Abundance of Harbour Seals (<I>Phoca vitulina</I>) in Norway Based on Aerial Surveys and Photographic Documentation of Hauled-Out Seals During the Moulting Season, 1996 to 1999

Abstract

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.

Status of harbour seals (<i>Phoca vitulina</i>) in mainland Norway

“…Results from the present study are comparable with the results from a similar study carried out during the moult in 1996-1999, which resulted in a minimum estimate of 7,465 harbour seals along the entire Norwegian coast (Bjørge et al 2007). Known harbour seal colonies in the inner part of the Sognefjord in Sogn og Fjordane County were not surveyed, but an earlier study suggests an abundance of approximately 50 seals in that area in 1996 (Sørland and Dale 2003).…”

“…It is, therefore, difficult to give a corrected estimate of the total current abundance of the species along the Norwegian coast. Nevertheless, in an attempt to estimate the total population from the number of hauled out seals recorded in 1996-1999, Bjørge et al (2007) applied assumed correction factors, based on regional visual observations of harbour seal haulout behaviour in relation to tidal amplitude and diurnal light conditions, and suggested a total population of approximately 10,000 animals based on 7,465 harbour seals counted along the Norwegian coast. Assuming that this estimate reflected the approximate size of the harbour seal population in Norwegian waters when the study was conducted, Bjørge et al (2007) concluded that subsequent anthropogenic removals were most likely above sustainable levels and would reduce the population unless the current management regime was changed.…”

“…The likely magnitude of current anthropogenic removals, combined with the results from the most recent survey, emphasize the need for improved methodology to monitor population trends and effect of current management. Bjørge et al (2007) suggested the use of repetitive flights to improve the reliability of abundance estimation and associated variances with the estimates. Additionally, the proportion of the population hauled out at any time, as well as the age and sex composition of the hauled out seals, changes with area and season (Thompson and Rothery 1987, Kovacs et al 1990, Huber et al 2001, Daniel et al 2003, Jemison and Kelly 2003, Reder et al 2003.…”

“…Harbour seal abundance was investigated along the entire Norwegian coast at previously known moulting haulout sites (Øynes 1964, 1966, Bjørge 1991, Roen and Bjørge 1995, Bjørge and Øien 1999, Bjørge et al 2007 In the areas from Finnmark to Vestfold counties, the aerial photographic surveys were conducted using a PA31 Piper Navajo aircraft fitted with the gyro-mounted Leica RC 30 camera with 15.3 cm lens and Agfa Pan 400 aerographic black-and-white film and Agfa Aviphot colour HX100 PE1 film. The surveys were conducted at altitudes of 800-900 ft (243-274 m).…”

“…Harbour seals are usually counted during pupping and/or moult, when most of the species haul out (Heide-Jørgensen and Härkönen 1988, Thompson and Harwood 1990, Reijndeers et al 1997, Huber et al 2001, Gilbert et al 2005. A first harbour seal survey that included the entire Norwegian coast, primarily based on aerial photographic surveys, but supplemented with boat based visual counts in a few areas, was conducted during moult (August-September) in 1996-1999 and resulted in a point estimate of 7,465 harbour seals for the entire Norwegian coast (Bjørge et al 2007). Even though the methodology applied in 1996-1999 differed from previous studies, the new point estimate was taken as an indication of recent growth in the harbour seal population.…”