SEASONAL OCCURRENCE AND MOVEMENTS OF SEALS IN NORTH‐WEST BRITAIN

Abstract

The breeding distribution of the grey seal (Halichoerus grypus) in north‐west Britain is well known. The general distribution of the common seal (Phoca vitulina) is not well known. The seasonal movements of seals in the area are unknown, though some postulates about them have been made. The work gives two sets of results: (i) an analysis of response of observers, and (ii) the analysis of the data which they reported to suggest seasonal occurence and movements of seals.A postal survey of seal occurrence in the area was carried out between June 1958 and May 1959. The co‐operation of over 100 observers at different stations was obtained. The response in terms of numbers of forms returned and of their content is analysed and found to be satisfactory. Owing to factors which affected observers in different ways, only about half the response could be used in studying seasonal changes. The common seal and grey seal are both involved. Periods of three months and over of high numbers can be grouped in two distinct classes: (i) those occurring from spring to autumn associated with the summer range of the grey seal, and (ii) from autumn to winter associated with the influx of young into the common seal population. Observations from the Firth of Clyde show a predominance of common seals. There is an assembly mostly of grey seals near Innellan in spring and summer, and large assemblies of commons, particularly in autumn and winter at Ardrossan (north shore), Lady Isle and Heads of Ayr. Seasonal movements are discussed. Observations from the North Channel coast show a mixed population of greys and commone from the Copeland Islands, Maidens (Larne), Rathlin, Davaar, Sanda and Mull of Kintyre, with commons only at Inishowen and Fanad Head. Seasonal movements are discuased. Observations from the Atlantic coasts of Scotland show a predominance of grey seals. The occurrence of both species is discussed, together with movements associated with heavy seas and movements of fish. The movements of grey seals from a wide summer range to the breeding islands takes place in August and September; movement back takes from January to June. There is a peak in seal movements (both species considered) in north‐west Britain in May and June. The upsurge in activity starts in March, increases greatly in April and is at maximum in May and June. Thereafter it declines gradually through summer and autumn to a minimum in winter. A subsidiary peak in September may be associated with movements of grey seals to breeding grounds. The suggested peak of movements is synchronous with the periods of finest weather in north‐west Britain. No connection between movements of seals and the disposition offish throughout the range of the survey, can be made.