Sunken WWII Hospital Ship Po Becomes a Living Laboratory in Albania’s Vlora Bay

On the night of March 14, 1941, a British Swordfish torpedo tore into the stern of the Italian hospital ship Po, sending the 134.98-meter vessel to the seabed of Vlora Bay in just ten minutes. More than eight decades later, the wreck has found a second life as a scientific instrument. A two-year monitoring program led by Italian and Albanian researchers, working with CMAS diving federations, has produced the first quantitative portrait of how this World War II casualty reshapes the seafloor and gathers marine life around it, revealing a structure that now functions as an artificial reef, a sediment trap, and an occasional refuge for one of the Mediterranean’s most endangered mammals.

The Po rests in 33 to 37 meters of water on a gently inclined shelf inside the Karaburun-Sazan Marine Protected Area, Albania’s first and largest. The team combined high-resolution multibeam echosounder and side-scan sonar surveys with Structure-from-Motion photogrammetry, sediment sampling, and biological censuses to capture the wreck’s interaction with its environment. Over 32 scientific dives conducted between June 2022 and September 2024, divers and a small remotely operated vehicle documented more than 2,000 high-resolution images per survey, building sub-centimetric 3D models of the upper decks and hull. Twelve sediment samples were collected with a Van Veen grab at stations flanking the wreck and at reference sites 200 meters away, while twelve visual census dives cataloged the fish and invertebrate communities.

The geophysical data revealed a striking asymmetry in the seabed around the hull. On the upstream, northwest-facing side, a scour hollow plunges 2.1 meters below the reference seabed level, carved out by currents that accelerate around the obstruction. Downstream, a depositional tail of accumulated sediment stretches roughly 45 meters from the stern. Side-scan sonar backscatter showed small sedimentary ripples with wavelengths of 0.6 to 1.2 meters aligned with the dominant current, but these bedforms vanish within a five-meter radius of the wreckage, evidence that the ship acts as a local hydrodynamic obstacle disrupting near-bed flow.

Sediment analysis sharpened the picture. The reference sites 200 meters away consist of medium sand with a median grain size of 350 micrometers, while samples taken immediately adjacent to the hull contain fine silt and clay fractions smaller than 63 micrometers. The researchers interpret this gradient as consistent with deposition under regional sediment-supply conditions influenced by the Vjosa River, one of Europe’s last undammed rivers, whose delta lies about 15 kilometers to the northwest. During flood events, the Vjosa discharges an average of roughly 195 cubic meters per second, increasing turbidity and sediment transport across the bay. The team is careful to note that direct attribution of the fine sediments to the river requires further validation through time-series monitoring or sediment-trap data, but the spatial pattern strongly suggests the wreck intercepts part of the suspended load and shelters it in its wake.

Biologically, the wreck has become what the researchers describe as a living structure. More than 80 percent of its hard surfaces are covered by a dense, multi-layered biogenic crust comprising 54 identified taxa across eleven phyla. Encrusting sponges such as Crambe crambe and Spirastrella cunctatrix dominate, alongside sea squirts like Botryllus schlosseri and calcified bryozoans including Myriapora truncata and Reteporella septentrionalis. Mediterranean mussels and European flat oysters cluster on vertical surfaces and upper hull sections, while photophilic algae and turf-forming species favor the horizontal decks exposed to light. This zonation reflects eight decades of ecological succession on an artificial substrate in a region where natural hard bottoms are scarce.

Statistical analysis confirmed that the wreck is not uniformly colonized. A one-way PERMANOVA on species abundance data, using a Bray-Curtis resemblance matrix and 9,999 permutations, showed that the bow, midship, stern, and adjacent seabed host significantly different assemblages. The bow section, which faces the prevailing northwest flow, exhibited the highest structural complexity and a Shannon-Wiener diversity index of 2.91, compared with 2.14 in the more sheltered midship and stern sections. Vertical, current-exposed surfaces support richer filter-feeding communities, a pattern consistent with established findings on artificial reefs throughout the Mediterranean. An exploratory Spearman rank correlation between sediment texture and biological cover yielded a coefficient of negative 0.83, suggesting that finer sediments in the wreck’s wake are associated with greater biological cover, hinting at a coupled eco-geomorphological feedback.

The visual censuses recorded 28 fish species from 14 families, including dusky grouper, Mediterranean moray, red scorpionfish, and greater amberjack. Juvenile Diplodus vulgaris, Serranus scriba, and Mullus surmuletus were observed using the structure, with the striped red mullet foraging preferentially in the fine-grained sediments of the depositional tail rather than on the surrounding medium sands. More remarkable were three exceptional records. Large gelatinous egg clusters, likely from the squid Illex coindetii, adhered to vertical surfaces at depths of 22 to 25 meters, evidence of spawning activity. The non-indigenous lionfish Pterois miles was confirmed through direct observation and citizen science photography, with up to seven individuals seen simultaneously, one of the northernmost occurrences yet reported in the Adriatic Sea. And on August 12, 2023, a camera captured a transient Mediterranean monk seal, Monachus monachus, resting within the sheltered cavities of the wreck’s upper structure, the first confirmed sighting in Vlora Bay since 1996.

To distill these heterogeneous observations into usable numbers, the team proposed two preliminary indices. The Geometric-Geomorphological Wreck Effect Index, or IGEOM, combines three dimensionless ratios: the length of the depositional tail relative to the wreck’s length, the scour depth relative to local water depth, and the contrast in median grain size between the far-field reference and the wake zone. Using the measured values, the index yields 0.52. The Ecological Hotspot Value Index, or IECO, integrates normalized benthic diversity, percentage cover, and the presence of key indicator species, producing a score of 0.83. Both indices use geometric means so that no single component can compensate for the absence of another. The researchers stress that these are site-specific descriptive tools, not calibrated thresholds, and that multi-site validation would be needed before they can support cross-wreck comparisons or formal ecological classifications.

The findings carry weight beyond a single wreck. Around 45 ships sank in and around Vlora Bay during the two World Wars and later years, 27 of them inside the bay, most at depths shallower than 50 meters. These structures, largely unstudied until now, represent a distributed network of hard substrates in a soft-bottom environment. The Po demonstrates that such wrecks can serve as long-term observatories of ecological and geomorphological change, fixed reference points against which the effects of river inputs, warming waters, and shifting currents can be measured. The presence of Pterois miles, a thermophilic invader expanding northward through the Adriatic, suggests that wrecks may function as stepping stones for non-indigenous species, while the monk seal sighting hints that artificial structures could provide occasional resting or foraging opportunities where natural habitats are fragmented.

The authors frame the work as a hypothesis-generating baseline rather than a definitive causal analysis. The survey window was short, lacking time-series current data or seasonal biological monitoring, and benthic coverage estimates remain sensitive to spatial variability. Future directions include extending surveys to other wrecks in the bay, deploying current meters and sediment traps, completing the 3D digital twin of the Po, and developing virtual reality models for public engagement and Marine Spatial Planning. By coupling the methodological workflow with high-resolution hydrodynamic modeling, the researchers aim to establish a standardized protocol for evaluating the wreck-effect across the Adriatic-Ionian region, transforming historical shipwrecks from isolated artifacts into integrated tools for conservation, monitoring, and the sustainable management of the Mediterranean’s submerged cultural heritage.

Subject of Research: Marine geomorphology and bio-colonization of the Hospital Ship Po wreck in Vlora Bay, Albania

Article Title: First assessments on marine geomorphology and bio-colonization through the study and survey of the wreck of the Hospital Ship “Po” along the Albanian coasts (Vlora Bay)

Article References: Modugno, S., Castro, R., Pustina, I., Kromidha, G., Dedej, Z., & Meçöllari, A. (2026). First assessments on marine geomorphology and bio-colonization through the study and survey of the wreck of the Hospital Ship “Po” along the Albanian coasts (Vlora Bay). Frontiers in Ocean Sustainability, 4, Article 1762680. https://doi.org/10.3389/focsu.2026.1762680

Image Credits: AI Generated

DOI: 10.3389/focsu.2026.1762680

Keywords: shipwreck, artificial reef, marine geomorphology, benthic colonization, Vlora Bay, Albania, Mediterranean monk seal, lionfish, Pterois miles, sediment dynamics, Marine Protected Area, photogrammetry