Visual prey availability and distribution of foraging little auks (Alle alle) in the shelf waters of West Spitsbergen

Abstract

As diving seabirds use vision underwater, it is presumed they should preferentially select sites where their preferred food items are not only abundant but also clearly visible. To test this, we studied the optical properties of the seawater in the West Spitsbergen Shelf, in combination with zooplankton abundance in the feeding grounds of the planktivorous little auks from the nearby colonies in Hornsund. We estimated the relative attractiveness of the foraging sites using a novel parameter-visual prey availability (VPAv), which relates density and proportion of the preferred food item (Calanus glacialis) of the little auk, in total zooplankton, to the optical properties of the seawater. We found a significant positive correlation between the density of foraging little auks and VPAv values. Birds chose areas where C. glacialis was both abundant and clearly visible, because of the clarity of the water and low proportion of other zooplankton species. The birds avoided foraging over the warmer Atlantic-type waters, characterised by a high abundance of zooplankton taxa mostly ignored by birds and where VPAv values were low. VPAv values could potentially also be applied to other visual planktivores for which prey preference and visual acuity are known.

Intra-seasonal variation in zooplankton availability, chick diet and breeding performance of a high Arctic planktivorous seabird

“…The two fjords are the main breeding areas of the little auk on Svalbard (Isaksen 1995). We studied the zooplankton community composition on known foraging grounds of little auks breeding in Hornsund (fjord and shelf zone) and Magdalenefjorden areas (fjord and shelf zone close to Magdalenefjorden and Smeerenburgfjorden) , 2014Stempniewicz et al 2013;Hovinen et al 2014a) (Fig. 1).…”

Large versus small zooplankton in relation to temperature in the Arctic shelf region

“…In addition, since the differences in the percentage of large zooplankton in the three tested zones in 2013 and 2014 were very small, up to a maximum of 4%, it is very difficult to demonstrate a clear relationship, in contrast to the three colder years, when the maximum difference was up to 30%. The availability of preferred prey, expressed as its percentage in the total zooplankton abundance, seems to be a reliable proxy for assessing the quality of little auk foraging grounds Stempniewicz et al 2013). In this context, our study may suggest that the concentration of potential highquality food in the Arctic and Frontal Zone over colder years and close to the surface in the upper 20 m water layer may be related to potentially more favourable foraging conditions for little auks (Karnovsky et al 2003;Karnovsky et al 2010;Brown et al 2012) …”

“…This study provides only high-resolution LOPC measurements -information about zooplankton species composition is based on numerous previous studies conducted in the same study area (e.g., Kwasniewski et al 2010;Kwasniewski et al 2012;Jakubas et al 2013;Stempniewicz et al 2013;Trudnowska et al 2014;Trudnowska et al 2015;Trudnowska et al 2016). Using the abundance estimates and size structure data derived from the LOPC, we grouped plankton into two size fractions: a small size fraction (0.25-1.0 mm ESD), consisting of, e.g., Calanus nauplii, Oithona spp., Microcalanus spp., Pseudocalanus spp., and a large size fraction (1.0-2.5 mm ESD) -hereafter Calanus -represented predominantly by the older life stages (CIV, CV, adult) of Calanus spp.…”

“…To estimate the relative attractiveness of a foraging ground for little auk, it is important to consider the proportion of preferred species abundance in relation to total zooplankton abundance Stempniewicz et al 2013). Visual planktivores such as little auk need their prey to be abundant and also easily available, i.e., to occur in a high proportion compared to other zooplankton species.…”

“…Cold Arctic type waters provide particularly favourable conditions for feeding, where rather low total zooplankton abundance is compensated for by very high proportions of C. glacialis. On the other hand, Atlantic type water masses, which usually contain high total zooplankton abundance but a relatively low proportion of preferred copepods, are typically avoided by little auk Stempniewicz et al 2013).…”