Thames estuary revealed as critical habitat for harbour porpoises

Conservationists call for greater protections for Europe’s smallest whale in the Thames estuary as ZSL-led survey shows high presence there. 

The Greater Thames Estuary is a critical habitat for the harbour porpoise (Phocoena phocoena), and greater protection for marine mammals in the busy waterway is essential, according to a new report by ZSL (Zoological Society London). 
 
Published today (Wednesday 17 August), the report confirms that the Thames Estuary is an important habitat with significant densities of porpoises present in the area, over multiple years. Acoustic and visual surveys conducted by ZSL and Marine Conservation Research International (MCRI) in Spring 2022 along the south-east coast of the UK - from Suffolk to Kent - found several groups of porpoises, with a significant number residing in the outer Thames where the river meets the sea. 
 
Harbour porpoises are notoriously shy animals, often evading human presence, making them hard to see – and even harder to count. However, the small cetaceans use echolocation clicks at a very high frequency (around 120 kHz - more than six times higher than human hearing) to communicate with each other.  This unique communication method gave researchers an opportunity to detect them, employing specialist equipment including a hydrophone array, to pick up the porpoises’ sounds and provide accurate information on their presence.  
 
Two surveys, conducted over a seven-year period demonstrated high numbers of porpoise in the Thames Estuary with 31 individual detections of porpoise groups and 16 sightings (of these encounters, seven were both seen and heard) recorded in spring 2022.  
 
At just under two metres in length, the harbour porpoise is Europe’s smallest cetacean. Their small body size, coupled with life in temperate waters means that they must feed almost continuously to maintain their high metabolic rate. The reasons behind their presence in the Thames estuary requires further research, but the team believe that a good source of fish and use of the estuary as a nursery to raise their young could be two of them. 
 
Due to their hypersensitive hearing, harbour porpoises are easily disturbed by noise in the water from human activities such as boat engines or construction noise. They are also threatened by bycatch in fishing gear.  
 
In 2019, five Special Areas of Conservation (SACs) were designated as areas of importance for the species in the UK. This includes the Southern North Sea to the east of England, covering an area of almost 37,000 km2. This new report says that the continued rapid development in the Thames is a cause for concern, given the declines in densities of harbour porpoise documented in other regions. 

A harbour porpoise swimming

ZSL ecosystem restoration project manager, Anna Cucknell said: “Estuaries are important for many marine species, particularly for rearing young. The harbour porpoise is a ‘sentinel species’ - like the old adage ‘canary in the coalmine’, its presence is an indication of how healthy an ecosystem is. The porpoise numbers we’ve recorded in the Thames are comparable to those recorded in European protected sites, suggesting that the Thames, and potentially other UK estuaries are vitally important habitats for harbour porpoises.” 
 
Anna continued: "Detecting a high number of harbour porpoises in the Thames estuary this year signifies that large numbers are spending time outside of protected, offshore zones to find food and breed. 
 
“Harbour porpoise are a protected species in the UK under the Wildlife and Countryside Act, but the Thames waterway is a busy gateway- with regular ship traffic, important fisheries and offshore windfarms. Our intention is that our report will inform future conservation actions for this elusive marine mammal and provide evidence for wider coastal and estuary protection zones in the UK.” 
 
MCRI scientist Dr Oliver Boisseau said: “We are delighted to discover that porpoises are still utilising the Thames estuary in significant numbers, highlighting the importance of estuaries for this charismatic British marine mammal, and emphasising the need to undertake baseline research on protected species in other little studied UK coastal waters”.  
 
As we approach a landmark season for nature with global conferences such as CBD COP15 on the horizon, ZSL is calling for world leaders to put nature at the heart of all global decision making to ensure mitigations against biodiversity loss is part of wider policy decision-making. 

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