In late September 2021, an unprecedented mass die-off of crustaceans including brown crab (Cancer pagurus) occurred along 70 km of the North East England coastline. The Defra-appointed Crustacean Mortality Expert Panel (CMEP) Report concluded that a “novel pathogen” was “about as likely as not” to be the cause, while discounting dredging and chemical toxicity as “very unlikely” based on the assumption of routine maintenance dredging in a “clean” area.

This study reassesses the physical, chemical, and epidemiological evidence surrounding the event. Between September 25 and October 4, 2021, the trailing suction hopper dredger UKD Orca conducted an atypical, highly intensive operation, dredging 420 km across a 2.5 km x 300 m navigation channel and dumping ~150,000 tonnes of sediment in 10 days - an operational intensity exceeding typical annual maintenance schedules. Operating at 2.5 knots, the vessel resuspended an estimated 50,000 tonnes of sediment directly into coastal waters. Contemporary October 2021 sediment sampling revealed elevated level of low molecular weight polycyclic aromatic hydrocarbons (PAHs), including 2-fold increase in phenanthrene (>2,200 µg/kg) and 5-fold increase in fluoranthene (>2,100 µg/kg).

While finfish rapidly biotransform parent PAHs via inducible CYP1A1 mixed-function oxygenase systems, crustaceans possess low, non-inducible CYP450 activity. Continuous branchial pumping of contaminated slurry, combined with digestive processing in the gastric mill, forces rapid uptake and bioaccumulation of lipophilic PAHs into the lipid-rich hepatopancreas. The observed clinical presentation, somersaulting, continuous twitching, and paralysis, directly matches the neurotoxic mode of action of phenanthrene via acetylcholinesterase (AChE) inhibition. Furthermore, southward hydrodynamic plume dispersion aligns chronologically with initial mortality reports starting on day zero at Greatham Creek and propagating down to Whitby within days.

These findings demonstrate that an acute, dredging-induced toxic pulse of bioavailable PAHs carried on fine sediment particles, rather than a hypothetical pathogen, provides a robust, mechanistically supported explanation for the 2021 crustacean die-off.

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Read the Defra-appointed Crustacean Mortality Expert Panel (CMEP) Report. Certainly question remain but are there also some questionable conclusions?

Novel virus “as likely as not” but no evidence, pyridine “very unlikely” but present at dump site, a 10 day continuous dredge in the littoral flow was normal, contaminated chemicals can't impact crabs.

The speed of the die-off appears to rule out a novel virus, but the extent of the die-off would fit with a disease.

The extent of the die-off was used to rule out a chemical route.

• Henderson, G. et al. Independent Expert Assessment of Unusual Crustacean Mortality in the North-East of England in 2021 and 2022. 66 (2023).
• Department for Enviroment Food & Rural Affairs. Joint Agency Investigation into Teesside and Yorkshire Coast Crab and Lobster Mortalities Investigation Summary. https://assets.publishing.service.gov.uk/media/62a70931e90e07039ae3ecf0/Joint_agency_investigation_into_Teesside_and_Yorkshire_Coast_Crab_and_Lobster_mortalities.pdf.

Using AIS data it is possible to estimate the distances travelled while dredging, using these estimates it is possible to compare a typical months dredging with the atypical dredging the UKD Orca carried out.

The UKD Orca operated continuously for 10 days in the littoral flow, resuspending sediment while dredging, overflowing and disposing. UKD Orca dredged at around 2.5kts for 470.10 km over 2.5km navigation channel releasing 50,000tonnes of fine sediment while dredging and disposing of 150,000tonnes at the disposal site. AIS UKD Orca 2021 September / October.

Whereas in March 2025 the Heortnesse dredged at an average speed of 1.25kts for 60.39km and the Emerald Duchess dredged at an average speed of 1.28kts for 109.27km. AIS Hoertnesse and Emerald Duchess 2025 March. Over the whole of 2025 only 68,000tonnes was disposed of, so less than 60,000tonnes will have been disposed of, with less than 15,000 tonnes of fine sediment being resuspended during the dredging operations.

Dredging of this area in the mouth of the River Tees is supposed to be composed of clean sand brought in by sea with a small amount of contaminated material carried down river. In fact in sediment when tested in October 2021 was more contaminated than in previous or subsequent years in Chart 11 (where 90% of sediment was dredged from).

Data from the marine licence application MLA-2015-00088(L/2021/00333/7) 3 yearly sediment test data spreadsheets for whole river and the number of samples taken in Charts 10, 11 and 12:

• MMO Results_Template MAR00356_2.xlsm Sampled in 2019/08/14 has 3 samples.
• MMO_Results_Template - MAR01178.xlsmSampled on 2021/10/13 has 3 samples.
• MMO_Results_ - MAR02085 UPDATED.xlsmSampled on 2023/10/19 has 2 samples.
• MMO Results Template MAR02499 V2.xlsm Sampled on 2024/10/07 has 2 samples.
• MAR02481 V2.xlsm Sampled in 2024/10/28 has 1 sample

Sediment data explorer - 2019, 2021, 2023 and 2024 data

Fluoranthene, phenanthrene and pyrene showed the largest increases compared to the other hydrocarbons.

The 3 highlighted PAHs are all hydrophobic and as such will be held mainly either within small carbonaceous particles or adsorbed on the surface of fine sediment particles. Both the carbonaceous particles and the fine sediment particles will be respired crabs and also some material will be ingested.

Fluoranthene

Phenanthrene

Pyrene

Eastabrook et al modelled the spread of pyridine from the dredging and disposal sites. The model showed that pyridine would have travelled south along the coast which is consistent with the timeline of crab mortality travelling south in late September / early October 2021.

Fine sediment particles would take over 7 days to sediment and as such would have travelled at roughly the same rate as the pyridine. While larger particles would have settled more rapidly, these larger particle have a low surface area to volume and so carry a lower proportion of the contamination, meaning that the majority of the contamination would have remained suspended for the full time.

While storm action has also been posited as a cause any contaminated particles to be swamped by relatively clean sand. In fact storm action would preferentially resuspend the finest most contaminated particles acting as pump for continuing to move the contamination along the coast.

• Eastabrook, C. L. et al. Determining the toxicity and potential for environmental transport of pyridine using the brown crab Cancer pagurus (L.). Preprint at https://doi.org/10.1101/2022.11.17.516169 (2022).

• Losada, S., Pitchers, E. P., Potter, K., Katsiadaki, I. & Barber, J. L. Quantitative Determination of Pyridine Content in Crustacean Tissues and Marine Sediments by Headspace Gas Chromatography/Tandem Mass Spectrometry (HS-GC-MS/MS). Anal. Lett. 58, 3011–3027 (2025).
• Peters, A. Comment on “Why there is no evidence that pyridine killed the English crabs” by A. T. Ford, M. F. Fitzsimons and C. Halsall, Environ. Sci.: Adv., 2024, 3, 1385, DOI: https://doi.org/10.1039/ D4VA00006D. Environ. Sci.
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• Watson, G. M., Andersen, O.-K., Galloway, T. S. & Depledge, M. H. Rapid assessment of polycyclic aromatic hydrocarbon (PAH) exposure in decapod crustaceans by fluorimetric analysis of urine and haemolymph. Aquat. Toxicol. 67, 127–142 (2004).
• Hansen, D. J. et al. EPA-Procedures for the Derivation of Equilibrium Partitioning Sediment Benchmarks ESBs for the Protection of Benthic Organisms - PAH Mixtures. (2003).
• Stegeman, J. J. & Lecht, J. J. Cytochrome P-450 Monooxygenase Systems in Aquatic Species: Carcinogen Metabolism and Biomarkers for Carcinogen and Pollutant Exposure.
• Fogaça, F. H. D. S. et al. Polycyclic Aromatic Hydrocarbons in Seafood: Occurrence, Trophic Bioaccumulation, and Human Health Risks. Fishes 10, 591 (2025).
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• Yildirim Cikcikoglu, N., Serdar, O. & Derman, T. Individual and Combined Toxicity of Polycyclic Aromatic Hydrocarbons Phenanthrene and Fluoranthene in Freshwater Amphipod Gammarus pulex (L., 1758) (Amphipoda: Gammaridae). Acta Zool. Bulg. 3, 002704 (2023).
• Sanz-Lázaro, C., Marin, A. & Borredat, M. Toxicity Studies of Polynuclear Aromatic Hydrocarbons (PAHs) on European Amphipods. Toxicol. Mech. Methods 18, 323–327 (2008).