Arne Bjørge

Habitat use and diving behaviour of harbour seals in a coastal archipelago in Norway

Abstract Background: the harbour seal is coastal non-migratory and suitable for behavioural studies using short-range telemetry and tracking. Methods: a combination of VHF radio telemetry and underwater ultrasonic telemetry was used to obain behavioural and physiological data from 13 harbour seals tagged at an archipelago in Norway. VHF signals were used to locate seals, and they were tracked at close proximity by inflatable boats. Results: transit and foraging activity were identified based on differences in dive profiles. When foraging, all tagged seals operated solitarily, and they returned repeatedly to the same or approximately the same foraging sites. The radio tagged seals used different types of foraging habitats, ranging from shallow kelp areas located 20 km offshore to 150–200 m deep basins with muddy sea bed located a few kilometres from the respective haul-out sites. The seals were typically foraging at or close to the sea bed. Display behaviour including underwater vocalization was recorded for sexually mature males in june and July. Conclusion: the combination of VHF and ultrasonic telemetry is useful for studies of resident seals. Tracking free ranging seals at close proximity made it possible to identify and describe their resting, foraging and display areas.

A Landscape Ecological Approach to Coastal Zone Applications

Managing complex environments requires suitable tools to integrate data from a variety of sources and efficiently analyse and present them within a geographical context. Recently there has been a growing interest in the integration of geographical, environmental and behavioural data for use in coastal zone management and planning. Our study shows how easily accessible information on bathymetry, terrain variation and wind conditions may be integrated into a georeferenced model applied to the study of key species and ecosystems of the Norwegian coastal zone. Through case studies, we predicted kelp forest ( Laminaria hyperborea ) distribution and analysed harbour seal ( Phoca vitulina ) habitat selection. Combining information on depth and wind exposure derived from a digital terrain model was a suitable approach to predict kelp forest distribution, even though the prediction showed deviations from information provided by kelp harvesters. Including information on sea-bed sediment improved the predictive ability drastically, and more investigation is needed to continue this kind of modelling. This approach is relevant for making decisions concerning site selection of kelp forest harvesting and restoration. Integrating the kelp forest prediction model with information on depth and the presence of slopes, islands and georeferenced behavioural data, we developed a technique for classifying habitats and studying resource selection.

Status of the harbour seal phoca vitulina L. in Norway

Abstract The current estimate of the number of harbour seal Phoca vitulina in Norwegian waters, including Svalbard, is 4129, based on actual counts of seals at the haul-out sites during the period 1977–1988. Comparison with an estimate obtained during the early 1960s indicates that there has been little change in the overall number of harbour seals in Norway during the last 25 years. Three distinct types of habitats are utilized by harbour seals in Norway—open rocky coasts, deep fjords and estuarine sandbanks. The habitat types and the status of legislative protection of harbour seals and their habitats are described.

Haul-out behaviour of the Norwegian harbour seal during summer

Abstract Background: the haul-out behaviour of the Norwegian harbour, seals had previously not been studied throughly. Methods: the intent was to examine the haul-out behaviour in relation to three factors: (1) the diel light cycle, (2) the tidal cycle and (3) the interaction between these two cycles. Observations were therefore made on days with low tide around noon, and later repeated on days with high tide around noon. The number of hauled-out seals was counted every hour, day and night. Three places were selected on the basis of their differing characteristics in diel light and tidal variation during summer. Results: the diel light cycle, the tidal cycle, and the interaction between the two cycles, all showed a significant relation with the haul-out behaviour both at Froan and in Kongsfjord. At Hvaler, the haulout pattern was very inconsistent, with large fluctuations from day to day. Conclusions: the higher numbers of hauled-out seals in the daytime, especially in Froan but to a certain degree also in Kongsfjord, may indicate that most seals feed at night. Similarly, the higher numbers hauled out at low tide, especially in Kongsfjord but also in Froan, may indicate that most seals feed around high tides. At Hvaler, the seals may be less at ease by the great deal of commercial and recreational boat traffic.

Seasonal and regional variations in the diet of harbour seal in Norwegian waters

Abstract Background: in order to describe the fish prey in the diet of the harbour seal Phoca vitulina, field studies were carried out in the Hvaler area in outer Oslofjord in 1990 and 1991, and in Froan in mid-Norway in 1991. Method: the studies were based on analysis of harbour seal faeces with identification of fish prey otoliths. The studies also included trawling in the Hvaler area to study the availability of prey. Results: analysis of faeces showed that the harbour seal feed mainly on shooling fish, predominantly benthic species. Most of the individual fish in the diet of the harbour seals were estimated not to exceed 30 cm in length. The most important groups of fish in the diet were the Gadidae, Clupeidae and Ammodytidae; Norway pout (Trisopterus esmarkii) was the single most important species both at Hvaler and at Froan. There was a substantial variation in the diet during the year, but to a lesser degree between the years. Regional variation in the diet was shown to occur between Hvaler and Froan. Conclusion: the harbour seals seem to have an opportunistic feeding strategy, but not feeding on all the species of fish found in the area. The variation both seasonally and regionally was probably due to variation in availability of species.

The reproductive biology of the harbour seal, Phoca vitulina L., in Norwegian waters

Abstract This study is aimed at establishing the mean age at attainment of sexual maturity and birth rates of harbour seals at the Norwegian coast, using information from 172 female and 184 male harbour seals sampled between 62 and 66° N during the period 1977–1982. Based on the occurrence of first time ovulation, the mean age at attainment of female sexual maturity was 3.75 years. The mass of ovaries showed maximum rate of increase between three and five years of age. Testicular growth indicates that males matured between five and seven years of age. The birth rate increased from 0.2 in four-year-old females to 0.5 in six-year-olds and to 0.9 in females eight years of age and older.

The effect of stochasticity in birth and survival on small populations of the harbour seal Phoca vitulina L.

Abstract Small groups of harbour seals are scattered along the entire Norwegian coastline and in some fjords. Some of these groups are separated by long distances from larger populations, and these groups may run a risk of extinction due to demographic and environmental stochasticity. Here we studied the viability of small groups of harbour seals assuming no migration between the smaller, isolated groups and larger populations. We applied a stochastic population model and population parameters recorded for harbour seals at the Norwegian coast. The model was divided into twenty-one different age categories (0–20 years of age), each with potential different survival and reproduction probabilities. Survival and reproduction were modelled at the level of the individual. The Minimum Viable Population (MVP) was defined as the minimum population size that ensures ≥95 % probability that the population will survive for a period of 100 years. For a hypothetical population that was stable using a deterministic popul...

Joint diving behaviour of harbour seal (Phoca vitulina) females and pups in the lactation period

We studied the behaviour of harbour seal (Phoca vitulina) female–pup pairs during joint diving sessions in the lactation period. During this period, females and pups most often started diving simultaneously. However, at night the pups most often started the diving events. The pair rarely resurfaced simultaneously, and there was no pattern in who resurfaced first. There was no difference in the dive duration of females and pups when they were swimming together. The pups occasionally dived alone, leaving their mothers at the surface. However, they returned faster to the surface when diving alone than when diving with their mothers. This was also the case when the females dived without the pups.