TESS stranded seal pup investigation 2025

For the past few years dying harbour (or common) seal pups have been stranding along the Tees Bay coastline, north as far as Northumberland and south to north Yorkshire. This has been happening in late summer - from the end of July to September, with a peak around mid-August - when the pups are newly weaned from their mothers. 

This little pup, born in the Tees seal colony in late June 2024 and cared for by its mother, had little chance of reaching two months of age.  

In late June-early July 2024 there were ~23 pups born at Seal Sands and cared for by their mothers. 

 Seal pups following their mothers at Seal Sands (June-July 2024) 

After weaning at about 3½ – 4 weeks old, the pups leave the Tees estuary and disperse along the coastline where they gradually learn to forage for themselves on small fish and shrimps. Sadly in 2024 – as in the previous few years, by late July and mid-August many of the pups had washed up along the coast – between north Yorkshire and Northumberland - in a starving condition, mostly less than normal birth weight - and nowhere near normal healthy weaning weight. Pups stranding later in August and September were a little bigger, but were still barely more than birth weight. 

 Body weights of pups stranding in north-east England in the summers of 2021, 2022 and 2024

Left: Healthy weaned pups (Co. Down, N. Ireland), more than 20 kg. Right: stranded emaciated Tees pup, weighing only 7 kg 

As well as being underweight, most of these pups also had "mouth rot" - where the soft tissue in and around the mouth had rotted away due to pathogenic bacteria. The pups are thought to be infected by the bacteria in the coastal waters, on the sea bed and in the shrimps and small fish that they are eating. The infection is thought to spread very rapidly, in a matter of days;  then the pup can no longer feed.   

However, the question was not only what bugs are responsible for the mouth rot - but why the stranded pups were so small and emaciated, likely even before the mouth rot infection set in. We suspected PCBs in the Tees Estuary and Tees Bay might be underlying this. PCBs were manufactured in the Tees until the production was banned in the 1980s and pups dying in the Tees estuary in the late 1980s/early1990s had high PCB levels in the fat under the skin. PCBs will have been lying in the Tees muddy sediments for the past 50-60 years.

So last summer (2024) our small group of five volunteers (Tees Estuary Seal Study – TESS) began to analyse tissue samples from a few pups that had been euthanised by vets. Samples of the mouth rot tissue were sent for bacterial culture, and skin samples from six pups were sent for analysis of PCBs in the fat layer. The highly pathogenic bacteria Serratia and Vibrio were isolated from two pups dying in early August, while Streptococcus was isolated from three pups stranding a few weeks later. 

Stranded pups from the Tees Bay area, underweight and with mouth rot - Left: pup has mouth rot on the lower lip, Right: mouth rot has extended to the pup's face. Both pups were so severely affected  they had to be euthanised by a vet.

The six pups we analysed for PCBs had levels above that which pups tend to be born underweight, fail to grow and thrive, and have poor immunity to infections – such as the mouth rot bacteria. 

PCB levels in the fat of six pups analysed by TESS in 2024 - the yellow dotted line indicates the approximate level above which growth and immune suppression are thought to occur. 

The pups get their PCBs mostly from their mothers' milk, which is contaminated by PCBs in the fish they eat during pregancy. The Pups' PCB levels at weaning may be topped up after weaning if they start to eat PCB contaminated food in the Tees Bay area.

This is what we think may be happening to the Tees pups, but clearly we need analyses of more dying pups before we can be sure, and the plan for 2025 is to have both the bacteria isolation and PCB analysis from each pup dying. If our theory on PCB contamination of fish in Tees Bay turns out to be correct, what can be done to help the Tees common seal colony in the future? This could be used as a scientific basis for reviewing policy decisions on offshore activities in the Tees.    

We expect this heart-breaking common seal pup mortality to recur this coming summer. This is why we are hoping to raise funds by crowdfunding to carry out the further analyses necessary for clarification of the causes of the pup deaths. 

The cost of the bacterial culture analysis is ~£65 (incl. VAT) per sample (at 2024 prices); the PCB analysis is ~£252 (incl. VAT), also at 2024 prices. There will also be costs of travel to and from Teesside,  and temporary local accommodation exacerbated by summer prices and availability. Even with vets and our volunteer team donating their expertise and time, funding is urgently needed for sufficient information to be collected this summer to ensure action is taken to save future Tees seal pups.