole Mediterranean, including inside the PSSA, independent of the 2023 designation. The PSSA reinforces them by adding the vulnerability rationale and the vigilance regime, but it does not create a new discharge Special Area boundary.

For a watchkeeper the practical point is that several distinct regimes apply at once inside the PSSA polygon: the MARPOL Annex I and Annex V Special Area discharge rules (mandatory), the Annex VI air-emission regime including the new sulfur regime described below (mandatory), and the PSSA vigilance and speed APM (recommendatory). Confusing the recommendatory APM with the mandatory discharge rules, in either direction, is the most common compliance error and is treated in the Limitations section.

Relationship to the Mediterranean SECA effective 1 May 2025

A second regime now overlaps the PSSA. The whole Mediterranean Sea became an Emission Control Area for sulfur oxides under MARPOL Annex VI, with the 0.10 percent m/m fuel-sulfur limit effective 1 May 2025, the subject of the Mediterranean SECA 2025 article. The Mediterranean SECA was adopted by Resolution MEPC.361(79) in December 2022 and is mandatory across the basin, including inside the NW Med PSSA.

The two measures are independent in law but reinforce each other in effect, and the interaction with speed is worth spelling out. The SECA cut sulfur and particulate emissions; the PSSA asks ships to slow down where whales are present. Slower steaming reduces fuel burn and so reduces the total mass of all combustion emissions for a given voyage, while also cutting underwater radiated noise, which is a separate cetacean stressor the PSSA proposal flagged alongside strike risk. A ship complying with both regimes runs on 0.10 percent sulfur fuel and, where cetaceans are reported, at 10 to 13 knots. The SECA is enforced through bunker delivery notes, fuel sampling and the FONAR exception regime; the PSSA speed band is not enforced at all. The contrast in enforceability between two regimes sharing the same water is the clearest illustration of what recommendatory status means in practice.

Relationship to the Strait of Bonifacio PSSA

The north-western Mediterranean already held a PSSA before 2023. The Strait of Bonifacio, the narrow channel between Corsica and Sardinia, was designated a PSSA by Resolution MEPC.204(62) in 2011 on a joint French and Italian proposal, with a recommended pilotage measure for the strait. The Strait of Bonifacio lies inside the geographic envelope of the larger NW Med PSSA, so a ship passing through the strait is now inside two PSSAs at once.

The two designations address different risks. The Strait of Bonifacio PSSA targets grounding and collision risk in a confined, current-swept channel with a recommendation that ships, especially those carrying dangerous or polluting cargoes, take a pilot. The NW Med PSSA targets ship strikes and noise across the open basin. They are complementary: a laden tanker transiting from the Tyrrhenian toward the Gulf of Lion would respond to the Bonifacio pilotage recommendation in the strait and to the cetacean vigilance and speed recommendation across the wider basin. Both are recommendatory, so the combined regime is a layered set of professional expectations rather than a stack of enforceable rules.

ACCOBAMS and monitoring voluntary uptake

Because the APM has no enforcement mechanism, the designation’s success rests on measuring whether ships actually comply, and that monitoring falls largely to ACCOBAMS, the Agreement on the Conservation of Cetaceans of the Black Sea, Mediterranean Sea and Contiguous Atlantic Area, a regional treaty under the Bonn Convention with a secretariat in Monaco.

ACCOBAMS ran the aerial and vessel surveys that produced the population figures behind the proposal, and it now tracks compliance. Its Scientific Committee paper SC16.Doc18, an analysis of maritime traffic speed in 2023 inside the PSSA, uses AIS data to measure how many ships are already in the 10 to 13 knot band and how many run faster. That kind of AIS-based speed analysis is the only practical way to audit a voluntary measure across hundreds of thousands of movements, and it gives the coastal states and the IMO an evidence base for whether the recommendatory approach is working or whether a mandatory measure should follow.

ACCOBAMS has been consistent that the most effective ship-strike mitigation separates whales from vessels, through routeing or a binding speed limit, rather than relying on detection and voluntary slowing. That position, recorded in ACCOBAMS Resolution 5.11 on ship strikes and in its engagement at IMO, frames the recommendatory APM as a first step whose performance is on probation. If the AIS speed analyses show poor uptake and strike mortality holds steady, the case for a future mandatory APM, an area-to-be-avoided, a routeing measure or a mandatory speed zone, strengthens.

Enforcement and legal limits under UNCLOS

The recommendatory design is not only a political compromise; it tracks the limits of what a coastal state can impose on foreign-flag ships under the United Nations Convention on the Law of the Sea (UNCLOS) of 1982, the framework the PSSA overview sets out. Inside the territorial sea, a coastal state has broad regulatory authority but cannot deny innocent passage; in the exclusive economic zone, it may apply generally accepted international rules but not unilateral measures beyond them. Much of the PSSA, including the Pelagos high-seas component, lies in the EEZ or on the high seas where coastal-state prescriptive power over navigation is narrow.

A mandatory speed limit or routeing measure on foreign ships in those waters would need IMO adoption under SOLAS Chapter V to be enforceable, and securing that adoption is a higher bar than a recommendatory APM under the PSSA Guidelines. By choosing a recommendatory package, the four states obtained an IMO designation and charted notation quickly and across the whole multi-state basin, accepting weaker enforcement power in exchange. The legal reality is that even a transiting ship that ignores the APM entirely commits no enforceable breach, which is why the measure is best understood as IMO-endorsed guidance to the seafarer’s judgment rather than as a rule.

Implementation, charts and the bridge

The PSSA is promulgated to mariners through charted notation and nautical publications rather than through any reporting obligation. National hydrographic offices of the four proposing states, the French SHOM, the Italian IIM, the Spanish IHM and the Monegasque authorities, carry the PSSA boundary and an explanatory note on the relevant charts and in the sailing directions, and the IMO records the area in its PSSA list. A bridge team plans for the PSSA the way it plans for any charted advisory area: note the boundary on the passage plan, brief the speed and lookout expectations, and prepare the sighting-report procedure.

The operational footprint on a well-run bridge is light. There is no entry or exit report to file, no mandatory waypoint schedule, and no pilotage requirement outside the separate Bonifacio measure. The work is enhanced lookout, readiness to slow to 10 to 13 knots on a sighting, and the discipline to log and broadcast sightings. For an operator already running REPCET, the sighting workflow is built in; for one that is not, the recommendation is to report by VHF to the local coastal authority. The marginal voyage cost of complying is the time penalty of slowing on the segments where whales are actually reported, which for most transits is a small fraction of the passage, not a basin-wide speed cap.

For the company office, the PSSA belongs in the safety management system under the ISM Code rather than in any certificate or survey regime, since there is nothing to certify. A practical implementation is a standing instruction in the navigation procedures: brief the PSSA boundary at the passage-plan stage, set the enhanced-lookout and speed expectation in the master’s standing orders for transits of the area, equip the bridge to receive REPCET or VHF sighting traffic, and record sightings and any contact with a large animal in the deck log and the company incident system. None of this is compulsory under the resolution, so the office decides the level of commitment, and that decision is the real determinant of whether the ship contributes to the measure or merely passes through the polygon. Operators that have published whale-protection policies treat the 10-to-13-knot band as a firm internal instruction in the high-density season, which converts the IMO recommendation into an enforceable company rule even though it is not an enforceable IMO rule.

Comparison with the Hauraki Gulf and the role of voluntary speed reduction

The NW Med PSSA is part of a small group of cetacean-focused shipping measures that lean on voluntary speed reduction rather than mandatory exclusion. Voluntary speed-reduction programs run off the United States West Coast in the Santa Barbara Channel and the San Francisco approaches, and a recommendatory speed measure protects Bryde’s whales in New Zealand’s Hauraki Gulf. The common thread is the same lethality-versus-speed curve and the same political reality: a voluntary measure is faster to put in place and easier to agree than a mandatory one, but its effect depends on uptake.

The North American programs publish compliance rates derived from AIS and tie them to recognition schemes that reward operators for slowing, which has lifted uptake over time. The NW Med PSSA does not yet have an equivalent incentive layer beyond the ACCOBAMS monitoring; whether one develops will shape how the basin’s voluntary measure performs against its mandatory-measure alternatives. The comparison is instructive for any operator: the speed band of 10 to 13 knots is not an arbitrary Mediterranean figure but the same survivable-speed range that strike-mitigation science has converged on across ocean basins.

The MEPC 80 designation process

The designation followed the standard two-stage PSSA route under A.982(24). The four states submitted the proposal to the MEPC, the proposal was examined by a PSSA technical group, and the area was designated by the MEPC at its 80th session, held 3 to 7 July 2023, by Resolution MEPC.380(80) adopted on the final day. The submission was prepared over the preceding period with the backing of the Pelagos parties and ACCOBAMS, and timed to follow the December 2022 COP15 of the Convention on Biological Diversity, where the political case for marine biodiversity protection was prominent.

The proposal originally canvassed stronger measures. Conservation submissions to the MEPC, including from OceanCare, pressed for a mandatory speed limit or a routeing measure on the strength of the strike science, and ACCOBAMS had on record since its Resolution 5.11 the position that separating whales from ships is the most effective mitigation. The four coastal states settled on a recommendatory package to reach consensus at MEPC 80 and to cover the whole multi-state basin in one designation, rather than negotiate a mandatory SOLAS Chapter V measure that would have needed separate adoption and a harder agreement among the flag states represented at IMO. The outcome is a designation secured fast and wide, with the enforcement question deferred.

Because the APM carries no SOLAS routeing or reporting element, the proposal did not need a parallel referral to the Sub-Committee on Navigation, Communications and Search and Rescue (NCSR) or adoption by the Maritime Safety Committee, the dual-track path that the Galapagos and Western European Waters PSSAs followed for their mandatory measures. The whole instrument lives in the MEPC resolution. That single-track route is part of why a recommendatory PSSA is quicker to designate than a mandatory one, and it is the procedural signature of the choice the four states made.

Fin whale and sperm whale: the species at risk

The Mediterranean fin whale is the second-largest animal alive, reaching about 20 meters and 40 to 50 tonnes, and the resident population is genetically distinct from the North Atlantic stock that occasionally enters the basin through Gibraltar. The IUCN lists the Mediterranean subpopulation as Vulnerable. The animals concentrate in the north-western basin in spring and summer to feed on the krill Meganyctiphanes norvegica, lunging through dense swarms near the surface, the behavior that puts them in the path of fast ships. ACCOBAMS survey work placed roughly 67 percent of the whole Mediterranean fin-whale population inside the PSSA at the time of the proposal, which is why the designation centers on this species.

The sperm whale of the Mediterranean is also a distinct, IUCN-Endangered subpopulation. Unlike the surface-feeding fin whale, the sperm whale is a deep diver that hunts squid below 400 meters, but it rests and socializes at the surface in long horizontal periods between dives, and during those surface intervals it is exposed to strikes and is slow to react. The 2017 estimate for about half the proposed PSSA put the number at 300 to 600 animals, concentrated along the steep continental slopes where deep water lies close to the coast and to the shipping lanes. Sperm whales also bear the brunt of underwater noise, since their echolocation and communication occupy the frequency bands that merchant traffic floods.

Both species share the trait that makes them hard to protect by detection alone: they spend extended time at or near the surface, do not reliably flee an oncoming hull, and are dark against a dark sea at night. The PSSA’s lookout and infrared recommendations target detection, but the speed recommendation is the part that protects the animal the lookout never sees, by lowering the lethality of the strikes that detection fails to prevent.

Traffic profile and the speed problem

The north-western Mediterranean carries some of the densest mixed traffic in the sea. The roughly 220,000 annual vessel movements include the Gulf of Lion and Ligurian merchant lanes feeding Marseille, Genoa, Barcelona and the Italian Tyrrhenian ports, the dense passenger and ro-pax ferry network linking the French and Italian mainlands with Corsica, Sardinia and the Balearics, cruise traffic, and the high-speed craft that run the island routes in summer. Merchant ships typically transit at 14 to 20 knots; high-speed craft reach about 35 knots. Summer is the worst overlap: peak ferry and high-speed sailings coincide with the peak fin-whale feeding season, so the fastest ships run through the densest whale aggregations in the same months.

That overlap is the operational heart of the problem. The strike-risk hotspot mapping for fin and sperm whales along the main north-western Mediterranean lanes shows risk concentrating where high vessel speed crosses high whale density, and the high-speed ferry corridors score worst. A measure that addressed only slow merchant traffic would miss the fastest, highest-risk vessels; the APM’s 300 GT scoping captures commercial ships and the larger yachts, and the speed band of 10 to 13 knots is set well below the 14-to-20-knot merchant cruising range and far below the 35-knot high-speed-craft range, so compliance means a real reduction for exactly the vessels that pose the most risk.

The cost of complying is the time penalty on the segments where ships actually slow. A high-speed craft dropping from 35 to 13 knots on a leg through a reported whale zone loses substantial schedule, which is the friction that makes voluntary compliance hardest for the operators whose vessels matter most to the whales. For a merchant ship already near the band, the penalty is modest. This asymmetry, light for slow ships and heavy for fast ones, is the reason a voluntary measure may under-deliver precisely where it is needed, and the reason the ACCOBAMS AIS speed analyses focus on whether the fast cohort is slowing at all.

The speed-lethality relationship in numbers

The 10-to-13-knot recommendation rests on the published relationship between vessel speed and the probability that a struck large whale dies. The work most often cited, by Vanderlaan and Taggart, fitted strike records to a logistic curve in which the probability of a lethal injury rises through about 0.5 near 11.8 knots and exceeds about 0.9 above roughly 15 knots, while falling toward 0.2 or below as speed drops under 10 knots. The curve is steep through the 10-to-15-knot band, so a few knots of reduction in that range buys a large drop in lethality.

The relationship can be written as a logistic function of speed:

where $v$ is vessel speed in knots and $a$ and $b$ are fitted constants from the strike data, with $b > 0$ so the probability increases with speed. The practical reading for a bridge team is direct: holding to 13 knots rather than 18 moves the ship from the upper, near-certain-lethal part of the curve to the lower shoulder, turning a likely kill into a likely survivable encounter. That is the whole logic of a voluntary speed-reduction zone, and it is why the band tops out at 13 rather than at a higher “slow” figure.

Speed reduction also helps through detection, not only lethality. A ship at 13 knots covers about 1,300 meters in three minutes against about 1,800 meters at 18 knots, so the slower ship gives the bridge team more time and more distance to act on a sighting before the closest point of approach. The two effects compound: slowing both improves the chance the strike is avoided and lowers the chance it kills if it happens. Neither effect is captured by any enforceable threshold, which is the recurring tension of the recommendatory design.

Limitations and practitioner notes

The single most important caveat is the one stated at the top: the APM is recommendatory. It is not enforceable against a transiting ship by any authority. A vessel that maintains 18 knots through a reported whale aggregation breaks no rule, faces no detention, and incurs no penalty. Treating the speed band as a hard limit, or assuming a port state can act on a failure to slow, misreads the measure. The corollary error is the opposite: assuming that because the speed measure is voluntary, the mandatory regimes in the same water can also be ignored. They cannot. The Mediterranean SECA 0.10 percent sulfur limit, the MARPOL Annex I and Annex V Special Area discharge rules, and the rest of MARPOL apply inside the PSSA with full force and are PSC-detainable.

The 300 GT figure is the scoping threshold for which ships the measure is deemed to address, not a SOLAS routeing threshold and not an enforced cut-off. Smaller craft are encouraged to follow the same practice; larger ships are not exempted by any cargo or trade carve-out, because there is nothing to be exempt from.

The measure protects against a strike only where the whale is detected. Its weakest point is night and poor visibility, when even an alert bridge team cannot see a dark whale on a dark sea, and infrared coverage is far from universal. REPCET narrows the gap but only among equipped ships, and most of the 220,000 annual movements are not equipped. Compliance auditing through AIS speed analysis can show whether ships slowed, but it cannot show whether they slowed at the right place relative to an animal that was never reported.

The high-seas and EEZ components of the PSSA limit what any future tightening can look like. A mandatory speed zone or area-to-be-avoided would require fresh IMO adoption under SOLAS Chapter V, which is a slower and harder process than the recommendatory designation already achieved. Operators planning for the medium term should watch the ACCOBAMS speed-analysis series and the MEPC agenda: if voluntary uptake is judged inadequate, a mandatory measure is the foreseeable next step, and it would change the compliance calculus from professional judgment to enforceable rule.

Finally, the PSSA does not regulate underwater noise directly even though noise was part of the proposal’s vulnerability case. The speed recommendation reduces radiated noise as a side effect, but there is no noise limit, no quiet-ship requirement, and no measurement obligation. An operator seeking to reduce its acoustic footprint in the basin is acting beyond what the PSSA requires.

See also

• Particularly Sensitive Sea Areas: IMO PSSA Guide
• Mediterranean SECA effective 1 May 2025
• Barcelona Convention 1976 and 1995 protocols
• Particularly Sensitive Sea Area: Galapagos Archipelago
• Particularly Sensitive Sea Area: Western European Waters
• MARPOL Annex VI air pollution
• MARPOL Annex I oil pollution prevention
• MARPOL Annex V garbage
• MARPOL Convention
• Calculator catalogue