Haul-out behaviour, site fidelity and vigilance of common seals (Phoca vitulina) and grey seals (Halichoerus grypus) in the tees Estuary

Abstract

The project was carried out from the beginning of May to the beginning of July 1994, at Seal Sands, in the Tees estuary. The study aimed to examine (i) ecological and (ii) behavioural aspects of two species of seals, common seals (Phoca vitulina) and grey seals (Halichoerus grypus). The aims of the study were (i): (a) to document the seasonal fluctuation in number of seals using haul-out sites during May and June, (b) To compare activities between day and night time, (c) To examine possible factors influencing the proportion of the Tees population of common and grey seals hauling-out at Seal Sands, and factors conditioning haul- out site choice, (ii): (a) To examine distribution on haul-out sites and haul-out site fidelity, group and individual behaviour, (b) To determine vigilance behaviour in common and grey seals in relation to time of the day and group size. The number of seals hauled-out at Seal Sands, varied from day to day and night to night, and between successive low tides. The common seal number hauling-out, varied from a minimum of 13 to a maximum of 39, at day time, and from 9 to 15 at night time, these number increased gradually during the study period. The number of grey seals varied from nil on two occasions to 27 at daytime, and from 2 to 26 at night time. Both high and low number of common and grey seals tended to occur on the same days. The number of animals hauling out could be influenced by both environmental factors and population size at Teesmouth. Numbers of common seals at site C during day time was significantly and inversely related to the tide level at low water. The numbers of grey seals at site C at night time were inversely related to wind speed and positively related to maximum temperature. The changes in sites use are discussed in relation to physical characteristics of the sites and tide levels. Common and grey seals were observed to haul-out in restricted areas, although space availability was not a limiting factor. Common seals showed site fidelity both as individuals and as a group. Grey seals, show site fidelity as a group. Vigilance behaviour varied considerably within the two species of seals. In relation to the time of the day, the percentage of common seals alert, tended to decrease progressively after low tide. The percentage of grey seals alert was lower than in common seals with few peaks caused by sudden disturbance. Common seals show a strong negative correlation between the percentage of individuals alert and group size, whereas grey seals did not show any relation to the size of the group.