Environmental drivers of population-level variation in the migratory and diving ontogeny of an Arctic top predator

This work is an output of the ARISE project (NE/P006035/1 and NE/P00623X/1), part of the Changing Arctic Ocean programme jointly funded by the UKRI Natural Environment Research Council (NERC) and the German Federal Ministry of Education and Research (BMBF). Fieldwork in Canada was carried out under a Canadian Council on Animal Care permit no. NAFC2017–2 and funded by Fisheries and Oceans Canada and a bursary from Department for Business, Energy and Industrial Strategy (BEIS) administered by the NERC Arctic Office. Fieldwork in the Greenland Sea was approved by the Greenland Ministry of Fisheries, Hunting and Agriculture and the Norwegian Food Safety Authority (permit no. 11546) as part of the Northeast Greenland Environmental Study Program 2017–2018 (by the Danish Centre for Environment and Energy at Aarhus University, The Greenland Institute of Natural Resources and the Environmental Agency for Mineral Resource Activities of the Government of Greenland) and financed by oil licence holders in the area.The development of migratory strategies that enable juveniles to survive to sexual maturity is critical for species that exploit seasonal niches. For animals that forage via breath-hold diving, this requires a combination of both physiological and foraging skill development. Here, we assess how migratory and dive behaviour develop over the first year of life for a migratory Arctic top predator, the harp seal Pagophilus groenlandicus, tracked using animal-borne satellite relay data loggers. We reveal similarities in migratory movements and differences in diving behaviour between 38 juveniles tracked from the Greenland Sea and Northwest Atlantic breeding populations. In both regions, periods of resident and transitory behaviour during migration were associated with proxies for food availability: sea ice concentration and bathymetric depth. However, while ontogenetic development of dive behaviour was similar for both populations of juveniles over the first 25 days, after this time Greenland Sea animals performed shorter and shallower dives and were more closely associated with sea ice than Northwest Atlantic animals. Together, these results highlight the role of both intrinsic and extrinsic factors in shaping early life behaviour. Variation in the environmental conditions experienced during early life may shape how different populations respond to the rapid changes occurring in the Arctic ocean ecosystem.Publisher PDFPeer reviewe

Distribution, density and abundance of Antarctic ice seals off Queen Maud Land and the eastern Weddell Sea

The Antarctic Pack Ice Seal (APIS) Program was initiated in 1994 to estimate the abundance of four species of Antarctic phocids: the crabeater seal Lobodon carcinophaga , Weddell seal Leptonychotes weddellii , Ross seal Ommatophoca rossii and leopard seal Hydrurga leptonyx and to identify ecological relationships and habitat use patterns. The Atlantic sector of the Southern Ocean (the eastern sector of the Weddell Sea) was surveyed by research teams from Germany, Norway and South Africa using a range of aerial methods over five austral summers between 1996–1997 and 2000–2001. We used these observations to model densities of seals in the area, taking into account haul-out probabilities, survey-specific sighting probabilities and covariates derived from satellite-based ice concentrations and bathymetry. These models predicted the total abundance over the area bounded by the surveys (30° W and 10° E). In this sector of the coast, we estimated seal abundances of: 514 (95 % CI 337–886) 10^3 crabeater seals, 60.0 (43.2–94.4) 10^3 Weddell seals and 13.2 (5.50–39.7) 10^3 leopard seals. The crabeater seal densities, approximately 14,000 seals per degree longitude, are similar to estimates obtained by surveys in the Pacific and Indian sectors by other APIS researchers. Very few Ross seals were observed (24 total), leading to a conservative estimate of 830 (119–2894) individuals over the study area. These results provide an important baseline against which to compare future changes in seal distribution and abundance

High Mercury levels: are Arctic seals “what” or “where” they eat?

In contrast to other regions of the world or even sympatric terrestrial species, Arctic marine predators continue to accumulate increasing levels of Mercury (Hg) in their tissues. Hg bioaccumulation in Arctic seals may be linked to their particular life style, or to their extreme physiological adaptations, such as a short period of lactation with very fatty milk. The present study aimed at assessing how dietary resources and hunting distribution influence Hg exposure in Arctic true seals, through the integration of isotopic tracers with Hg levels. Indeed stable Carbon, Nitrogen and Sulfur isotope ratios can be successfully used to study species’ ecology and indicate potential contamination sources. For this reason hair was sampled from free-ranging hooded seal Cystophora cristata (Cc, n = 25) and harp seal Phoca groenlandicus (Pg, n = 36) in the pack ice of the Greenland Sea (near Jan Mayen). Stable isotope ratios were acquired via Isotope Ratio – Mass Spectrometry and used to model stable isotope niches (Standard Ellipses Areas; SEAs). Iterative Bayesian estimations were used to calculate the % of overlap between the ellipses. Total-Hg (T-Hg) concentrations were measured via Atomic absorption spectroscopy. The Cc δ15N-δ13C SEA (3.02‰2) was larger than that of Pg (2.64‰2) in 69% of model runs and did not overlap (22%). This may reflect Cc wide migrations down to warmer sub-Arctic waters compared to Pg that have an exclusively Arctic distribution. Moreover, while Cc hunt for a variety of bentho-pelagic prey (e.g., halibut, redfish, cod and squid) during long dives down to 1000m, Pg feed mostly on pelagic schooling fish between 100 and 400m of depth. The Cc δ15N-δ34S SEA (21‰2) was also larger than that of Pg (16‰2) in 85% of model runs; but this time the ellipses overlapped considerably (52%). Indeed both species presented two distinct groups along the δ34S axis: the most 34S enriched group included adult individuals, while the 34S depleted one included juveniles. This may result from the shallower hunting behavior of juvenile seals and their reliance on ice food webs. Finally, the larger migratory patterns and deep feeding behavior of Cc seem to determine significantly higher levels of T-Hg levels in this species (3.2±3.6 µg g-1) with respect to Pg (1.7±0.9µg g-1; U = 322, P = 0.01), as a consequence of (1) the higher number of Hg sources in sub-Arctic waters and (2) Hg remobilization from the sea bottom and its uptake by benthic food webs.SODYMARS (FRIA scholarship

Estimated food consumption of minke whales <i>Balaenoptera acutorostrata</i> in Northeast Atlantic waters in 1992-1995

Data on energy requirements, diet composition, and stock size were combined to estimate the consumption of various prey species by minke whales (Balaenoptera acutorostrata) in Northeast Atlantic waters. In the period 1992-1995, the stock of 85,000 minke whales appeared to have consumed more than 1.8 million tonnes of prey per year in coastal waters off northern Norway, in the Barents Sea and around Spitsbergen during an assumed 6 month stay between mid-April and mid-October. Uncertainties in stock estimates suggest a 95% confidence range of 1.4 - 2.1 million tonnes. The point estimate was composed of 602,000 tonnes of krill Thysanoessa spp., 633,000 tonnes of herring Clupea harengus, 142,000 tonnes of capelin Mallotus villosus, 256,000 tonnes of cod Gadus morhua, 128,000 tonnes of haddock Melanogrammus aeglefinus and 54,500 tonnes of other fish species, including saithe Pollaehius virens and sand eel Ammodytes sp. Consumption of various prey items by minke whales may represent an important mortality factor for some of the species. For example, the estimated annual consumption of herring corresponds to about 70% of the herring fisheries in the Northeast Atlantic in 1995. Minke whale diets are subject to year-to-year variations due to changes in the resource base in different feeding areas. Thus, the regional distribution of consumption of different prey items is highly dynamic

Novel insights into habitat use, diving and diet of the elusive Ross seal

Ross seals (Ommatophoca rossii) are the least studied and scarcest of the Antarctic pinnipeds. Only two studies exist on its at-sea movements: four and eight individuals tracked in the Amundsen and Weddell seas respectively. Diving behaviour has only been recorded for seven individuals and no longitudinal stable isotope data exist. Between 2016 and 2019, we deployed 15 satellite trackers of which seven measured diving behaviour and collected whiskers for bulk-stable isotope analyses from 25 individuals, making this the single largest study on Ross seals to date. Tracking data was combined with the eight animals previously tracked in the Weddell Sea to build the first habitat model for the species. Ross seals travelled away from the Antarctic pack-ice to forage pelagically on myctophid fish and cephalopods. This is reflected in the sequentially sampled bulk stable-isotope data from collected whiskers, with oscillations in δ13C and δ15N values reflecting their south-north movements. During winter, they spend most of their time tracking the marginal sea ice while summer is spent in open water. Ross seals dive deeper, but not longer, during the day presumably following the diel vertical migrations of their preferred prey and haul-out behaviour is influenced by lunar phases. The habitat model shows that sea-surface temperature is the most important indicator of foraging behaviour and they prefer to forage in a very narrow temperature band. This contrasts with suggestions that Ross seals might benefit from climate change due to the receding ice and reduced travel distances required to reach the open water

Arctic seals as tracers of environmental and ecological change

This work resulted from the ARISE project (NE/P006035/1, NE/P006310/1, and NE/P006000/2), part of the Changing Arctic Ocean programme, funded by the UKRI Natural Environment Research Council (NERC).Knowledge of species trophic position (TP) is an essential component of ecosystem management. Determining TP from stable nitrogen isotopes (δ15N) in predators requires understanding how these tracers vary across environments and how they relate to predator isotope composition. We used two seal species as a model for determining TP across large spatial scales in the Arctic. δ15N in seawater nitrate (δ15NNO3) and seal muscle amino acids (δ15NAA) were determined to independently characterize the base of the food web and the TP of harp and ringed seals, demonstrating a direct link between δ15NNO3 and δ15NAA. Our results show that the spatial variation in δ15NAA in seals reflects the δ15NNO3 end members in Pacific vs. Atlantic waters. This study provides a reference for best practice on accurate comparison of TP in predators and as such, provides a framework to assess the impact of environmental and human-induced changes on ecosystems at pan-Arctic scales.Publisher PDFPeer reviewe