Food consumption estimates of Barents Sea harp seals
Abstract and Figures
The consumption of various prey species, required by the Barents Sea harp seal (Phoca groenlandica) stock in order to cover their energy demands, has been estimated by combining data on the energy density of prey species and on seasonal variations in the energy expenditure and body condition of the seals. Data on diet composition and body condition were collected in the period 1990-1996 by sampling harp seals during different seasons, in various areas of the Barents Sea. All diet composition data were based on reconstructed prey biomass, and adjustments were made for differences in digestibility of crustaceans and fish. The number of seals representing different age and sex groups were calculated for the entire population, and the monthly food requirements were estimated.
In 1998, the total Barents Sea harp seal stock was estimated to comprise 2.22 million seals based on a mean production of 301,000 pups. After adjustments for a pup mortality of 30% its total annual food consumption was estimated to be in the range of 3.35-5.05 million tonnes (depending on choice of input parameters). Assuming that there are seasonal changes in basal metabolic rate associated with
changes in body mass, and that the field metabolic rate of the seals corresponded to two times their predicted basal metabolic rate, the annual food consumption of the Barents Sea harp seal stock was estimated. If capelin (Mallolus villosus) was assumed to be abundant, the annual total consumption was estimated to be 3.35 million tonnes, of which 1,223,800 tonnes were crustaceans, 807,800 tonnes
were capelin, 605,300 tonnes were polar cod (Boreogadus saida), 212,400 tonnes were herring (Clupea harengus), 100,500 tonnes were cod (Gadus morhua) and 404,200 tonnes were "other fish". A very low capelin stock in the Barents Sea (as it was in the period 1993-1996) led to switches in seal diet composition, with increased consumption of polar cod (from ca. 16%-18 % to ca. 23%-25 % of
total consumption), other gadoids (dominated by cod, but also including haddock (Melanogrammus aeglefinus) and saithe (Pollachius virens)), herring, and "other fish". Using the same set of assumptions as in the previous estimate, the total consumption would have been 3.47 million tonnes, divided between various prey species as follows (in tonnes): polar cod 876,000, codfish (cod, saithe and haddock) 359,700, "other fish" 618,800, herring 392,500, and crustaceans 1,204,200. Overall, the largest quantities of food were estimated to be consumed in the period June-September.
In 1999, the total Barents Sea harp seal stock size was estimated to be 2.18 (95% CI, 1.79 to 2.58) million animals, which would give an annual food consumption in the range of 2,69 - 3.96 million tonnes (based on upper and lower 95% confidence limits and adjusted for a pup mortality rate of 0.3) if capelin is assumed to be abundant.
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... and the pelagic amphipod Themisto libellula. Polar cod, some demersal Arctic fish species, and pelagic crustaceans appear to be of particular importance during summer and autumn feeding in the northern parts of the Barents Sea (May-October) (Nilssen et al. 2000;Lindstrøm et al. 2013). The summer and autumn ice edge has retreated northwards to areas with much deeper waters north of Svalbard in recent years, and it appears that this has resulted in replacement of the demersal Arctic fish species with more boreal species such as northeast Arctic cod Gadus morhua and blue whiting Micromesistius poutassou on the harp seal menu (Haug et al. 2021). ...
... The species also plays a role in the Greenland and Norwegian Sea where, however, minke (Balaenoptera acutorostrata) and fin whales (Balaenoptera physalus) are the most prominent consumers. Using a bioenergetic model, Nilssen et al. (2000) estimated a total food consumption by 2.2 million (ca. 700,000 more than today) harp seals in the Barents Sea in the 1990s. ...
... During summer and autumn, their diet is dominated by invertebrate species such as krill (Thysanoessa spp.) and sea iceassociated amphipods (e.g. Themisto libellula), and Arctic cod Lindstrøm et al., 2013;Nilssen et al., 2000;Ogloff et al., 2019). During winter, forage fish, such as Atlantic herring, Arctic cod and especially capelin, are the primary prey, although the proportion of capelin in the diet varies among years reflecting the local abundance of other prey species, such as Atlantic cod, Greenland halibut, haddock, sand eels (Ammodytes sp.), sculpins, redfish (Sebastes spp.), gadoids, mysids and shrimp Nilssen et al., 2000;Nilssen, Haug, Potelov, Stasenkov, et al., 1995;Ogloff et al., 2019). ...
... Themisto libellula), and Arctic cod Lindstrøm et al., 2013;Nilssen et al., 2000;Ogloff et al., 2019). During winter, forage fish, such as Atlantic herring, Arctic cod and especially capelin, are the primary prey, although the proportion of capelin in the diet varies among years reflecting the local abundance of other prey species, such as Atlantic cod, Greenland halibut, haddock, sand eels (Ammodytes sp.), sculpins, redfish (Sebastes spp.), gadoids, mysids and shrimp Nilssen et al., 2000;Nilssen, Haug, Potelov, Stasenkov, et al., 1995;Ogloff et al., 2019). In general, demersal species such as Atlantic cod, Greenland halibut and haddock are at a higher trophic position, and have higher δ 15 N values than schooling fish such as capelin and ...
Arctic food webs are being impacted by borealisation and environmental change. To quantify the impact of these multiple forcings, it is crucial to accurately determine the temporal change in key ecosystem metrics, such as trophic position of top predators. Here, we measured stable nitrogen isotopes (δ15 N) in amino acids in harp seal teeth from across the North Atlantic spanning a period of 60 years to robustly assess multi-decadal trends in harp seal trophic position, accounting for changes in δ15 N at the base of the food web. We reveal long-term variations in trophic position of harp seals which are likely to reflect fluctuations in prey availability, specifically fish- or invertebrate-dominated diets. We show that the temporal trends in harp seal trophic position differ between the Northwest Atlantic, Greenland Sea and Barents Sea, suggesting divergent changes in each local ecosystem. Our results provide invaluable data for population dynamic and ecotoxicology studies.
... Since most marine mammals and boreal fish species are considered to be generalist predators (including feeders, grazers, etc.), a sigmoidal (type III) functional response might be more appropriate (Magnusson and Palsson, 1991). This has been proposed as being likely for minke whales, harp seals, and cod in the Barents Sea where switching between prey species has been hypothesized (Nilssen et al., 2000;Schweder et al., 2000). Sigmoidal functional response arising due to heterogeneity of space is less addressed in the literature. ...
In this paper, we investigate the complex dynamics of a spatial plankton-fish system with Holling type III functional responses. We have carried out the analytical study for both one and two dimensional system in details and found out a condition for diffusive instability of a locally stable equilibrium. Furthermore, we present a theoretical analysis of processes of pattern formation that involves organism distribution and their interaction of spatially distributed population with local diffusion. The results of numerical simulations reveal that, on increasing the value of the fish predation rates, the sequences spots spot-stripe mixtures stripes hole-stripe mixtures holes wave pattern is observed. Our study shows that the spatially extended model system has not only more complex dynamic patterns in the space, but also has spiral waves.
... The positive effect of polar cod on capelin indicates a covariation or release of predation pressure when polar cod is abundant. The latter, linked to the low effect of capelin on polar cod, may indicate that capelin is not a mitigation factor for the predation effect on the polar cod dynamics as previously assumed [12,18,56]. However, we cannot exclude such a mechanism since we did not account for other predators in our model. ...
Population dynamics depend on trophic interactions that are affected by climate change. The rise in sea temperature is associated with the disappearance of sea ice in the Arctic. In the Arctic part of the Barents Sea, Atlantic cod, capelin and polar cod are three fish populations that interact and are confronted with climate-induced sea ice reductions. The first is a major predator in the system, while the last two are key species in Arctic and sub-Arctic ecosystems, respectively. There are still many unknowns regarding how predicted environmental change may influence the joint dynamics of these populations. Using time series from a 32 year long survey, we developed a state-space model that jointly modelled the dynamics of cod, capelin and polar cod. Using a hindcast scenario approach, we projected the effect of reduced sea ice on these populations. We show that the impact of sea ice reduction and concomitant sea temperature increase may lead to a decrease of polar cod abundance at the benefit of capelin but not of cod which may decrease, resulting in strong changes in the food web. Our analyses show that climate change in the Arcto-boreal system can generate different species assemblages and new trophic interactions, which is the knowledge needed for effective management measures.
... To date, studies of marine mammal consumption in the Nordic and the Barents Seas have focused predominantly on only a few commercially harvested species, primarily common minke whales and harp seals (e.g. Sigurjónsson and Víkingsson, 1997;Stefánsson et al., 1997;Bogstad et al., 2000;Folkow et al., 2000;Nilssen et al., 2000;Lindstrøm et al., 2009), and considered consumption of only a few key fish species such as Northeast Atlantic (NEA) cod, herring, and capelin. However, the broad array of marine mammal species inhabiting these systems, together with the volume and range of fishery removals, warrants a more comprehensive assessment of marine mammal-fisheries interactions. ...
In this study, we assess prey consumption by the marine mammal community in the northeast Atlantic [including 21 taxa, across three regions: (I) the Icelandic shelf, Denmark Strait, and Iceland Sea (ICE); (II) the Greenland and Norwegian Seas (GN); and (III) the Barents Sea (BS)], and compare mammal requirements with removals by fisheries. To determine prey needs, estimates of energetic requirements were combined with diet and abundance information for parameterizing simple allometric scaling models, taking uncertainties into account through bootstrapping procedures. In total, marine mammals in the ICE, GN, and BS consumed 13.4 [Confidence Interval (CI): 5.6–25.0], 4.6 (CI: 1.9–8.6), and 7.1 (CI: 2.8–13.8) million tonnes of prey year–1. Fisheries removed 1.55, 1.45, and 1.16 million tonnes year–1 from these three areas, respectively. While fisheries generally operate at significantly higher trophic levels than marine mammals, we find that the potential for direct competition between marine mammals and fisheries is strongest in the GN and weakest in the BS. Furthermore, our results also demonstrate significant changes in mammal consumption compared to previous and more focused studies over the last decades. These changes likely reflect both ongoing population recoveries from historic whaling and the current rapid physical and biological changes of these high-latitude systems. We argue that changing distributions and abundances of mammals should be considered when establishing fisheries harvesting strategies, to ensure effective fisheries management and good conservation practices of top predators in such rapidly changing systems.
... Similar evaluations of populations' overall energy requirements and prey consumption have continued over the years (Banas et al., 2021;Bejarano et al., 2017;Benoit-Bird, 2004;Costa et al., 1989;Fortune et al., 2013;Gallagher et al., 2018;Guilpin et al., 2019;Kriete, 1995;Lockyer, 2007;Malavear, 2002;McHuron et al., 2017b;Noren, 2011;Noren et al., 2012Noren et al., , 2014Rechsteiner et al., 2013;Reisinger et al., 2011;Williams et al., 2004;Winship et al., 2002). Often, accounting models have been developed with the applied management goal of quantifying the levels of predation on prey stocks and potential competition with local fisheries (Acevedo and Urbán, 2021;Boyd, 2002;Cornick et al., 2006;Faure et al., 2021;Forcada et al., 2009;Markussen et al., 1992;McHuron et al., 2020;Mohn and Bowen, 1996;Nilssen et al., 2014;Olesiuk, 1993;Queiros et al., 2018;Trzcinski et al., 2006;Weise and Harvey, 2008). ...
Bioenergetic models describe the processes through which animals acquire energy from resources in the environment and allocate it to different life history functions. They capture some of the fundamental mechanisms regulating individuals, populations and ecosystems and have thus been used in a wide variety of theoretical and applied contexts. Here, I review the development of bioenergetic models for marine mammals and their application to management and conservation. For these long-lived, wide-ranging species, bioenergetic approaches were initially used to assess the energy requirements and prey consumption of individuals and populations. Increasingly, models are developed to describe the dynamics of energy intake and allocation and predict how resulting body reserves, vital rates and population dynamics might change as external conditions vary. The building blocks required to develop such models include estimates of intake rate, maintenance costs, growth patterns, energy storage and the dynamics of gestation and lactation, as well as rules for prioritizing allocation. I describe how these components have been parameterized for marine mammals and highlight critical research gaps. Large variation exists among available analytical approaches, reflecting the large range of life histories, management needs and data availability across studies. Flexibility in modelling strategy has supported tailored applications to specific case studies but has resulted in limited generality. Despite the many empirical and theoretical uncertainties that remain, bioenergetic models can be used to predict individual and population responses to environmental change and other anthropogenic impacts, thus providing powerful tools to inform effective management and conservation.
The UNCOVER project ‘Understanding the mechanisms of stock recovery’ has produced a rational scientific basis for developing Long-Term Management Plans (LTMPs) and recovery strategies for 11 of the ecologically and socioeconomically most important fish stocks/ fisheries in the Norwegian and Barents Seas (Northeast Arctic cod,
Norwegian spring-spawning herring, Barents Sea capelin), the North Sea (North Sea cod, Autumn spawning herring, North Sea plaice), the Baltic Sea (Eastern Baltic cod, Baltic sprat) and the Bay of Biscay and Iberian Peninsula (Northern hake, Southern hake, Bay of Biscay anchovy). UNCOVER’s objectives were to identify changes
experienced during stock depletion/collapses, to understand prospects for recovery, to enhance the scientific understanding of the mechanisms of fish stock/fishery recovery, and to formulate recommendations how best to implement LTMPs/recovery plans.
This UNCOVER report is aimed at a knowledgeable readership comprising, in particular, scientists, scientific advisors and administrators/managers in the fishery and environmental fields. The report provides an overview of the project’s aims and scope, approaches and methodologies, and detailed documentation of the deliverables and results which places these in relation to current and emerging challenges, constraints and opportunities.
UNCOVER emphasizes that it is essential to set ‘realistic’ long-term objectives and strategies for achieving successful LTMPs/recovery plans. It is recommended that such plans ideally should include:
1) Consideration of stock-regulating environmental processes;
2) Incorporation of fisheries effects on stock structure and reproductive potential;
3) Consideration of changes in habitat dynamics due to global change;
4) Incorporation of biological multispecies interactions;
5) Incorporation of technical multispecies interactions and mixed-fisheries issues;
6) Integration of economically optimized harvesting;
7) Exploration of the socio-economic implications and political constraints from the implementation of existing and alternative recovery plans;
8) Investigations on the acceptance of the plans by stakeholders and specifically incentives for compliance by the fishery;
9) Agreements with and among stakeholders.
UNCOVER has provided imperative policy support underpinning the following fundamental areas: a) Evolution of the Common Fisheries Policy with respect to several aims of the ‘Green Paper’; b) Contributing to the Marine Strategy Framework Directive with respect to fish stocks/communities; c) Furthering the aims of the 2002 Johannesburg Declaration of the World Summit on Sustainable Development regarding achieving Maximum Sustainable Yield (MSY) for depleted fish stocks. This has been done by contributing to LTMPs/recovery plans for fish stocks/fisheries, demonstrating how to shift from scientific advice based on limit reference points towards setting and attaining targets such as MSY, and furthering ecosystem-based management through incorporating multispecies, environmental and habitat, climate variability/change, and human dimensions into these plans.
The harp seal (Pagophilus groenlandicus) is the most abundant pinniped in the northern hemisphere, with an estimated total of 9.5 million animals. Commercially exploited since the eighteenth century, there is a large historical body of ecological knowledge that has provided insights into environmental factors that affect this species’ behavioral dynamics. Often referred to as the ice-loving seal from Greenland, harp seals breed and rest in spring on the drifting pack ice at the southern limits of their range, then migrate northwards to summer at the edge of the Arctic polar ice pack. Harp seals are gregarious during the breeding season. Ice-based research opportunities have provided insights into how harp seals locate conspecifics, care for their young, and how young transition from a ‘terrestrial’ to a marine environment. Fine-scale observations of animals outside of the breeding season have been more limited as animals disperse over hundreds of kilometers to the north of the breeding areas to molt and then feed. Nonetheless, the deployment of biologgers, working with seal hunters, and multidisciplinary studies have provided insights into factors affecting productivity and how environmental factors such as climate change may impact harp seals in the longer term.Keywords
Pagophilus groenlandicus
Ice sealLoup marinPack iceCapelinLactationDivingBehaviorHarp sealSaddleback seal
Climate change has large effects on population dynamics of fish species in high latitude ecosystems. Arctic fish stocks experience multiple pressures with changing abiotic living conditions and increased competition and predation from boreal species. However, there are many unknowns regarding how environmental change influences the dynamics of those populations. Here, we focused on the Barents Sea polar cod, a pan-Arctic zooplanktivorous key fish species physiologically and ecologically adapted to the presence of sea ice. We developed an age-resolved Bayesian state-space model of the dynamics of polar cod based on 30 yr of survey data (1986-2015). Using this model, we quantified how inter-annual changes in abundance were associated with abiotic variables (temperature and sea ice cover) and biotic variables (prey biomasses and a predation index). Using the model output, we used a hindcast scenario approach to investigate to which degree the observed variations in total population size were related to the abiotic or biotic variables. Our results showed that variation in abundance of young polar cod (ages 0 and 1) was best explained by abiotic variables while variation in the older age groups (ages 3 and 4) was best explained by predation. Hindcast scenarios showed that the abiotic variables had a more evident effect than predation on population dynamics, but none of the variables we considered could explain the drastic population decline observed in recent years. Our work shows the advantages of studying age-specific responses as a stepping stone to understand changes at the population level.
The distributional patterns of Barents Sea harp seals (Phoca groenlandica) throughout the year are presented based on existing literature and recent Norwegian and Russian field observations. The harp seals breed in February-March in the White Sea. Moulting occurs during April to June in the White Sea and southern Barents Sea. Feeding.behaviour is closely related to the configuration and localisation of the drifting sea-ice during summer and autumn (June-October) when the seals follow the receding ice edge, retiring gradually northwards and northeastwards in the Barents Sea. The southward movement of the population in autumn probably takes place in November prior to the advance of the ice edge, and is likely related to food-search. Apparently, most Barents Sea harp seals seems to concentrate at the southern end of their range in winter and spring.
The harp seal (Phoca groenlandica) population of the Barents and White seas has probably decreased from about one million individuals to half this size the last few years. Energy requirements of the population have been estimated by use of the simulation model SEAERG. In this model the energy requirements of an individual seal from each age group is multiplied with the group size and summed to provide the requirements of the population. In addition to population size and age structure the total food and energy requirement is sensitive to individual activity levels as well as metabolic levels and other specified physiological functions. The interactions between the seal population and fisheries depends on the caloric density of the prey species which varies with season and location. Realistic simulations of interactions between seals and fisheries require more information about spatial and temporal variations in the prey selection of harp seals than is available today. Present estimates indicate average maintenance requirements of about 13,600 and ll,150kcal/day for adult female and male harp seals respectively. The high value for the females is due to the costs of pregnancy and lactation. With a mean energy density of prey of 1500 kcal/kg, the corresponding food consumption is 9 kg/day for females and 7.4 kg/day for males.
Since 1978, and in particular in 1986-1988, large numbers of harp seals Phoca groenlandica ERXLEBEN have invaded coastal waters of North Norway during winter and spring. After 1988 the harp seal invasions have been restricted to the northeasternmost parts of the coast of Norway. In 1995, however, a significant increase occurred in both the magnitude and the spatial extent of the harp seal invasions. Diet composition, age structure and body condition parameters were examined on seals taken incidentally in Norwegian gill net fisheries during winter and spring in 1995. In early winter immature animals were taken, while mature females dominated in the spring. Analyses of stomach contents suggested that the diet mainly contained fish, in particular saithe Pollachius virens (L.), haddock Melanogrammus aeglefinus (L.) and cod Gadus morhua (L.) Body condition parameters revealed that the one year old seals taken in February 1995 were in significantly poorer condition than harp seals of the same age taken in the southeastern Barents Sea in February 1993. Also the mature females taken in April 1995 had significantly lower condition compared to adult females collected in April 1992. Recaptures of 39 tagged harp seals showed that some of the invading immature seals in the winter of 1995 belonged to the Barents Sea stock. Comparisons of age compositions of the Barents Sea harp seals based on material collected during Norwegian commercial sealing in the East Ice moulting lairs in the period 1978-1993 with samples from 1995 could suggest a low recruitment to this stock of the 1993 and in particular the 1994 year classes.
Nineteen hooded seals (Cystophora cristata) were tagged with satellite-linked platform terminal transmitters (PTT) on the sea ice near Jan Mayen. Fifteen were instrumented after completion of the mouly in July 1992 (five males, ten females, at 71°N, 12°W), and four during breeding in March 1993 (four females, at 69°N, 20°W). Sixteen of the seals were tagged with Satellite-Linked Time-Depth-Recorders (SLTDR), yielding location, dive depth and dive duration data. The average (±SD) longevity of all PTTs was 199±84 days (n=19; range: 43–340 days), and they yielded 12,834 location fixes. Between tagging in July 1992 and pupping in March 1993, two seals remained in or near the ice off the east coast of Greenland for most of the tracking period. However, most of the seals made one or several trips away from the ice edge, mostly to distant waters. These excursions had an average (±SD) duration of 47±22 days (n=46; range: 4–99 days). Eight seals travelled to waters off the Faeroe Islands, three to the continental shelf break south of Bear Island, and three to the Irminger Sea southwest of Iceland. Eleven seals were tracked in the period between breeding (March/April) and moulting (July). Several of these spent extended periods at sea west of the British Isles, or in the Norwegian Sea.