Age- and sex-specific behaviour in harbour seals Phoca vitulina leads to biased estimates of vital population parameters

1. Modelling of the population dynamics of seals require data on an array o f vital parameters (fecundity, mortality, age structure, migrations, popula tion growth rate). The most common way to obtain these data is to estimate the parameters from samples taken from the population. However, the influen ce from skewed samples can be substantial in populations with age- and sex- specific features. By quantifying the behavioural differences among age and sex classes, data from skewed samples can be compensated retrospectively. Awareness of the existence and the potential magnitude of such biases is hi ghly relevant for the design of surveys, sampling programmes and the implem entation of management plans of age structured populations. 2. The age- and sex-specific behaviour of harbour seals Phoca vitulina and grey seals Halichoerus grypus can be studied by using freeze-branded animal s. Since the brand is permanent and visible up to a distance of 500 m, the harassment is limited to one occasion in the lifetime of the seal (the catc hing day). 3. A method is also given for analysing data arising from re-sighted brande d animals, where re-sightings of individual seals were transformed to estim ates of relative haul-out frequencies of seals by age and sex. 4. The composition of harbour seal groups on land exhibit a conspicuous sea sonal flux, and the fraction on land was not representative of the entire p opulation at any time during the summer. The results have far-reaching impl ications since most studies of seals are carried out at haul-out sites, and differential behaviour between the sexes and among age classes is expected in all populations and species of seals. Skewed samples generate biases in estimates of population growth rate, age-specific mortality and fecundity. 5. Age-specific haul-out patterns must be taken into account when analysing data from populations with non-stable age structures. As a consequence of changes in age structure after the 1988 seal epizootic, surveys under-estim ated the size of the Swedish-Danish harbour seal population by 6% in 1988 a nd over-estimated the same parameter by up to 16% during the following year s. 6. The present paper establishes the complications of sampling natural popu lations that are structured by age and sex, and presents a method on how to quantify sampling errors.