, Volume 40, Issue 1, pp 52–59 | Cite as

Detecting Density Dependence in Recovering Seal Populations

  • Carl Johan Svensson
  • Anders Eriksson
  • Tero Harkonen
  • Karin C. Harding


Time series of abundance estimates are commonly used for analyses of population trends and possible shifts in growth rate. We investigate if trends in age composition can be used as an alternative to abundance estimates for detection of decelerated population growth. Both methods were tested under two forms of density dependence and different levels of environmental variation in simulated time series of growth in Baltic gray seals. Under logistic growth, decelerating growth could be statistically confirmed after 16 years based on population counts and 14 years based on age composition. When density dependence sets in first at larger population sizes, the age composition method performed dramatically better than population counts, and a decline could be detected after 4 years (versus 10 years). Consequently, age composition analysis provides a complementary method to detect density dependence, particularly in populations where density dependence sets in late.


Density-dependent population growth Age-structured populations Population management Detecting population trends Environmental stochasticity 



Financial support was provided by the Swedish Environmental Protection Agency, the Royal Society of Arts and Sciences in Göteborg, and the Centre of Theoretical Biology at the University of Göteborg. Many thanks to Ailsa J. Hall for useful comments on earlier drafts.


  1. Bengtsson, J.L., and D.B. Siniff. 1981. Reproductive aspects of female Crabeater seals (Lobodon carcinophagus) along the Antarctic Peninsula. Canadian Journal of Zoology 59: 92–102.CrossRefGoogle Scholar
  2. Benton, T.G., C.T. Lapsley, and A.P. Beckerman. 2002. The population response to environmental noise: Population size, variance and correlation in an experimental system. Journal of Animal Ecology 71: 320–332.CrossRefGoogle Scholar
  3. Bjornstad, O.N., and B. Grenfell. 2001. Noisy clockwork: Time series analysis of population fluctuations in animals. Science 293: 638–643.CrossRefGoogle Scholar
  4. Boveng, P.L., L.M. Hiruki, M.K. Schwartz, and J.L. Bengtson. 1998. Population growth of Antarctic fur seals: Limitation by a top predator, the leopard seal? Ecology 79: 2863–2877.CrossRefGoogle Scholar
  5. Bowen, W.D., J.I. McMillan, and W. Blanchard. 2007. Reduced population growth of gray seals at Sable Island: Evidence from pup production and age of primiparity. Marine Mammal Science 23: 48–64.CrossRefGoogle Scholar
  6. Brynjarsdottir, J., and G. Stefansson. 2004. Analysis of cod catch data from Icelandic groundfish surveys using generalized linear models. Fisheries Research 70: 195–208.CrossRefGoogle Scholar
  7. Cadigan, N.G., and R.A. Myers. 2001. A comparison of gamma and lognormal maximum likelihood estimators in a sequential population analysis. Canadian Journal of Fisheries and Aquatic Sciences 58: 560–567.CrossRefGoogle Scholar
  8. Carlens, H., C. Lydersen, B.A. Kraffts, and K.M. Kovacs. 2006. Spring haul-out behaviour of ringed seals (Pusa Hispida) in Kongsfjorden, Svalbard. Marine Mammal Science 22: 379–393.CrossRefGoogle Scholar
  9. Caswell, H., ed. 2001. Matrix population models, 2nd ed., 727 pp. Sunderland, MA: Sinauer.Google Scholar
  10. Caughley, G. 1974. Interpretation of age-ratios. Journal of Wildlife Management 38: 557–562.CrossRefGoogle Scholar
  11. Cornell, H.V., R.H. Karlson, and T.P. Hughes. 2007. Scale-dependent variation in coral community similarity across sites, islands, and island groups. Ecology 88: 1707–1715.CrossRefGoogle Scholar
  12. Dennis, B., J.M. Ponciano, S.R. Lele, M.L. Taper, and D.F. Staples. 2006. Estimating density dependence, process noise, and observational error. Ecological Monographs 76: 323–341.CrossRefGoogle Scholar
  13. Eberhardt, L.L. 1977. Optimal policies for conservation of large mammals, with special reference to marine ecosystems. Environmental Conservation 4: 205–212.CrossRefGoogle Scholar
  14. Forcada, J. 2000. Can population surveys show if the Mediterranean Monk Seal colony at Cap Blanc is declining in abundance? The Journal of Applied Ecology 37: 171–181.Google Scholar
  15. Fowler, C.W. 1981. Density dependence as related to life history strategy. Ecology 62: 602–610.CrossRefGoogle Scholar
  16. Freckleton, R.P., A.R. Watkinson, R. Green, and W.J. Sutherland. 2006. Census error and the detection of density dependence. Journal of Animal Ecology 75: 837–851.CrossRefGoogle Scholar
  17. Gerrodette, T. 1987. A power analysis for detecting trends. Ecology 68: 1364–1372.CrossRefGoogle Scholar
  18. Harding, K.C., and T.J. Harkonen. 1999. Development in the Baltic grey seal (Halichoerus grypus) and ringed seal (Pusa hispida) populations during the 20th century. Ambio 28: 619–627.Google Scholar
  19. Harding, K.C., T. Harkonen, and H. Caswell. 2002. The 2002 European seal plague: Epidemiology and population consequences. Ecology Letters 5: 727–732.CrossRefGoogle Scholar
  20. Harding, K.C., T. Harkonen, B. Helander, and O. Karlsson. 2007. Status of Baltic grey seals: Population assessment and risk analysis. NAMMCO Scientific Publications 6: 33–56.Google Scholar
  21. Harkonen, T., and K.C. Harding. 2001. Spatial structure of harbour seal populations and the implications thereof. Canadian Journal of Zoology 79: 2115–2127.CrossRefGoogle Scholar
  22. Harkonen, T., K.C. Harding, and M.P. Heide-Jorgensen. 2002. Rates of increase in age-structured populations: A lesson from the European harbour seals. Canadian Journal of Zoology 80: 1498–1510.CrossRefGoogle Scholar
  23. Harwood, J., and J. Prime. 1978. Some factors affecting the size of British grey seal populations. Journal of Applied Ecology 15: 401–411.CrossRefGoogle Scholar
  24. Heide-Jørgensen, M.P., T. Harkonen, and P. Aberg. 1992. Long-term effects of epizootic in harbour seals in the Kettegatt-Skagerak and adjacent areas. Ambio 21: 511–516.Google Scholar
  25. Holmes, E.E., and A.E. York. 2003. Using age structures to detect impacts on threatened populations: A case study with Steller sea lions. Conservation Biology 17: 1794–1806.CrossRefGoogle Scholar
  26. ICES Advice. 2007. Book 8, pp 31.Google Scholar
  27. Innes, S., R.E.A. Stewart, and D.M. Lavigne. 1981. Growth in northwest Atlantic harp seals, Phoca groenlandica. Journal of Zoology 194: 11–24.CrossRefGoogle Scholar
  28. Jussi, M., T. Harkonen, E. Helle, and I. Jussi. 2008. Decreasing ice coverage will reduce the fitness of Baltic grey seal (Halichoerus grypus) females. Ambio 37: 80–85.CrossRefGoogle Scholar
  29. Kjellqwist, S.A., T. Haug, and T. Oritsland. 1995. Trends in age-composition, growth and reproductive parameters of Barents Sea harp seals, Phoca groenlandica. ICES Journal of Marine Science 52: 197–208.CrossRefGoogle Scholar
  30. Kokko, H., J. Lindstrom, and E. Ranta. 1997. Risk analysis of hunting seal populations in the Baltic. Conservation Biology 11: 917–927.CrossRefGoogle Scholar
  31. Krafft, B.A., K.M. Kovacs, A.K. Frie, T. Haug, and C. Lydersen. 2006. Growth and population parameters of ringed seals (Pusa hispida) from Svalbard, Norway, 2002–2004. ICES Journal of Marine Science 63: 1136–1144.Google Scholar
  32. Lande, R. 1993. Risks of population extinction from demographic and environmental stochasticity and random catastrophes. American Naturalist 142: 911–927.CrossRefGoogle Scholar
  33. Lande, R., S. Engen, and B.-E. Saether. 2002. Estimating density dependence in time-series of age-structured populations. Philosophical Transects of the Royal Society of London, Biological Sciences 357: 1179–1184.CrossRefGoogle Scholar
  34. Lett, P.F., R.K. Mohn, and D.F. Gray. 1979. Density dependent processes and management strategy for the north-west Atlantic harp seal population. ICNAF Selected Papers 5: 61–79.Google Scholar
  35. Mathews, E.A., and G.W. Pendleton. 2006. Declines in harbor seal (Phoca vitulina) numbers in Glacier Bay national park, Alaska, 1992–2002. Marine Mammal Science 22: 167–189.CrossRefGoogle Scholar
  36. McCain, C.M. 2005. Elevation gradients in diversity of small mammals. Ecology 8: 366–372.CrossRefGoogle Scholar
  37. Merrick, R.L., M.K. Chumbley, and G.V. Byrd. 1997. Diet diversity of Steller sea lions (Eumatopias jubatos) and their population decline in Alaska: A potential relationship. Canadian Journal of Fisheries and Aquatic Sciences 54: 1342–1348.CrossRefGoogle Scholar
  38. Murdoch, W.W. 1994. Population regulation in theory and practise. Ecology 75: 271–287.CrossRefGoogle Scholar
  39. Saether, B.-E., and S. Engen. 2002. Pattern of avian population growth rates. Philosophical Transects of the Royal Society of London, Biological Sciences 357: 1185–1195.CrossRefGoogle Scholar
  40. Saether, B.-E., R. Lande, S. Engen, H. Weimerskirch, M. Lillegard, R. Altwegg, P.H. Becker, T. Bregnballe, et al. 2005. Generation time and temporal scaling of bird population dynamics. Nature 436: 99–102.CrossRefGoogle Scholar
  41. Shenk, T.M., G.C. White, and K.P. Burnham. 1998. Sampling-variance effects on detecting density dependence from temporal trends in natural populations. Ecological Monographs 68: 445–463.CrossRefGoogle Scholar
  42. Sibly, R.M., D. Barker, M.C. Denham, J. Hone, and M. Pagel. 2005. On the regulation of populations of mammals, birds, fish and insects. Science 309: 607–610.CrossRefGoogle Scholar
  43. Sibly, R.M., and J. Hone. 2002. Population growth rate and its determinants: An overview. Philosophical Transects of the Royal Society of London, Biological Sciences 357: 1153–1170.CrossRefGoogle Scholar
  44. Stenseth, N.C., W. Falck, K.S. Chan, O.N. Bjornstad, M. O’Donoghue, H. Tong, R. Boonstra, S. Boutin, et al. 1998. From patterns to processes: Phase and density dependencies in the Canadian lynx cycle. Proceedings of the National Academy of Science of the USA 95: 15430–15435.CrossRefGoogle Scholar
  45. Taylor, B.L., M. Martinez, T. Gerrodette, and J. Barlow. 2007. Lessons from monitoring trends in abundance of marine mammals. Marine Mammal Science 23: 157–175.CrossRefGoogle Scholar
  46. Thompson, P.M., B. Mackey, T.R. Barton, C. Duck, and J.R.A. Butler. 2007. Assessing the potential impact of salmon fisheries management on the conservation status of harbour seals (Phoca vitulina) in north-east Scotland. Animal Conservation 10: 48–56.CrossRefGoogle Scholar
  47. Tuljapurkar, S.D. 1990. Population dynamics in variable environments, vol. 1. New York: Springer.Google Scholar
  48. Turchin, P. 1999. Population regulation: A synthetic view. Oikos 84: 153–159.CrossRefGoogle Scholar
  49. Turchin, P., ed. 2003. Complex population dynamics, 450 pp. Princeton, NJ: Princeton University Press.Google Scholar

Copyright information

© Royal Swedish Academy of Sciences 2010

Authors and Affiliations

  • Carl Johan Svensson
    • 1
  • Anders Eriksson
    • 2
  • Tero Harkonen
    • 3
  • Karin C. Harding
    • 1
  1. 1.Department of Marine EcologyGothenburg UniversityGothenburgSweden
  2. 2.Division of Physical Resource Theory, Department of Energy and EnvironmentChalmers University of TechnologyGothenburgSweden
  3. 3.Swedish Museum of Natural HistoryStockholmSweden

Personalised recommendations