Abundance surveys – Counting seals
Updated: September 2018
In order to effectively manage human impacts on marine mammals, we need to know how many marine mammals are in the management area. We also need to know if this number is changing over time: is the population going up or down? The best way to get this information is to carry out surveys to estimate the abundance of whales or seals in specific areas. This must be done repeatedly over time to determine if the population is stable, rising or falling.
Why do we count seals?
Surveys are carried out to generate estimates of absolute abundance, which is the number of animals in a specific area at a specific time. In some cases estimates of relative abundance (a fraction of absolute abundance that is assumed to be constant) are also useful. If surveys are repeated over time, trends in abundance (whether the number of animals is increasing, decreasing or stable over time) can be estimated. The main use of these estimates is for the management of whale and seal populations. They are also used for general environmental monitoring and ecosystem research.
Marine Mammal Management
“Management” does not mean telling seals what to do! It is actually the management of human impacts on seal populations, mainly direct catch (hunting) and indirect catch (by-catch, ship strikes), but also other sub-lethal impacts such as pollution and climate change. The main goal of management is usually to ensure that human impacts on marine mammal populations are sustainable, meaning that they do not cause the populations to decrease below a pre-defined threshold. To do this, estimates of abundance are combined with past, present and projected future catch levels in a population model, a mathematical model that mimics the response of the population to catch. This enables managers to set allowable total removal levels: levels of direct and/or indirect catch that will not endanger the population.
How do we count seals?
Pinnipeds are counted differently than cetaceans. Both adults and pups can be counted to estimate the total population. This is usually done during the whelping or moulting seasons when the seals are congregated on land or ice. The counts are performed differently between species and location. Different seal species have different whelping and moulting times, and this can even vary within species, depending on location.
Which type of count?
Pup production surveys
Pups are counted at breeding colonies to estimate pup production. The pup production and factors like female fecundity, mortality and catch rates are then used in a population model to estimate the total population size. Grey and harp seals are among the species for which the population size is based on pup production surveys.
Seals are counted during the moulting season, when most of the population is hauled out on land or ice and thus available for counting. Different age classes usually start moulting at different times, so surveys must be done multiple times throughout the moulting period, or when the highest proportion of seals are hauled out. Correction factors are then applied to account for the portion of seals not present on land or ice and therefore not available for counting. Harbour and ringed seals are among the species for which the population size is estimated from counts during the moulting season.
Ships versus air
Aerial surveys can be done in several ways. Seals can be counted directly by observers from a plane, and/or photographed from an airplane, with counts performed on the photographs. Lately, it has also become possible to use drones/unmanned aerial vehicles (UVAs) to photograph seals. This is both safer, less expensive, and more environmentally friendly than using airplanes.
In shipboard or land-based surveys, observers count hauled-out seals from ships or nearby islets. This can be difficult because of the angle, as seals can be hidden behind each other or in the back, and can thus be missed. The Institute of Marine Research (IMR) in Norway has had success combining this survey method with drone footage for harbour seal counts, especially when there are many seals on each site (IMR 2016).
Successful surveys; land-based, shipboard and aerial, are heavily dependent on good weather and sighting conditions.
Ice seals are those seal species whose life cycle is completed largely on or about the sea ice: harp, hooded, ringed and bearded seals in the North Atlantic/Arctic. They are nearly impossible to count directly because the seals are widely distributed and dispersed across the north Atlantic and Arctic Oceans at most times of the year. Also they spend much of their time underwater. Methods to estimate the population sizes of ice seal species usually use some combination of direct counts and population modelling to obtain the total number of seals.
Harp and hooded seals
Harp and hooded seals congregate during the whelping season, which makes counting them possible at that period. However, not all of the population is present at the surface at any one time or place. Therefore, direct surveys of the whelping grounds (counting seal pups on the ice) are used in combination with population modelling.
Surveys for harp and hooded seals use a combination of visual and photographic methods (Hammill et al. 2014, 2015, Øigard et al. 2014). Visual reconnaissance surveys, using helicopters and fixed wing aircraft, are used to locate and map the whelping patches. Photographic survey transects are then flown over the patches. Pups are counted on the photos by experienced readers, and the counts are verified by repeated readings. The pups are classified by “stages” according to their apparent age, from newborn to weaned pup. These data are used in combination with information from tagged pups to estimate the proportion of pups that are on the ice, and not in the water, and thus “available” to be seen, at any time. The corrected pup density on the photographic transects are applied to the entire survey area using standard strip transect methodology.
The total population is estimated from the pup production using population models. These models incorporate data on pregnancy rate at age, natural mortality rate and catch at age to calculate the number of adult seals needed to produce the number of pups observed during the survey (Hammill et al. 2014, 2015, Øigard et al. 2014).
Ringed seals are widely distributed throughout the Arctic, and they have never been counted throughout their range. However, ringed seal abundance has been estimated for various areas using many different methods: 1) direct counts from a ship or from land, 2) acoustic monitoring (estimating the number of seals based on number of recorded vocalizations), 3) estimating how big the population size must be to sustain the polar bear populations, 4) using dogs to find breeding lairs and counting the lairs, 5) looking at the type of sea ice that ringed seals typically use and then using known sea ice density to estimate ringed seal numbers, and 5) the most common method of using aerial surveys, similar to harp and hooded seals.
Rather than during the breeding period (when ringed seals are often hidden in their “birth lairs”), ringed seals are counted during the moulting period (when seals lose their hair and grow a new coat), as the most seals are then on the ice. Airplanes are flown over concentrations of seals and photographs are taken. Information from tagged seals are used to estimate how many seals were likely not counted because they were in the water and not available to be photographed.
Similar to ringed seals, bearded seals are widely distributed throughout the Arctic, and the total population has never been counted. However, some aerial surveys have been conducted in some areas, and ice density has also been used to estimate population size. For the ice density estimation, aerial surveys were conducted in various areas, and the average density of seals on various types of ice (e.g. fast ice versus pack ice) were documented. Then, using satellite maps of ice density, the number of bearded seals was estimated.
Coastal seals are seal species for which the life cycle is not obligatory associated with ice. Coastal seals; harbour and grey seals in the North Atlantic, are counted by visual and photographic methods during either the pupping season or the annual moult when most of the population haul out on land. In Norway, only parts of the coastline are surveyed each year, giving a complete count of coastal seals approximately every fifth year (Nilssen and Haug 2007, Nilssen et al 2010).
Pup production surveys are used to estimate the total grey seal population. In Norway, pups are counted using mainly shipboard and land-based surveys, and sometimes also aerial photographic surveys. The grey seal pupping season lasts for approximately three weeks, and thus every pupping area needs to be counted twice or three times during that period (Nilssen and Haug 2007). To prevent double-counting of pups on subsequent surveys, developmental stage of each pup is recorded (Nilssen and Haug 2007). The total population is then estimated using population modelling (Øigård et al. 2012). In Iceland, mainly aerial surveys are used to count grey seal pups, supplemented with shipboard and land-based surveys (CSWG 2016, Hauksson et al. 2014).
Harbour seal population investigations along the Norwegian coast began with interviews and questionnaires in the 1960s (Øynes 1964, 1966, cited in Nilssen et al. 2010).
Boat-based counts in the breeding season were later conducted. Presently, harbour seals are counted during the moulting season, using aerial photographic surveys (fixed-wing and drones). Also, visual counts using binoculars from boat and land are used in areas with difficult topography, or as a supplement in areas with low seal numbers (Nilssen et al. 2010, IMR 2016).
In Iceland, aerial population surveys of harbour seals have been conducted from 1980, using both photography and direct counts. Two observers count and photograph all observed seals. The surveys are done under standardized conditions at the peak of the moulting season, using haul-out sites from previous surveys (Þorbjörnsson et al. 2017). Icelandic harbour seals mostly haul out in smaller groups of <4 animals, meaning that the entire coastline needs to be surveyed in order to obtain an accurate population estimate (Þorbjörnsson et al. 2017). This has proved difficult due to the amount of coastline needed to be covered in quite a short time (approximately 3 weeks), and often few possible days airborne due to bad weather (CSWG 2016). Correction factors used to account for biases are not yet optimized for Icelandic conditions. However, because similar correction factors are used for all years, the yearly results are comparable between years (Þorbjörnsson et al. 2017) and represent an index of relative abundance.
(CSWG) Report of the Coastal Seals Working Group (2016) Reykjavik, Iceland. Available at: https://nammco.no/topics/sc-working-group-reports/
Hammill, M.O., Stenson, G.B., Mosnier A., and Doniol-Valcroze, T. (2014). Abundance Estimates of Northwest Atlantic Harp seals and Management advice for 2014. DFO Can. Sci. Advis. Sec. Res. Doc. 2014/022. v + 33 p.
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IMR (2016). Omfattende telling av sel i sør, Institute of Marine Research [online] Available at: https://www.hi.no/nyhetsarkiv/2016/september/omfattende_telling_av_sel_i_sor/nb-no [Accessed 27 June 2018]
Nilssen, K.T. and Haug, T. (2007). Status of grey seals (Halichoerus grypus) in Norway. NAMMCO Sci. Publ. 6:23-31
Nilssen, K.T., Skavberg, N.-E., Poltermann, M., Haug, T., Härkönen, T., and Henriksen, G. (2010). Status of harbour seals (Phoca vitulina) in mainland Norway. NAMMCO Sci. Publ. 8:61-70
Þorbjörnsson, J. G., Hauksson, E., Sigurðsson, G. M. and Granquist, S. M. (2017), Aerial census of the Icelandic harbour seal (Phoca vitulina) population in 2016: Population estimate, trends and current status. Available at: https://www.hafogvatn.is/is/midlun/utgafa/haf-og-vatnarannsoknir/aerial-census-of-the-icelandic-harbour-seal-phoca-vitulina-population-in-2016-population-estimate-trends-and-current-status-landselstalning-2016-stofnstaerdarmat-sveiflur-og-astand-stofns
Øigård, T. A., Haug, T., and Nilssen, K. T. (2014). From pup production to quotas: current status of harp seals in the Greenland Sea. – ICES Journal of Marine Science, 71: 537–545.
Øigård, T.A., Frie, A.K., Nilssen, K.T. & Hammill, M.O. 2012. Modelling the abundance of grey seals (Halichoerus gryupus) along the Norwegian coast. ICES Journal of Marine Science 69: 1446-1447. doi: 10.1093/icesjms/fss013