The Grey seal (Halichoerus grypus) at North Rona, 1959 to 1968
Authors: Boyd J. Morton , Campbell R. Niall 
Published: August 2009

North Rona (59° 06′ N, 05° 50′ W) has an area of about 120 ha (300 acres) and rises to about 108 m (355 feet) above sea level. Together with the neighbouring islet of Sula Sgeir, declared a National Nature Reserve in 1956, mainly because of the breeding assembly of Grey seals (Halichoerus grypus) which breeds there. This seal assembly has been visited annually in October‐November in 1959–68 inclusive and the following work was done:

(i) annual counts of all pups found alive and dead on the island on two days in late October;

(ii) establishment of the sex ratios of newly‐born, moulted, ailing and dead pups;

(iii) tagging of 783 pups in 1960–61;

(iv) branding of 4672 pups in 1960–68;

(v) weighing of newly‐born and moulted pups;

(vi) observations on the morbidity of pups;

(vii) miscellaneous observations on distribution, behaviour and health of adults and pups.

Tagging was by plastic discs attached to the tail with Monel wire. Thirty‐one sightings or recoveries of tags were obtained, ranging from east Iceland, about 756 km (470 miles) from Rona, to Ireland, and St. Kilda to Norway, about 724 km (450 miles). The oldest seal carrying a tailtag recovered (in 1963) was two years of age.

Branding was by hot iron with cyphers placed on different parts of the body to indicate the year of branding. So far 51 seals branded at Rona have been sighted at places ranging from Norway, about 640 km (400 miles) to Rona itself, in subsequent years.

The main seal assembly is concentrated unevenly on the low‐lying northern peninsula of Fianuis. There is a smaller assembly on Sceapull in the south‐west, and a few on the main ridge over 61 m (200 feet) above sea level. Large areas of the island are unoccupied by seals. Censuses were done by a standard method using study areas on the island and marking counted pups with dye. Between 1900 and 2700 pups were estimated as born annually on North Rona in 1959–68, the average being 2250. Mortality of pups ranged from 14 to 25% of the total born annually, with an average of 18 %. Approximately 112 males were born to every 100 females, but differential mortality resulted in about 93 males going to sea to every 100 females. Numbers of pups born in the various study areas fluctuated from year to year, an increase in one area being usually accompanied by a decrease in a neighbouring one.

Newly‐born male pups averaged 15–8 kg (34.8 lb) and females 14–9 kg (32.8 lb). Completely moulted male pups (moulters) averaged 39–8 kg (87.8 lb) and females 39–7 kg (87.4 lb). The average daily increment recorded over 17 days' suckling was 1 3 kg (2‐9 lb) in males and 1–4 kg (3.1 lb) in females. The average daily increment recorded in pups weighed at frequent intervals was 0–41 kg (0‐9 lb) in males and 1–1 kg (2.4 lb) in females. These differences may be due to disturbance.

The future of the branding and census work is discussed in relationship to conservation of the assembly. Monitoring needs require to be balanced against the disturbance and concomitant mortality of pups and other effects on breeding success. A watch must now be kept at North Rona, and at other seal assemblies throughout the British Isles, to record the breeding of seals born at North Rona 1960–68. The survival of cohorts of marked seals should throw light on the accuracy of Hewer's (1964) provisional life table for the Grey seal.