MEPS 581:199-214 (2017)  -  DOI: https://doi.org/10.3354/meps12285

ABSTRACT: Resource selection is a hierarchical and dynamic process. Selection can occur at different spatial and temporal scales, and can change over time with shifts in resource availability. The temporal dynamic of selection was investigated for a harbour seal Phoca vitulina population in Porsangerfjord, Norway, a subarctic fjord with large seasonal fluctuations in resource distribution. The availability of potential harbour seal prey was assumed to be dependent on the prey’s biomass densities, distance from the seals’ haulout sites and accessibility of the areas where prey was located, restricted by the presence of sea ice during winter and spring. The foraging behaviour of seals was investigated by assessing their preference and foraging response to the seasonal dynamics of prey distribution. The movement and dive patterns of individual seals (n = 10) were tracked with GPS devices. Foraging locations were compared to the availability of potential prey species in the fjord. Results suggested that harbour seals in Porsangerfjord had a preference for small-sized fish (<25 cm). Small codfish were preferred during autumn, but a response to the presence of pelagic fish was seen when the latter aggregated to overwinter in cold deep waters in the inner parts of the fjord. The formation of ice during late winter, however, provoked a shift in preference for small codfish, due to the sudden inaccessibility of pelagic fish. A strong reversed trend was observed in spring when the ice melted. The results indicate preference for small aggregated fish and the presence of a foraging response to changes in resource distribution.

KEY WORDS: Feeding response · Resource selection function · Resource availability · Preference · Predator-prey overlap · Home range · Haulout · Ice coverage · Phoca vitulina