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  • Oxford University Press  (9)
  • 1
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    Error patterns in age estimation and tooth readability assignment of grey seals (Halichoerus grypus): results from a transatlantic, image-based, blind-reading study using known-age animals (2013)
    Oxford University Press
    Details
    Publication Date: 2013-02-21
    Description: Frie, A. K., Hammill, M. O., Hauksson, E., Lind, Y., Lockyer, C., Stenman, O., and Svetocheva, O. 2013. Error patterns in age estimation and tooth readability assignment of grey seals ( Halichoerus grypus ): results from a transatlantic, image-based, blind-reading study using known-age animals – ICES Journal of Marine Science, 70: 418–430. We analysed error patterns in a first interlaboratory grey seal ( Halichoerus grypus ) age-reading experiment. The experiment involved ten readers, who estimated age using images of cementum growth layers from teeth of 68 known-age seals (0–22 years). The percentages of correct estimates ranged from 32.4% to 60.3% among readers, and 89.3% of all errors were by ±1–2 years. Six readers showed increasing underageing with increasing seal age. An elevated risk of underestimation by 1 year occurred in teeth collected 0–5 months after breeding and was attributed to more frequent absence of a distinct growth layer for the new year and lack of information on months between the last birthday and the date of sample collection (plusmonths). For plusmonths 6–11, positive bias was predominant, suggesting that overestimation is the more common error when plusmonth information is available. Readers assigned readability scores to the tooth sections, and 79.1% of all ageing errors occurred in sections of low or intermediate readability. Excluding these sections would, however, also exclude 43.0% of all correct estimates. Neither levels of age estimation error nor predictive values of readability assignments were associated with reader experience levels. Analyses of image markings identified common errors in delineations of annual increment layers.
    Print ISSN: 1054-3139
    Electronic ISSN: 1095-9289
    Topics: Biology , Geosciences , Physics
    Published by Oxford University Press
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  • 2
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    Habitat selection by hooded seals (Cystophora cristata) in the Northwest Atlantic Ocean (2012)
    Oxford University Press
    Details
    Publication Date: 2012-12-12
    Description: Andersen, J. M., Wiersma, Y. F., Stenson, G. B., Hammill, M. O., Rosing-Asvid, A., and Skern-Maurizen, M. 2013. Habitat selection by hooded seals ( Cystophora cristata ) in the Northwest Atlantic Ocean. – ICES Journal of Marine Science, 70:173–185. We examined annual habitat use for 65 hooded seals (32 adult females, 17 adult males, and 16 juveniles) equipped with satellite relay data loggers (SRDLs) in spring or summer during five field seasons (2004–2008). A combined approach using first passage time (FPT) analysis and a generalized additive model (GAM) was applied to test for habitat selection, with a focus on environmental parameters of depth, slope, ice, sea surface temperature (SST), and chlorophyll. The models were run on adult males, adult females, and juveniles separately, and the results identified SST, depth, and chlorophyll as the most important factors influencing habitat selection across all categories. Furthermore, males and females preferred similar habitat conditions, but were separated geographically, and by depth, at various times of the year. Males appeared to be more localized in their habitat use patterns, focusing their search effort in areas of complex seabed relief such as Baffin Bay, Davis Strait, and the Flemish cap, while females concentrated their search effort along shelf areas (e.g. the Labrador shelf). These findings support our hypothesis that hooded seals prefer areas where topography and oceanographic processes create favourable foraging conditions.
    Print ISSN: 1054-3139
    Electronic ISSN: 1095-9289
    Topics: Biology , Geosciences , Physics
    Published by Oxford University Press
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  • 3
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    Modelling the abundance of grey seals (Halichoerus grypus) along the Norwegian coast (2012)
    Oxford University Press
    Details
    Publication Date: 2012-08-22
    Description: Øigård, T. A., Frie, A. K., Nilssen, K. T., and Hammill, M. O. 2012. Modelling the abundance of grey seals ( Halichoerus grypus ) along the Norwegian coast. – ICES Journal of Marine Science, 69: . An age-structured population dynamics model of the Norwegian grey seal ( Halichoerus grypus ) population has been developed. The model is of a Bayesian character in the sense that priors for various parameters were used. Model runs indicated an increase in the abundance of the total Norwegian grey seal population during the last 30 years, suggesting a total of 8740 (95% confidence interval: 7320–10 170) animals in 2011. A total catch of 707 (95% confidence interval: 532–882) grey seals would maintain the population size at the 2011 level. Model runs suggest that current catch levels will likely result in a reduction in the population size in Sør-Trøndelag and Nord-Trøndelag counties, and an increase in the population size in Rogaland, Nordland, Troms, and Finnmark counties. The model runs assumed that 80% of the seals taken in Rogaland came from the UK and that 50 and 55% of the catches in Troms and Finnmark, respectively, were immigrants from Russia.
    Print ISSN: 1054-3139
    Electronic ISSN: 1095-9289
    Topics: Biology , Geosciences , Physics
    Published by Oxford University Press
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  • 4
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    Error patterns in age estimation of harp seals (Pagophilus groenlandicus): results from a transatlantic, image-based, blind-reading experiment using known-age teeth (2011)
    Oxford University Press
    Details
    Publication Date: 2011-11-24
    Description: Frie, A. K., Fagerheim, K-A., Hammill, M. O., Kapel, F. O., Lockyer, C., Stenson, G. B., Rosing-Asvid, A., and Svetochev, V. 2011. Error patterns in age estimation of harp seals ( Pagophilus groenlandicus ): results from a transatlantic, image-based, blind-reading experiment using known-age teeth. – ICES Journal of Marine Science, 68: 1942–1953. Blind readings of known-age samples are the ultimate quality control method for age estimates based on hard tissues. Unfortunately, this is often not feasible for many species because of the scarcity of known-age samples. Based on a unique collection of known-age teeth of harp seals (age range: 1–18 years), ageing errors were evaluated in relation to true age, reader experience, sex, and tooth format (images vs. originals). Bias was estimated by linear models fitted to deviations from true age, and precision was estimated as their residual standard error. Image-based blind readings of 98 tooth sections by 14 readers, representing different levels of experience, generally showed high accuracy and precision up to a seal age of ~8 years, followed by an increasingly negative bias and increased variance. Separate analyses were therefore conducted for young seals (1–7 years) and older seals. For young seals, moderate associations were found between reader experience and levels of bias, precision, and proportions of correct readings. For older seals, only precision levels showed a significant association with reader experience. Minor effects of sex and tooth format are unlikely to affect these main patterns. Observed errors, even for highly experienced readers, may affect important age-related parameters, emphasizing the importance of known-age calibration of the output from all readers.
    Print ISSN: 1054-3139
    Electronic ISSN: 1095-9289
    Topics: Biology , Geosciences , Physics
    Published by Oxford University Press
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  • 5
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    Can ice breeding seals adapt to habitat loss in a time of climate change? (2014)
    Oxford University Press
    Details
    Publication Date: 2014-09-30
    Description: Climate change associated declines in sea ice will have serious impact on species that rely on ice for reproduction and/or feeding. Little is known about the impacts on ice-dependent, sub-Arctic species or on how these species may adapt, although the ecosystem changes are likely to be most rapid along the ice edge. Harp seals ( Pagophilus groenlandicus ) require stable ice for pupping, nursing and the first weeks after weaning when the young develop the capacity to swim and feed. Although ice conditions in the Northwest Atlantic have varied over the past 40 years, in 2010 and 2011, the total extent of ice suitable for whelping harp seals was at, or near, the lowest ever recorded. These years of exceptionally poor ice provided us with an opportunity to improve our understanding about how ice breeding seals may respond to the conditions expected in the future. Harp seals responded to poor ice conditions differently, depending on the presence or absence of ice at the beginning of the pupping period. If no ice was present, females moved away from their traditional whelping areas to find suitable ice. If small amounts of ice were present, females gave birth even if the ice was too thin to sustain the pups, resulting in high pup mortality. There was no evidence to indicate that harp seals pupped on land even in areas where ice was absent. Young seals that drifted to shore had high levels of abandonment and mortality. If the predicted warming trends continue, ice-breeding harp seals will encounter more years with poor ice conditions and may eventually adapt by moving north. Until then, they will continue to have increased levels of mortality that could result in the disappearance of the most southern breeding component in the Gulf of St Lawrence.
    Print ISSN: 1054-3139
    Electronic ISSN: 1095-9289
    Topics: Biology , Geosciences , Physics
    Published by Oxford University Press
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  • 6
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    A correction to "Error patterns in age estimation and tooth readability assignment of grey seals (Halichoerus grypus) - results from a transatlantic, image-based blind-reading study using known-age animals" (2013)
    Oxford University Press
    Details
    Publication Date: 2013-06-29
    Print ISSN: 1054-3139
    Electronic ISSN: 1095-9289
    Topics: Biology , Geosciences , Physics
    Published by Oxford University Press
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  • 7
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    Application of the precautionary approach and conservation reference points to management of Atlantic seals (2007)
    Oxford University Press
    Details
    Publication Date: 2007-05-01
    Description: Hammill, M. O., and Stenson, G. B. 2007. Application of the precautionary approach and conservation reference points to management of Atlantic seals. – ICES Journal of Marine Science, 64: 702–706. Resource management requires a trade-off between conservation, economic, and political concerns in establishing harvest levels. The precautionary approach (PA) brings scientists, resource managers, and stakeholders together to identify clear management objectives and to agree on population benchmarks that would initiate certain management actions when those benchmarks are exceeded. A conceptual framework for applying the PA to marine mammals is outlined. For a data-rich species, precautionary and conservation reference levels are proposed. When a population falls below the precautionary reference level, increasingly risk-averse conservation measures are applied. A more conservative, risk-averse approach is required for managing data-poor species. The framework has been implemented for the management of commercial seal harvests in Atlantic Canada.
    Print ISSN: 1054-3139
    Electronic ISSN: 1095-9289
    Topics: Biology , Geosciences , Physics
    Published by Oxford University Press on behalf of International Council for the Exploration of the Sea.
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  • 8
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    Growth and condition in harp seals: evidence of density-dependent and density-independent influences (2017)
    Oxford University Press
    Details
    Publication Date: 2017-01-04
    Print ISSN: 1054-3139
    Electronic ISSN: 1095-9289
    Topics: Biology , Geosciences , Physics
    Published by Oxford University Press on behalf of International Council for the Exploration of the Sea.
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  • 9
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    Can ice breeding seals adapt to habitat loss in a time of climate change? (2014)
    Oxford University Press
    Details
    Publication Date: 2014-05-15
    Description: Climate change associated declines in sea ice will have serious impact on species that rely on ice for reproduction and/or feeding. Little is known about the impacts on ice-dependent, sub-Arctic species or on how these species may adapt, although the ecosystem changes are likely to be most rapid along the ice edge. Harp seals (Pagophilus groenlandicus) require stable ice for pupping, nursing and the first weeks after weaning when the young develop the capacity to swim and feed. Although ice conditions in the Northwest Atlantic have varied over the past 40 years, in 2010 and 2011, the total extent of ice suitable for whelping harp seals was at, or near, the lowest ever recorded. These years of exceptionally poor ice provided us with an opportunity to improve our understanding about how ice breeding seals may respond to the conditions expected in the future. Harp seals responded to poor ice conditions differently, depending on the presence or absence of ice at the beginning of the pupping period. If no ice was present, females moved away from their traditional whelping areas to find suitable ice. If small amounts of ice were present, females gave birth even if the ice was too thin to sustain the pups, resulting in high pup mortality. There was no evidence to indicate that harp seals pupped on land even in areas where ice was absent. Young seals that drifted to shore had high levels of abandonment and mortality. If the predicted warming trends continue, ice-breeding harp seals will encounter more years with poor ice conditions and may eventually adapt by moving north. Until then, they will continue to have increased levels of mortality that could result in the disappearance of the most southern breeding component in the Gulf of St Lawrence.
    Print ISSN: 1054-3139
    Electronic ISSN: 1095-9289
    Topics: Biology , Geosciences , Physics
    Published by Oxford University Press on behalf of International Council for the Exploration of the Sea.
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