The Harbour Seal Phoca vitulina is much smaller than the Grey Seal Halichoerus grypus, with male Harbour Seals weighing up to 170 kg and reaching 1.5 metres in length. In appearance the Harbour Seal has a smaller, more rounded “dog-like” head, with smaller nostrils which are further apart and more horizontal than those of the Grey Seal.
The Harbour Seal is the most widespread of the northern hemisphere pinnipeds, existing as five subspecies in the temperate and sub-arctic coastal areas of the North Atlantic and North Pacific (Seal Conservation Society (SCS), 2012). The worldwide population estimate for Harbour Seal is around 350,000-500,000 animals (Thompson and Härkönen, 2008), while the UK population in 2010 was estimated to be 36,050, of which 79% reside in Scottish waters, 16% in England and the remainder in Northern Ireland (Sea Mammal Research Unit (SMRU), 2011).
In English waters, Harbour Seals mainly use the east coast and are known from regular haul-out sites between Sussex on the south coast and then along the east coast from southeast Kent to north Northumberland. By far the largest population of Harbour Seals occurs around The Wash in Lincolnshire and Norfolk. In 2010 it was estimated that out of the 4,200 Harbour Seals resident in England 3,100 use The Wash (SMRU, 2011). In this context the proportion of the total UK (and England) population using the northeast coast of England is very small. The largest known population of Harbour Seals along the coastline from the Tees to the Tweed is in the Tees estuary, where there is a breeding population of around 70 to 80 animals.
Harbour Seal have lived at the mouth of the River Tees for many hundreds of years and it is estimated that the population in the early 1800s was as high as 1,000 animals (Lofthouse, 1900). This population had declined rapidly by the mid-1800s. As the industrial use of the estuary increased, large areas of habitat were lost due to land reclamation, and an increase in the volume of shipping using the river led to further habitat loss due to dredging. Industrial pollution led to a drastic reduction in fish populations and the final demise of the resident seal colony. By the 1930s seals had totally disappeared from the Tees estuary.
The mid 20th century saw old-style steel and coke plants being replaced by newer, less polluting works. In the late 1960s and early 1970s there began a concerted effort by regulators, statutory authorities and industry to reduce the pollution load. Eventually Harbour Seals began to reappear and by the mid-1980s there was once again a resident population of seals. Teesmouth is thought to be the only known estuary in Europe where Harbour Seals have re-colonised as a direct result of environmental improvements.
The Tees Seal population today is focused upon Seal Sands, which is an area of tidally inundated sand and mudflats. Seals haul-out here over the low tide period and move between different areas of the sands as they become exposed and inundated as the tide ebbs and flows. They often move along Seaton Channel to the mudflats at Greatham Creek where they haul out most often, though not exclusively, at high tide. The story for the Harbour Seal population today is positive, with a slow and steady increase in the number of adults observed at the peak season in August when the seals gather for their annual moult. The number rose from 23 in 1989 when monitoring by the Industry Nature Conservation Association (INCA) first began, to the current maximum of 88 in August 2012 (Woods, 2012).
Pup births generally occur in the last week of June and the first week of July at Teesmouth. The first pup birth recorded for this colony was in 1989. This pup and singletons in 1991 and 1993 were born live and at full-term, but all died within a few days (Wilson, 1994). In 1994 two seals were born and survived. Subsequently there has been a steady rise in birth rate to the current maximum of 18 pups in 2012. It is generally accepted that newborn pups should form between 20-25% of the population for a healthy and balanced population (Reijnders, 1981; Helander and Bignert, 1992). This is now almost the case at Seal Sands.
In 2004 the Tees Valley Wildlife Trust reported that seals were starting to use other areas along the Tees. A few animals had started to haul-out on intertidal mudflats at Billingham Beck, 14.5 km upstream of the Tees estuary (Gibson, 2005). Numbers of Harbour Seal hauling out here are small, usually around 10 individuals at peak season. Several Harbour Seals are also regularly seen in the water at the Tees Barrage, which is the now the maximum extent of tidal flow on the Tees, 16 km from the estuary. Individual seals are regularly reported hauling out in random locations along the coast near Hartlepool to the north of the Tees estuary and at Redcar and Saltburn to the south.
In addition to the Tees there are occasional sightings of seals using both the River Wear and the River Tyne, often some miles inland. Anecdotal records exist of seals being seen as far as the tidal limit of the River Wear at Cox Green (R. Ball, pers. comm., 2009), but the most unusual observation on this river is of a single Harbour Seal seen at Chester-le-Street in December 2011 (E. Haswell, pers. comm., 2011), some 21 km from the sea and around 3 km upstream of the tidal limit.
Ornithologists from the Gateshead Birders Group have reported Harbour Seals in the River Tyne at all times of the year since 2003 (Environmental Records Information Centre (ERIC), pers. comm., 2012). Occasional observations are from as far west as Clara Vale near Ryton, which is about 1 km east of the tidal limit of the Tyne at Wylam Bridge and 40 km from the sea; from Stella Haugh (Pinnock, 2012) and Newburn Bridge (R. Ball, pers. comm., 2009) a little further downstream and there are more regular sightings further downstream at the “timber beach”, Dunston (ERIC, pers. comm., 2012).
Further north along the coast there is a small resident population of Harbour Seal at Holy Island in north Northumberland (A. Craggs, pers. comm., 2012). The seals are known to haul-out regularly here on exposed sandbars at Fenham Flats among a larger population of Grey Seals. A maximum of nine adult Harbour Seal were seen at this location in August 2010. There is also an unconfirmed report of two pups.
Dietary studies of Harbour Seals living in the Tees estuary indicate that they are opportunistic feeders, taking advantage of the seasonal abundance of available prey, preferring the gadid species Cod Gadus morhua, Whiting Merlangius merlangus, and Poor Cod Trisopterus minutus (Smurthwaite, 2006). They will also prey upon benthic fish such as Flounder Platichthys flesus and crustaceans such as Shore Crab Carcinus maenas.
Written by Robert Woods (last updated Nov 12)