Harp seal body condition and trophic interactions with prey in Norwegian high Arctic waters in early autumn

Tore Haug,Martin Biuw,Harald Gjøsæter,Tor Knutsen,Ulf Lindstrøm,Kirsteen M Mackenzie,Sonnich Meier,Kjell T Nilssen

doi:10.1016/j.pocean.2020.102498

Tore Haug, Martin Biuw + Show 6 more

Open Access

https://doi.org/10.1016/j.pocean.2020.102498

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Abstract

In September 2016, a marine ecosystem survey covered all trophic levels from phytoplankton to seals in the Arctic Ocean to the west and north of Svalbard. At the ice edge, 26 harp seals were sampled to assess whether recent environmental changes had affected their diets and body condition by comparing our current results with previous investigations conducted 2–3 decades ago in the northern Barents Sea, when the ice edge was located much further south. Our results suggest that the body condition was slightly but significantly lower for one year and older seals in 2016 compared with seals sampled in the early 1990s. Furthermore, we confirmed previous findings that polar cod (Boreogadus saida) and the pelagic hyperiid amphipod Themisto libellula still dominate the seal diet. One consequence of current ice edge localisation north of Svalbard is that the water depth underneath is now 500 m and deeper, which probably explains the absence of bottom associated species, and the presence of species such as Atlantic cod (Gadus morhua) and blue whiting (Micromesistius poutassou) as alternative species in addition to polar cod and T. libellula in the seal diets. Stable isotope data also suggest possible long-term importance in the seal diet of T. libellula and of low trophic level benthopelagic prey such as the squid Gonatus fabricii over mid-trophic level pelagic fishes, but with a strong component of small, benthopelagic fish such as blue whiting. The long-term importance of pelagic crustaceans was also suggested from the fatty acid analyses. Assessment of the abundance of prey showed that T. libellula was by far the most abundant prey species in the upper water layers, followed by krill (mainly Thysanoessa inermis), Atlantic cod and polar cod. Prey-preference analyses indicated that polar cod was the most preferred prey species for the seals.

Highlights

The harp seal (Pagophilus groenlandicus) is the most important top predator in the Barents Sea ecosystem after the northeast Atlantic cod (Gadus morhua) (Bogstad et al, 2015)
From observed seasonal variation in the harp seal body condition, it is evident that the June-September period is when harp seals have the greatest increase in blubber mass, presumably due to increased food intake combined with increased energy content of prey (Nilssen et al, 1997, 2000)
The fatty acids trophic markers (FATM) analysis confirm previous and current instant short-term observations based on gastro-intestinal contents that harp seals feed much more on pelagic crustaceans, amphi pods, at their northern feeding grounds than in the south

Summary

Introduction

The harp seal (Pagophilus groenlandicus) is the most important top predator in the Barents Sea ecosystem after the northeast Atlantic cod (Gadus morhua) (Bogstad et al, 2015). Harp seals are pagophilic and prefer to be near sea ice at virtually all times of the year They are highly mobile predators that undertake extensive seasonal migrations, from breeding and moulting areas in the White Sea (southeast of the Barents Sea) in February-May to seasonally ice-covered areas in the northern Barents Sea, following the biological productivity of the ecosystem (e.g., Haug et al, 1994; Nilssen et al, 1995a, b; Nordøy et al, 2008; Lindstrøm et al, 2013). The availability of high-energetic food, such as krill and amphipods in the northern areas in summer and autumn presumably provide the energetic advantage necessary to account for the long migrations of harp seals from their more sub-Arctic winter and spring distributions (Lindstrøm et al, 2013)

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