Frie, A. K., Fagerheim, K-A., Hammill, M. O., Kapel, F. O., Lockyer, C., Stenson, G. B., Rosing-Asvid, A., and Svetochev, V. 2011. Error patterns in age estimation of harp seals (Pagophilus groenlandicus): results from a transatlantic, image-based, blind-reading experiment using known-age teeth. – ICES Journal of Marine Science, 68: 1942–1953. Blind readings of known-age samples are the ultimate quality control method for age estimates based on hard tissues. Unfortunately, this is often not feasible for many species because of the scarcity of known-age samples. Based on a unique collection of known-age teeth of harp seals (age range: 1–18 years), ageing errors were evaluated in relation to true age, reader experience, sex, and tooth format (images vs. originals). Bias was estimated by linear models fitted to deviations from true age, and precision was estimated as their residual standard error. Image-based blind readings of 98 tooth sections by 14 readers, representing different levels of experience, generally showed high accuracy and precision up to a seal age of ∼8 years, followed by an increasingly negative bias and increased variance. Separate analyses were therefore conducted for young seals (1–7 years) and older seals. For young seals, moderate associations were found between reader experience and levels of bias, precision, and proportions of correct readings. For older seals, only precision levels showed a significant association with reader experience. Minor effects of sex and tooth format are unlikely to affect these main patterns. Observed errors, even for highly experienced readers, may affect important age-related parameters, emphasizing the importance of known-age calibration of the output from all readers.

The first nationwide aerial survey to count harbour seals (Phoca vitulina) in Norway was conducted between 1996 and 1999. Haulout sites were surveyed during the early moult period of the seals in the second half of August, and most of the known haulout sites along the Norwegian coast (excluding Svalbard) were covered. The haulout sites were photographed, and the films were subsequently analysed at the Institute of Marine Research (IMR), Bergen. The number of seals hauled-out and documented during aerial surveys numbered 7,272. Some haulout sites in fjords of the alpine landscape of western Norway were difficult to cover by aerial surveys, so 193 hauledout seals counted from boat surveys in these fjords were added, and the total counted population was 7,465.Applying a correction factor for estimating the total population from the number of hauled-out harbour seals in the adjacent Swedish Skagerrak, the total estimate was 13,000 seals in Norway. We assumed that this was possibly an overestimate of the true population, however, because the tidal amplitude and diurnal light variation differ significantly along the Norwegian coast and are reported to affect the haulout patterns of harbour seals. When correction factors based on regional studies in Norwegian Skagerrak, western Norway, and Finnmark in northern Norway of haulout behaviour in relation to tidal amplitude and diurnal light variation were applied, the total population was estimated at 10,000 harbour seals. Most sites were surveyed only once; consequently, there is no estimate of variance.Because survey methods have changed from previous questionnaire studies and boat-based surveys, the current estimate cannot be used to assess trends in population size. Harbour seals in Norway are currently intensively hunted, however, and they are subject to high by-catch levels. A decline in numbers is expected under the current management regime, and there is an urgent demand for a new abundance estimate and improved survey design and methodology to account for bias, as well as the appropriate measures of uncertainty involved.

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