This painstaking analysis is part of the first comprehensive study of the breeding population of grey seals along the south-western coast of Wales. The results are published today by the Countryside Council for Wales. Seals are often seen as a threat by fishermen who believe they eat commercially valuable fish species, but the survey found that the Welsh seals have such a catholic diet that they pose no threat to fisheries.
The Welsh population is at the species' southern limit in the north-east Atlantic. The survey has revealed that more than 1,300 pups were born along 75 miles of coastline - in sea caves and on tiny, boulder-strewn beaches - each year from 1992 to 1994. Calculating from this number of pups, the researchers estimate that 5,000 seals inhabit the area, scattered in unusually small breeding groups. The three-year census, combined with unique information on breeding success, diet and the threat of disturbance, has been carried out by a team working for the Dyfed Wildlife Trust under contract to the countryside council. It cost more than pounds 200,000.
Grey seal males grow to nearly 13ft; femalesgrow to 8ft. Males are dark brown or black with pale blotches; females are usually paler, while the pups are almost white. Common seals - a separate species found off the Scottish and eastern English coast - are much smaller and are not found off south-west Wales.
Britain is home to half the world's grey seals, which breed mainly on beaches and small islands in the Hebrides and Orkney, where counting pups is relatively easy. The Welsh population may be the largest colony in the world exploiting caves as breeding sites.
"Counting them was a formidable task," says Mick Baines, who led the census team. "The breeding sites are, on the whole, inaccessible. Sea caves can only be reached by a specially equipped boat and using dry-suited swimmers to get in, while very few of the beaches can be reached from land. The pups can't even be seen from the clifftops in several places."
From Aberystwyth in the north to Caldy Island in the south, the team recorded 225 pupping sites. The number of pups proved similar in each of the three years, averaging 1,331, although there a slight upward trend suggests the population is slowly increasing. About 40 per cent of pups were in caves on tiny, dark shingle and boulder beaches above the tidal limit. Born between September and December, with a peak in early October, the pups are suckled for up to three weeks and lie docile for a few days. After that, with growing teeth and hardening claws, they learn to snarl and go into attack mode when approached too closely. After three weeks, they moult their white fur and disperse at sea.
Caves and inaccessible, cliff-backed boulder beaches are dangerous places for seal pups. At high tide, these (at first immobile) youngsters may be as little as 3ft above the water, and storms may drown some before they are able to swim. Recorded mortality in the census averaged just under 10 per cent. Mr Baines says this may be an underestimate because the number washed away is not known. Seals have bad years - for instance, the island of Skomer suffered a 43 per cent mortality rate in the 1975 season.
Some beaches had more than their fair share of rubbish, including discarded fishing nets and ropes which could be death traps for pups. There was evidence of females avoiding litter-strewn beaches when about to give birth.
But giving birth a hair's breadth from the sea has plus points. Dense breeding colonies well back from high water - as in many colonies in northern Britain - can be muddy, smelly and riddled with pathogens. Pups succumb readily to infections. Welsh beaches and caves, with their more scattered pups, are cleaned by salt-water washes.
A more dramatic threat to these grey seals is posed by the millions of tons of crude oil that pass nearby each year on their way into Milford Haven. When the Norwegian-registered Borga ran aground in October, tearing its hull, no oil was spilt - the tanker was double hulled. Most such ships using these waters have one hull.
Wales's grey seals - and much other marine life - are vulnerable. Canoes sometimes enter caves, not knowing what is inside. Seals avoid beaches used by the public, even though many of them have excellent pupping sites. Pleasure cruises to look at seals are increasing. A voluntary code of conduct has been agreed with the main tour operators. But Mr Baines thinks that earlier pupping in caves, rather than on beaches, may be linked to higher disturbance levels on beaches.
Powell Strong's encounter with piles of smelly seal droppings has produced valuable information. By identifying undigested hard parts of fish, crustaceans and molluscs, Mr Strong found that 70 per cent of the seals' diet consisted of whiting, Norway pout, bib, poor cod and sole. Herring made up just six per cent, but occasionally dominated the diet on some stretches of coast. Sometimes these fish were of commercial size, sometimes not.
The seals' menu is extremely varied. Mr Strong recorded about 40 species of fish - from little black goby and ugly lumpsucker to octopus and squid. Salmon and sea trout - of concern to river fishermen - hardly registered.
As the winter storms begin, the last moulted pups will swim away from their places of birth; the seals will spend most of the time until next September at sea. But the breeding females are already pregnant, ready to return next autumn.
The writer is director of policy and science with the Countryside Council for Wales.